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![Alexandre Courbot](/assets/img/avatar_default.png)
GENERIC_GPIO has been made equivalent to GPIOLIB in architecture code and all driver code has been switch to depend on GPIOLIB. It is thus safe to have GENERIC_GPIO removed. Signed-off-by: Alexandre Courbot <acourbot@nvidia.com> Acked-by: Linus Walleij <linus.walleij@linaro.org> Acked-by: Grant Likely <grant.likely@secretlab.ca>
775 lines
34 KiB
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775 lines
34 KiB
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GPIO Interfaces
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This provides an overview of GPIO access conventions on Linux.
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These calls use the gpio_* naming prefix. No other calls should use that
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prefix, or the related __gpio_* prefix.
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What is a GPIO?
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===============
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A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
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digital signal. They are provided from many kinds of chip, and are familiar
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to Linux developers working with embedded and custom hardware. Each GPIO
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represents a bit connected to a particular pin, or "ball" on Ball Grid Array
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(BGA) packages. Board schematics show which external hardware connects to
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which GPIOs. Drivers can be written generically, so that board setup code
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passes such pin configuration data to drivers.
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System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
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non-dedicated pin can be configured as a GPIO; and most chips have at least
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several dozen of them. Programmable logic devices (like FPGAs) can easily
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provide GPIOs; multifunction chips like power managers, and audio codecs
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often have a few such pins to help with pin scarcity on SOCs; and there are
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also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
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Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
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firmware knowing how they're used).
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The exact capabilities of GPIOs vary between systems. Common options:
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- Output values are writable (high=1, low=0). Some chips also have
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options about how that value is driven, so that for example only one
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value might be driven ... supporting "wire-OR" and similar schemes
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for the other value (notably, "open drain" signaling).
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- Input values are likewise readable (1, 0). Some chips support readback
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of pins configured as "output", which is very useful in such "wire-OR"
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cases (to support bidirectional signaling). GPIO controllers may have
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input de-glitch/debounce logic, sometimes with software controls.
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- Inputs can often be used as IRQ signals, often edge triggered but
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sometimes level triggered. Such IRQs may be configurable as system
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wakeup events, to wake the system from a low power state.
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- Usually a GPIO will be configurable as either input or output, as needed
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by different product boards; single direction ones exist too.
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- Most GPIOs can be accessed while holding spinlocks, but those accessed
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through a serial bus normally can't. Some systems support both types.
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On a given board each GPIO is used for one specific purpose like monitoring
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MMC/SD card insertion/removal, detecting card writeprotect status, driving
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a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
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watchdog, sensing a switch, and so on.
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GPIO conventions
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================
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Note that this is called a "convention" because you don't need to do it this
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way, and it's no crime if you don't. There **are** cases where portability
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is not the main issue; GPIOs are often used for the kind of board-specific
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glue logic that may even change between board revisions, and can't ever be
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used on a board that's wired differently. Only least-common-denominator
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functionality can be very portable. Other features are platform-specific,
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and that can be critical for glue logic.
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Plus, this doesn't require any implementation framework, just an interface.
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One platform might implement it as simple inline functions accessing chip
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registers; another might implement it by delegating through abstractions
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used for several very different kinds of GPIO controller. (There is some
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optional code supporting such an implementation strategy, described later
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in this document, but drivers acting as clients to the GPIO interface must
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not care how it's implemented.)
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That said, if the convention is supported on their platform, drivers should
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use it when possible. Platforms must select ARCH_REQUIRE_GPIOLIB or
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ARCH_WANT_OPTIONAL_GPIOLIB in their Kconfig. Drivers that can't work without
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standard GPIO calls should have Kconfig entries which depend on GPIOLIB. The
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GPIO calls are available, either as "real code" or as optimized-away stubs,
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when drivers use the include file:
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#include <linux/gpio.h>
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If you stick to this convention then it'll be easier for other developers to
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see what your code is doing, and help maintain it.
