mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-25 02:09:42 +00:00
![Uwe Kleine-König](/assets/img/avatar_default.png)
Most low-level PWM drivers support duty_cycle == period, and so does the
sysfs API. Also polarity can be changed for enabled PWMs since commit
39100ceea7
("pwm: Switch to the atomic API").
Reported-by: Jens Gehrlein <J.Gehrlein@eckelmann.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230911154454.675057-1-u.kleine-koenig@pengutronix.de
176 lines
7.1 KiB
ReStructuredText
176 lines
7.1 KiB
ReStructuredText
======================================
|
|
Pulse Width Modulation (PWM) interface
|
|
======================================
|
|
|
|
This provides an overview about the Linux PWM interface
|
|
|
|
PWMs are commonly used for controlling LEDs, fans or vibrators in
|
|
cell phones. PWMs with a fixed purpose have no need implementing
|
|
the Linux PWM API (although they could). However, PWMs are often
|
|
found as discrete devices on SoCs which have no fixed purpose. It's
|
|
up to the board designer to connect them to LEDs or fans. To provide
|
|
this kind of flexibility the generic PWM API exists.
|
|
|
|
Identifying PWMs
|
|
----------------
|
|
|
|
Users of the legacy PWM API use unique IDs to refer to PWM devices.
|
|
|
|
Instead of referring to a PWM device via its unique ID, board setup code
|
|
should instead register a static mapping that can be used to match PWM
|
|
consumers to providers, as given in the following example::
|
|
|
|
static struct pwm_lookup board_pwm_lookup[] = {
|
|
PWM_LOOKUP("tegra-pwm", 0, "pwm-backlight", NULL,
|
|
50000, PWM_POLARITY_NORMAL),
|
|
};
|
|
|
|
static void __init board_init(void)
|
|
{
|
|
...
|
|
pwm_add_table(board_pwm_lookup, ARRAY_SIZE(board_pwm_lookup));
|
|
...
|
|
}
|
|
|
|
Using PWMs
|
|
----------
|
|
|
|
Consumers use the pwm_get() function and pass to it the consumer device or a
|
|
consumer name. pwm_put() is used to free the PWM device. Managed variants of
|
|
the getter, devm_pwm_get() and devm_fwnode_pwm_get(), also exist.
|
|
|
|
After being requested, a PWM has to be configured using::
|
|
|
|
int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state);
|
|
|
|
This API controls both the PWM period/duty_cycle config and the
|
|
enable/disable state.
|
|
|
|
As a consumer, don't rely on the output's state for a disabled PWM. If it's
|
|
easily possible, drivers are supposed to emit the inactive state, but some
|
|
drivers cannot. If you rely on getting the inactive state, use .duty_cycle=0,
|
|
.enabled=true.
|
|
|
|
There is also a usage_power setting: If set, the PWM driver is only required to
|
|
maintain the power output but has more freedom regarding signal form.
|
|
If supported by the driver, the signal can be optimized, for example to improve
|
|
EMI by phase shifting the individual channels of a chip.
|
|
|
|
The pwm_config(), pwm_enable() and pwm_disable() functions are just wrappers
|
|
around pwm_apply_state() and should not be used if the user wants to change
|
|
several parameter at once. For example, if you see pwm_config() and
|
|
pwm_{enable,disable}() calls in the same function, this probably means you
|
|
should switch to pwm_apply_state().
|
|
|
|
The PWM user API also allows one to query the PWM state that was passed to the
|
|
last invocation of pwm_apply_state() using pwm_get_state(). Note this is
|
|
different to what the driver has actually implemented if the request cannot be
|
|
satisfied exactly with the hardware in use. There is currently no way for
|
|
consumers to get the actually implemented settings.
|
|
|
|
In addition to the PWM state, the PWM API also exposes PWM arguments, which
|
|
are the reference PWM config one should use on this PWM.
|
|
PWM arguments are usually platform-specific and allows the PWM user to only
|
|
care about dutycycle relatively to the full period (like, duty = 50% of the
|
|
period). struct pwm_args contains 2 fields (period and polarity) and should
|
|
be used to set the initial PWM config (usually done in the probe function
|
|
of the PWM user). PWM arguments are retrieved with pwm_get_args().
