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ACPI: docs: enumeration: Clarify ACPI bus concepts
In some cases, ACPI drivers are implemented as a way to manage devices enumerated with the help of the platform firmware through ACPI. This might be confusing, since the preferred way to implement a driver for a device that cannot be enumerated natively, is a platform driver, as stated in the documentation. Clarify relationships between ACPI device objects, platform devices and ACPI Namespace entries. Suggested-by: Elena Reshetova <elena.reshetova@intel.com> Co-developed-by: Michal Wilczynski <michal.wilczynski@intel.com> Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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@ -64,6 +64,49 @@ If the driver needs to perform more complex initialization like getting and
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configuring GPIOs it can get its ACPI handle and extract this information
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configuring GPIOs it can get its ACPI handle and extract this information
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from ACPI tables.
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from ACPI tables.
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ACPI device objects
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===================
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Generally speaking, there are two categories of devices in a system in which
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ACPI is used as an interface between the platform firmware and the OS: Devices
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that can be discovered and enumerated natively, through a protocol defined for
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the specific bus that they are on (for example, configuration space in PCI),
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without the platform firmware assistance, and devices that need to be described
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by the platform firmware so that they can be discovered. Still, for any device
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known to the platform firmware, regardless of which category it falls into,
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there can be a corresponding ACPI device object in the ACPI Namespace in which
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case the Linux kernel will create a struct acpi_device object based on it for
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that device.
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Those struct acpi_device objects are never used for binding drivers to natively
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discoverable devices, because they are represented by other types of device
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objects (for example, struct pci_dev for PCI devices) that are bound to by
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device drivers (the corresponding struct acpi_device object is then used as
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an additional source of information on the configuration of the given device).
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Moreover, the core ACPI device enumeration code creates struct platform_device
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objects for the majority of devices that are discovered and enumerated with the
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help of the platform firmware and those platform device objects can be bound to
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by platform drivers in direct analogy with the natively enumerable devices
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case. Therefore it is logically inconsistent and so generally invalid to bind
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drivers to struct acpi_device objects, including drivers for devices that are
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discovered with the help of the platform firmware.
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Historically, ACPI drivers that bound directly to struct acpi_device objects
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were implemented for some devices enumerated with the help of the platform
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firmware, but this is not recommended for any new drivers. As explained above,
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platform device objects are created for those devices as a rule (with a few
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exceptions that are not relevant here) and so platform drivers should be used
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for handling them, even though the corresponding ACPI device objects are the
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only source of device configuration information in that case.
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For every device having a corresponding struct acpi_device object, the pointer
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to it is returned by the ACPI_COMPANION() macro, so it is always possible to
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get to the device configuration information stored in the ACPI device object
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this way. Accordingly, struct acpi_device can be regarded as a part of the
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interface between the kernel and the ACPI Namespace, whereas device objects of
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other types (for example, struct pci_dev or struct platform_device) are used
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for interacting with the rest of the system.
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DMA support
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DMA support
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===========
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===========
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