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Note that these operations include I/O barriers on platforms which need to
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use them; drivers don't need to add them explicitly.
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Identifying GPIOs
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-----------------
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GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
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reserves "negative" numbers for other purposes like marking signals as
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"not available on this board", or indicating faults. Code that doesn't
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touch the underlying hardware treats these integers as opaque cookies.
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Platforms define how they use those integers, and usually #define symbols
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for the GPIO lines so that board-specific setup code directly corresponds
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to the relevant schematics. In contrast, drivers should only use GPIO
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numbers passed to them from that setup code, using platform_data to hold
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board-specific pin configuration data (along with other board specific
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data they need). That avoids portability problems.
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So for example one platform uses numbers 32-159 for GPIOs; while another
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uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
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type of GPIO controller, and on one particular board 80-95 with an FPGA.
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The numbers need not be contiguous; either of those platforms could also
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use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
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If you want to initialize a structure with an invalid GPIO number, use
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some negative number (perhaps "-EINVAL"); that will never be valid. To
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test if such number from such a structure could reference a GPIO, you
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may use this predicate:
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int gpio_is_valid(int number);
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A number that's not valid will be rejected by calls which may request
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or free GPIOs (see below). Other numbers may also be rejected; for
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example, a number might be valid but temporarily unused on a given board.
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Whether a platform supports multiple GPIO controllers is a platform-specific
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implementation issue, as are whether that support can leave "holes" in the space
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of GPIO numbers, and whether new controllers can be added at runtime. Such issues
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can affect things including whether adjacent GPIO numbers are both valid.
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Using GPIOs
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-----------
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The first thing a system should do with a GPIO is allocate it, using
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the gpio_request() call; see later.
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One of the next things to do with a GPIO, often in board setup code when
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setting up a platform_device using the GPIO, is mark its direction:
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/* set as input or output, returning 0 or negative errno */
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int gpio_direction_input(unsigned gpio);
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int gpio_direction_output(unsigned gpio, int value);
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The return value is zero for success, else a negative errno. It should
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be checked, since the get/set calls don't have error returns and since
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misconfiguration is possible. You should normally issue these calls from
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a task context. However, for spinlock-safe GPIOs it's OK to use them
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before tasking is enabled, as part of early board setup.
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For output GPIOs, the value provided becomes the initial output value.
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This helps avoid signal glitching during system startup.
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For compatibility with legacy interfaces to GPIOs, setting the direction
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of a GPIO implicitly requests that GPIO (see below) if it has not been
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requested already. That compatibility is being removed from the optional
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gpiolib framework.
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Setting the direction can fail if the GPIO number is invalid, or when
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that particular GPIO can't be used in that mode. It's generally a bad
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idea to rely on boot firmware to have set the direction correctly, since
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it probably wasn't validated to do more than boot Linux. (Similarly,
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that board setup code probably needs to multiplex that pin as a GPIO,
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and configure pullups/pulldowns appropriately.)
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Spinlock-Safe GPIO access
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-------------------------
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Most GPIO controllers can be accessed with memory read/write instructions.
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Those don't need to sleep, and can safely be done from inside hard
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(nonthreaded) IRQ handlers and similar contexts.
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Use the following calls to access such GPIOs,
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for which gpio_cansleep() will always return false (see below):
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/* GPIO INPUT: return zero or nonzero */
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int gpio_get_value(unsigned gpio);
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/* GPIO OUTPUT */
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void gpio_set_value(unsigned gpio, int value);
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The values are boolean, zero for low, nonzero for high. When reading the
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value of an output pin, the value returned should be what's seen on the
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pin ... that won't always match the specified output value, because of
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issues including open-drain signaling and output latencies.
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The get/set calls have no error returns because "invalid GPIO" should have
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been reported earlier from gpio_direction_*(). However, note that not all
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platforms can read the value of output pins; those that can't should always
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return zero. Also, using these calls for GPIOs that can't safely be accessed
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without sleeping (see below) is an error.