|
|
|
|
All consumers should really be reconfiguring the PWM upon resume as
|
|
appropriate. This is the only way to ensure that everything is resumed in
|
|
the proper order.
|
|
|
|
Using PWMs with the sysfs interface
|
|
-----------------------------------
|
|
|
|
If CONFIG_SYSFS is enabled in your kernel configuration a simple sysfs
|
|
interface is provided to use the PWMs from userspace. It is exposed at
|
|
/sys/class/pwm/. Each probed PWM controller/chip will be exported as
|
|
pwmchipN, where N is the base of the PWM chip. Inside the directory you
|
|
will find:
|
|
|
|
npwm
|
|
The number of PWM channels this chip supports (read-only).
|
|
|
|
export
|
|
Exports a PWM channel for use with sysfs (write-only).
|
|
|
|
unexport
|
|
Unexports a PWM channel from sysfs (write-only).
|
|
|
|
The PWM channels are numbered using a per-chip index from 0 to npwm-1.
|
|
|
|
When a PWM channel is exported a pwmX directory will be created in the
|
|
pwmchipN directory it is associated with, where X is the number of the
|
|
channel that was exported. The following properties will then be available:
|
|
|
|
period
|
|
The total period of the PWM signal (read/write).
|
|
Value is in nanoseconds and is the sum of the active and inactive
|
|
time of the PWM.
|
|
|
|
duty_cycle
|
|
The active time of the PWM signal (read/write).
|
|
Value is in nanoseconds and must be less than or equal to the period.
|
|
|
|
polarity
|
|
Changes the polarity of the PWM signal (read/write).
|
|
Writes to this property only work if the PWM chip supports changing
|
|
the polarity.
|
|
Value is the string "normal" or "inversed".
|
|
|
|
enable
|
|
Enable/disable the PWM signal (read/write).
|
|
|
|
- 0 - disabled
|
|
- 1 - enabled
|
|
|
|
Implementing a PWM driver
|
|
-------------------------
|
|
|
|
Currently there are two ways to implement pwm drivers. Traditionally
|
|
there only has been the barebone API meaning that each driver has
|
|
to implement the pwm_*() functions itself. This means that it's impossible
|
|
to have multiple PWM drivers in the system. For this reason it's mandatory
|
|
for new drivers to use the generic PWM framework.
|
|
|
|
A new PWM controller/chip can be added using pwmchip_add() and removed
|
|
again with pwmchip_remove(). pwmchip_add() takes a filled in struct
|
|
pwm_chip as argument which provides a description of the PWM chip, the
|
|
number of PWM devices provided by the chip and the chip-specific
|
|
implementation of the supported PWM operations to the framework.
|
|
|
|
When implementing polarity support in a PWM driver, make sure to respect the
|
|
signal conventions in the PWM framework. By definition, normal polarity
|
|
characterizes a signal starts high for the duration of the duty cycle and
|
|
goes low for the remainder of the period. Conversely, a signal with inversed
|
|
polarity starts low for the duration of the duty cycle and goes high for the
|
|
remainder of the period.
|
|
|
|
Drivers are encouraged to implement ->apply() instead of the legacy
|
|
->enable(), ->disable() and ->config() methods. Doing that should provide
|
|
atomicity in the PWM config workflow, which is required when the PWM controls
|
|
a critical device (like a regulator).
|
|
|
|
The implementation of ->get_state() (a method used to retrieve initial PWM
|
|
state) is also encouraged for the same reason: letting the PWM user know
|
|
about the current PWM state would allow him to avoid glitches.
|
|
|
|
Drivers should not implement any power management. In other words,
|
|
consumers should implement it as described in the "Using PWMs" section.
|
|
|
|
Locking
|
|
-------
|
|
|
|
The PWM core list manipulations are protected by a mutex, so pwm_get()
|
|
and pwm_put() may not be called from an atomic context. Currently the
|
|
PWM core does not enforce any locking to pwm_enable(), pwm_disable() and
|
|
pwm_config(), so the calling context is currently driver specific. This
|
|
is an issue derived from the former barebone API and should be fixed soon.
|
|
|
|
Helpers
|
|
-------
|
|
|
|
Currently a PWM can only be configured with period_ns and duty_ns. For several
|
|
use cases freq_hz and duty_percent might be better. Instead of calculating
|
|
this in your driver please consider adding appropriate helpers to the framework.
|