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Platform-specific implementations are encouraged to optimize the two
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calls to access the GPIO value in cases where the GPIO number (and for
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output, value) are constant. It's normal for them to need only a couple
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of instructions in such cases (reading or writing a hardware register),
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and not to need spinlocks. Such optimized calls can make bitbanging
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applications a lot more efficient (in both space and time) than spending
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dozens of instructions on subroutine calls.
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GPIO access that may sleep
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--------------------------
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Some GPIO controllers must be accessed using message based busses like I2C
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or SPI. Commands to read or write those GPIO values require waiting to
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get to the head of a queue to transmit a command and get its response.
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This requires sleeping, which can't be done from inside IRQ handlers.
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Platforms that support this type of GPIO distinguish them from other GPIOs
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by returning nonzero from this call (which requires a valid GPIO number,
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which should have been previously allocated with gpio_request):
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int gpio_cansleep(unsigned gpio);
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To access such GPIOs, a different set of accessors is defined:
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/* GPIO INPUT: return zero or nonzero, might sleep */
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int gpio_get_value_cansleep(unsigned gpio);
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/* GPIO OUTPUT, might sleep */
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void gpio_set_value_cansleep(unsigned gpio, int value);
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Accessing such GPIOs requires a context which may sleep, for example
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a threaded IRQ handler, and those accessors must be used instead of
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spinlock-safe accessors without the cansleep() name suffix.
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Other than the fact that these accessors might sleep, and will work
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on GPIOs that can't be accessed from hardIRQ handlers, these calls act
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the same as the spinlock-safe calls.
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** IN ADDITION ** calls to setup and configure such GPIOs must be made
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from contexts which may sleep, since they may need to access the GPIO
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controller chip too: (These setup calls are usually made from board
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setup or driver probe/teardown code, so this is an easy constraint.)
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gpio_direction_input()
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gpio_direction_output()
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gpio_request()
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## gpio_request_one()
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## gpio_request_array()
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## gpio_free_array()
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gpio_free()
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gpio_set_debounce()
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Claiming and Releasing GPIOs
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----------------------------
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To help catch system configuration errors, two calls are defined.
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/* request GPIO, returning 0 or negative errno.
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* non-null labels may be useful for diagnostics.
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*/
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int gpio_request(unsigned gpio, const char *label);
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/* release previously-claimed GPIO */
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void gpio_free(unsigned gpio);
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Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
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GPIOs that have already been claimed with that call. The return value of
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gpio_request() must be checked. You should normally issue these calls from
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a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
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before tasking is enabled, as part of early board setup.
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These calls serve two basic purposes. One is marking the signals which
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are actually in use as GPIOs, for better diagnostics; systems may have
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several hundred potential GPIOs, but often only a dozen are used on any
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given board. Another is to catch conflicts, identifying errors when
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(a) two or more drivers wrongly think they have exclusive use of that
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signal, or (b) something wrongly believes it's safe to remove drivers
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needed to manage a signal that's in active use. That is, requesting a
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GPIO can serve as a kind of lock.
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Some platforms may also use knowledge about what GPIOs are active for
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power management, such as by powering down unused chip sectors and, more
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easily, gating off unused clocks.
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For GPIOs that use pins known to the pinctrl subsystem, that subsystem should
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be informed of their use; a gpiolib driver's .request() operation may call
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pinctrl_request_gpio(), and a gpiolib driver's .free() operation may call
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pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio()
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to succeed concurrently with a pin or pingroup being "owned" by a device for
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pin multiplexing.
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Any programming of pin multiplexing hardware that is needed to route the
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GPIO signal to the appropriate pin should occur within a GPIO driver's
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.direction_input() or .direction_output() operations, and occur after any
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setup of an output GPIO's value. This allows a glitch-free migration from a
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pin's special function to GPIO. This is sometimes required when using a GPIO
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to implement a workaround on signals typically driven by a non-GPIO HW block.
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Some platforms allow some or all GPIO signals to be routed to different pins.
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Similarly, other aspects of the GPIO or pin may need to be configured, such as
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pullup/pulldown. Platform software should arrange that any such details are
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configured prior to gpio_request() being called for those GPIOs, e.g. using
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the pinctrl subsystem's mapping table, so that GPIO users need not be aware
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of these details.
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Also note that it's your responsibility to have stopped using a GPIO
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before you free it.
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Considering in most cases GPIOs are actually configured right after they
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are claimed, three additional calls are defined:
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/* request a single GPIO, with initial configuration specified by
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* 'flags', identical to gpio_request() wrt other arguments and
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* return value
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*/
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int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
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/* request multiple GPIOs in a single call
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*/
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int gpio_request_array(struct gpio *array, size_t num);
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/* release multiple GPIOs in a single call
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*/
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void gpio_free_array(struct gpio *array, size_t num);
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where 'flags' is currently defined to specify the following properties:
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* GPIOF_DIR_IN - to configure direction as input
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* GPIOF_DIR_OUT - to configure direction as output
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* GPIOF_INIT_LOW - as output, set initial level to LOW
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* GPIOF_INIT_HIGH - as output, set initial level to HIGH
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* GPIOF_OPEN_DRAIN - gpio pin is open drain type.
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* GPIOF_OPEN_SOURCE - gpio pin is open source type.
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* GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction
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* GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction
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since GPIOF_INIT_* are only valid when configured as output, so group valid
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combinations as:
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* GPIOF_IN - configure as input
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* GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
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* GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
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When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is
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open drain type. Such pins will not be driven to 1 in output mode. It is
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require to connect pull-up on such pins. By enabling this flag, gpio lib will
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make the direction to input when it is asked to set value of 1 in output mode
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to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
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When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is
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open source type. Such pins will not be driven to 0 in output mode. It is
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require to connect pull-down on such pin. By enabling this flag, gpio lib will
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make the direction to input when it is asked to set value of 0 in output mode
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to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
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In the future, these flags can be extended to support more properties.
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Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
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introduced to encapsulate all three fields as:
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struct gpio {
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unsigned gpio;
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unsigned long flags;
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const char *label;
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};
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A typical example of usage:
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static struct gpio leds_gpios[] = {
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{ 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
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{ 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
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{ 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
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{ 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
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{ ... },
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};
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err = gpio_request_one(31, GPIOF_IN, "Reset Button");
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if (err)
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...
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err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
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if (err)
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...
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gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
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GPIOs mapped to IRQs
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--------------------
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GPIO numbers are unsigned integers; so are IRQ numbers. These make up
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two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
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map between them using calls like:
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/* map GPIO numbers to IRQ numbers */
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int gpio_to_irq(unsigned gpio);
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/* map IRQ numbers to GPIO numbers (avoid using this) */
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int irq_to_gpio(unsigned irq);
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Those return either the corresponding number in the other namespace, or
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else a negative errno code if the mapping can't be done. (For example,
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some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
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number that wasn't set up as an input using gpio_direction_input(), or
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to use an IRQ number that didn't originally come from gpio_to_irq().
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These two mapping calls are expected to cost on the order of a single
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addition or subtraction. They're not allowed to sleep.
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Non-error values returned from gpio_to_irq() can be passed to request_irq()
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or free_irq(). They will often be stored into IRQ resources for platform
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devices, by the board-specific initialization code. Note that IRQ trigger
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options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
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system wakeup capabilities.
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Non-error values returned from irq_to_gpio() would most commonly be used
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with gpio_get_value(), for example to initialize or update driver state
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when the IRQ is edge-triggered. Note that some platforms don't support
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this reverse mapping, so you should avoid using it.
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Emulating Open Drain Signals
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----------------------------
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Sometimes shared signals need to use "open drain" signaling, where only the
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low signal level is actually driven. (That term applies to CMOS transistors;
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"open collector" is used for TTL.) A pullup resistor causes the high signal
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level. This is sometimes called a "wire-AND"; or more practically, from the
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negative logic (low=true) perspective this is a "wire-OR".
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One common example of an open drain signal is a shared active-low IRQ line.
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Also, bidirectional data bus signals sometimes use open drain signals.
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Some GPIO controllers directly support open drain outputs; many don't. When
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you need open drain signaling but your hardware doesn't directly support it,
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there's a common idiom you can use to emulate it with any GPIO pin that can
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be used as either an input or an output:
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LOW: gpio_direction_output(gpio, 0) ... this drives the signal
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and overrides the pullup.
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HIGH: gpio_direction_input(gpio) ... this turns off the output,
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so the pullup (or some other device) controls the signal.
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If you are "driving" the signal high but gpio_get_value(gpio) reports a low
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value (after the appropriate rise time passes), you know some other component
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is driving the shared signal low. That's not necessarily an error. As one
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common example, that's how I2C clocks are stretched: a slave that needs a
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slower clock delays the rising edge of SCK, and the I2C master adjusts its
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signaling rate accordingly.
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GPIO controllers and the pinctrl subsystem
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------------------------------------------
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A GPIO controller on a SOC might be tightly coupled with the pinctrl
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subsystem, in the sense that the pins can be used by other functions
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together with an optional gpio feature. We have already covered the
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case where e.g. a GPIO controller need to reserve a pin or set the
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direction of a pin by calling any of:
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pinctrl_request_gpio()
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pinctrl_free_gpio()
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pinctrl_gpio_direction_input()
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pinctrl_gpio_direction_output()
|
|
|
|
But how does the pin control subsystem cross-correlate the GPIO
|
|
numbers (which are a global business) to a certain pin on a certain
|
|
pin controller?
|
|
|
|
This is done by registering "ranges" of pins, which are essentially
|
|
cross-reference tables. These are described in
|
|
Documentation/pinctrl.txt
|
|
|
|
While the pin allocation is totally managed by the pinctrl subsystem,
|
|
gpio (under gpiolib) is still maintained by gpio drivers. It may happen
|
|
that different pin ranges in a SoC is managed by different gpio drivers.
|
|
|
|
This makes it logical to let gpio drivers announce their pin ranges to
|
|
the pin ctrl subsystem before it will call 'pinctrl_request_gpio' in order
|
|
to request the corresponding pin to be prepared by the pinctrl subsystem
|
|
before any gpio usage.
|
|
|
|
For this, the gpio controller can register its pin range with pinctrl
|
|
subsystem. There are two ways of doing it currently: with or without DT.
|
|
|
|
For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt.
|
|
|
|
For non-DT support, user can call gpiochip_add_pin_range() with appropriate
|
|
parameters to register a range of gpio pins with a pinctrl driver. For this
|
|
exact name string of pinctrl device has to be passed as one of the
|
|
argument to this routine.
|
|
|
|
|
|
What do these conventions omit?
|
|
===============================
|
|
One of the biggest things these conventions omit is pin multiplexing, since
|
|
this is highly chip-specific and nonportable. One platform might not need
|
|
explicit multiplexing; another might have just two options for use of any
|
|
given pin; another might have eight options per pin; another might be able
|
|
to route a given GPIO to any one of several pins. (Yes, those examples all
|
|
come from systems that run Linux today.)
|
|
|
|
Related to multiplexing is configuration and enabling of the pullups or
|
|
pulldowns integrated on some platforms. Not all platforms support them,
|
|
or support them in the same way; and any given board might use external
|
|
pullups (or pulldowns) so that the on-chip ones should not be used.
|
|
(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
|
|
Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
|
|
platform-specific issue, as are models like (not) having a one-to-one
|
|
correspondence between configurable pins and GPIOs.
|
|
|
|
There are other system-specific mechanisms that are not specified here,
|
|
like the aforementioned options for input de-glitching and wire-OR output.
|
|
Hardware may support reading or writing GPIOs in gangs, but that's usually
|
|
configuration dependent: for GPIOs sharing the same bank. (GPIOs are
|
|
commonly grouped in banks of 16 or 32, with a given SOC having several such
|
|
banks.) Some systems can trigger IRQs from output GPIOs, or read values
|
|
from pins not managed as GPIOs. Code relying on such mechanisms will
|
|
necessarily be nonportable.
|
|
|
|
Dynamic definition of GPIOs is not currently standard; for example, as
|
|
a side effect of configuring an add-on board with some GPIO expanders.
|
|
|
|
|
|
GPIO implementor's framework (OPTIONAL)
|
|
=======================================
|
|
As noted earlier, there is an optional implementation framework making it
|
|
easier for platforms to support different kinds of GPIO controller using
|
|
the same programming interface. This framework is called "gpiolib".
|
|
|
|
As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
|
|
will be found there. That will list all the controllers registered through
|
|
this framework, and the state of the GPIOs currently in use.
|
|
|
|
|
|
Controller Drivers: gpio_chip
|
|
-----------------------------
|
|
In this framework each GPIO controller is packaged as a "struct gpio_chip"
|
|
with information common to each controller of that type:
|
|
|
|
- methods to establish GPIO direction
|
|
- methods used to access GPIO values
|
|
- flag saying whether calls to its methods may sleep
|
|
- optional debugfs dump method (showing extra state like pullup config)
|
|
- label for diagnostics
|
|
|
|
There is also per-instance data, which may come from device.platform_data:
|
|
the number of its first GPIO, and how many GPIOs it exposes.
|
|
|
|
The code implementing a gpio_chip should support multiple instances of the
|
|
controller, possibly using the driver model. That code will configure each
|
|
gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
|
|
rare; use gpiochip_remove() when it is unavoidable.
|
|
|
|
Most often a gpio_chip is part of an instance-specific structure with state
|
|
not exposed by the GPIO interfaces, such as addressing, power management,
|
|
and more. Chips such as codecs will have complex non-GPIO state.
|
|
|
|
Any debugfs dump method should normally ignore signals which haven't been
|
|
requested as GPIOs. They can use gpiochip_is_requested(), which returns
|
|
either NULL or the label associated with that GPIO when it was requested.
|
|
|
|
|
|
Platform Support
|
|
----------------
|
|
To support this framework, a platform's Kconfig will "select" either
|
|
ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
|
|
and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
|
|
three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
|
|
|
|
It may also provide a custom value for ARCH_NR_GPIOS, so that it better
|
|
reflects the number of GPIOs in actual use on that platform, without
|
|
wasting static table space. (It should count both built-in/SoC GPIOs and
|
|
also ones on GPIO expanders.
|
|
|
|
ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
|
|
into the kernel on that architecture.
|
|
|
|
ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user
|
|
can enable it and build it into the kernel optionally.
|
|
|
|
If neither of these options are selected, the platform does not support
|
|
GPIOs through GPIO-lib and the code cannot be enabled by the user.
|
|
|
|
Trivial implementations of those functions can directly use framework
|
|
code, which always dispatches through the gpio_chip:
|
|
|
|
#define gpio_get_value __gpio_get_value
|
|
#define gpio_set_value __gpio_set_value
|
|
#define gpio_cansleep __gpio_cansleep
|
|
|
|
Fancier implementations could instead define those as inline functions with
|
|
logic optimizing access to specific SOC-based GPIOs. For example, if the
|
|
referenced GPIO is the constant "12", getting or setting its value could
|
|
cost as little as two or three instructions, never sleeping. When such an
|
|
optimization is not possible those calls must delegate to the framework
|
|
code, costing at least a few dozen instructions. For bitbanged I/O, such
|
|
instruction savings can be significant.
|
|
|
|
For SOCs, platform-specific code defines and registers gpio_chip instances
|
|
for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
|
|
match chip vendor documentation, and directly match board schematics. They
|
|
may well start at zero and go up to a platform-specific limit. Such GPIOs
|
|
are normally integrated into platform initialization to make them always be
|
|
available, from arch_initcall() or earlier; they can often serve as IRQs.
|
|
|
|
|
|
Board Support
|
|
-------------
|
|
For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
|
|
function devices, FPGAs or CPLDs -- most often board-specific code handles
|
|
registering controller devices and ensures that their drivers know what GPIO
|
|
numbers to use with gpiochip_add(). Their numbers often start right after
|
|
platform-specific GPIOs.
|
|
|
|
For example, board setup code could create structures identifying the range
|
|
of GPIOs that chip will expose, and passes them to each GPIO expander chip
|
|
using platform_data. Then the chip driver's probe() routine could pass that
|
|
data to gpiochip_add().
|
|
|
|
Initialization order can be important. For example, when a device relies on
|
|
an I2C-based GPIO, its probe() routine should only be called after that GPIO
|
|
becomes available. That may mean the device should not be registered until
|
|
calls for that GPIO can work. One way to address such dependencies is for
|
|
such gpio_chip controllers to provide setup() and teardown() callbacks to
|
|
board specific code; those board specific callbacks would register devices
|
|
once all the necessary resources are available, and remove them later when
|
|
the GPIO controller device becomes unavailable.
|
|
|
|
|
|
Sysfs Interface for Userspace (OPTIONAL)
|
|
========================================
|
|
Platforms which use the "gpiolib" implementors framework may choose to
|
|
configure a sysfs user interface to GPIOs. This is different from the
|
|
debugfs interface, since it provides control over GPIO direction and
|
|
value instead of just showing a gpio state summary. Plus, it could be
|
|
present on production systems without debugging support.
|
|
|
|
Given appropriate hardware documentation for the system, userspace could
|
|
know for example that GPIO #23 controls the write protect line used to
|
|
protect boot loader segments in flash memory. System upgrade procedures
|
|
may need to temporarily remove that protection, first importing a GPIO,
|
|
then changing its output state, then updating the code before re-enabling
|
|
the write protection. In normal use, GPIO #23 would never be touched,
|
|
and the kernel would have no need to know about it.
|
|
|
|
Again depending on appropriate hardware documentation, on some systems
|
|
userspace GPIO can be used to determine system configuration data that
|
|
standard kernels won't know about. And for some tasks, simple userspace
|
|
GPIO drivers could be all that the system really needs.
|
|
|
|
Note that standard kernel drivers exist for common "LEDs and Buttons"
|
|
GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
|
|
instead of talking directly to the GPIOs; they integrate with kernel
|
|
frameworks better than your userspace code could.
|
|
|
|
|
|
Paths in Sysfs
|
|
--------------
|
|
There are three kinds of entry in /sys/class/gpio:
|
|
|
|
- Control interfaces used to get userspace control over GPIOs;
|
|
|
|
- GPIOs themselves; and
|
|
|
|
- GPIO controllers ("gpio_chip" instances).
|
|
|
|
That's in addition to standard files including the "device" symlink.
|
|
|
|
The control interfaces are write-only:
|
|
|
|
/sys/class/gpio/
|
|
|
|
"export" ... Userspace may ask the kernel to export control of
|
|
a GPIO to userspace by writing its number to this file.
|
|
|
|
Example: "echo 19 > export" will create a "gpio19" node
|
|
for GPIO #19, if that's not requested by kernel code.
|
|
|
|
"unexport" ... Reverses the effect of exporting to userspace.
|
|
|
|
Example: "echo 19 > unexport" will remove a "gpio19"
|
|
node exported using the "export" file.
|
|
|
|
GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
|
|
and have the following read/write attributes:
|
|
|
|
/sys/class/gpio/gpioN/
|
|
|
|
"direction" ... reads as either "in" or "out". This value may
|
|
normally be written. Writing as "out" defaults to
|
|
initializing the value as low. To ensure glitch free
|
|
operation, values "low" and "high" may be written to
|
|
configure the GPIO as an output with that initial value.
|
|
|
|
Note that this attribute *will not exist* if the kernel
|
|
doesn't support changing the direction of a GPIO, or
|
|
it was exported by kernel code that didn't explicitly
|
|
allow userspace to reconfigure this GPIO's direction.
|
|
|
|
"value" ... reads as either 0 (low) or 1 (high). If the GPIO
|
|
is configured as an output, this value may be written;
|
|
any nonzero value is treated as high.
|
|
|
|
If the pin can be configured as interrupt-generating interrupt
|
|
and if it has been configured to generate interrupts (see the
|
|
description of "edge"), you can poll(2) on that file and
|
|
poll(2) will return whenever the interrupt was triggered. If
|
|
you use poll(2), set the events POLLPRI and POLLERR. If you
|
|
use select(2), set the file descriptor in exceptfds. After
|
|
poll(2) returns, either lseek(2) to the beginning of the sysfs
|
|
file and read the new value or close the file and re-open it
|
|
to read the value.
|
|
|
|
"edge" ... reads as either "none", "rising", "falling", or
|
|
"both". Write these strings to select the signal edge(s)
|
|
that will make poll(2) on the "value" file return.
|
|
|
|
This file exists only if the pin can be configured as an
|
|
interrupt generating input pin.
|
|
|
|
"active_low" ... reads as either 0 (false) or 1 (true). Write
|
|
any nonzero value to invert the value attribute both
|
|
for reading and writing. Existing and subsequent
|
|
poll(2) support configuration via the edge attribute
|
|
for "rising" and "falling" edges will follow this
|
|
setting.
|
|
|
|
GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
|
|
controller implementing GPIOs starting at #42) and have the following
|
|
read-only attributes:
|
|
|
|
/sys/class/gpio/gpiochipN/
|
|
|
|
"base" ... same as N, the first GPIO managed by this chip
|
|
|
|
"label" ... provided for diagnostics (not always unique)
|
|
|
|
"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
|
|
|
|
Board documentation should in most cases cover what GPIOs are used for
|
|
what purposes. However, those numbers are not always stable; GPIOs on
|
|
a daughtercard might be different depending on the base board being used,
|
|
or other cards in the stack. In such cases, you may need to use the
|
|
gpiochip nodes (possibly in conjunction with schematics) to determine
|
|
the correct GPIO number to use for a given signal.
|
|
|
|
|
|
Exporting from Kernel code
|
|
--------------------------
|
|
Kernel code can explicitly manage exports of GPIOs which have already been
|
|
requested using gpio_request():
|
|
|
|
/* export the GPIO to userspace */
|
|
int gpio_export(unsigned gpio, bool direction_may_change);
|
|
|
|
/* reverse gpio_export() */
|
|
void gpio_unexport();
|
|
|
|
/* create a sysfs link to an exported GPIO node */
|
|
int gpio_export_link(struct device *dev, const char *name,
|
|
unsigned gpio)
|
|
|
|
/* change the polarity of a GPIO node in sysfs */
|
|
int gpio_sysfs_set_active_low(unsigned gpio, int value);
|
|
|
|
After a kernel driver requests a GPIO, it may only be made available in
|
|
the sysfs interface by gpio_export(). The driver can control whether the
|
|
signal direction may change. This helps drivers prevent userspace code
|
|
from accidentally clobbering important system state.
|
|
|
|
This explicit exporting can help with debugging (by making some kinds
|
|
of experiments easier), or can provide an always-there interface that's
|
|
suitable for documenting as part of a board support package.
|
|
|
|
After the GPIO has been exported, gpio_export_link() allows creating
|
|
symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
|
|
use this to provide the interface under their own device in sysfs with
|
|
a descriptive name.
|
|
|
|
Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
|
|
differences between boards from user space. This only affects the
|
|
sysfs interface. Polarity change can be done both before and after
|
|
gpio_export(), and previously enabled poll(2) support for either
|
|
rising or falling edge will be reconfigured to follow this setting.
|