Re: [PATCH v1 2/9] docs: firmware-guide: ACPI: Clarify ACPI bus concepts

[Wilczynski, Michal]

On 10/5/2023 7:57 PM, Rafael J. Wysocki wrote:
> On Monday, September 25, 2023 4:48:35 PM CEST Michal Wilczynski wrote:
>> Some devices implement ACPI driver as a way to manage devices
>> enumerated by the ACPI. This might be confusing as a preferred way to
>> implement a driver for devices not connected to any bus is a platform
>> driver, as stated in the documentation. Clarify relationships between
>> ACPI device, platform device and ACPI entries.
>>
>> Suggested-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
>> Signed-off-by: Michal Wilczynski <michal.wilczynski@xxxxxxxxx>
>> ---
>>  Documentation/firmware-guide/acpi/enumeration.rst | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/Documentation/firmware-guide/acpi/enumeration.rst b/Documentation/firmware-guide/acpi/enumeration.rst
>> index 56d9913a3370..f56cc79a9e83 100644
>> --- a/Documentation/firmware-guide/acpi/enumeration.rst
>> +++ b/Documentation/firmware-guide/acpi/enumeration.rst
>> @@ -64,6 +64,19 @@ If the driver needs to perform more complex initialization like getting and
>>  configuring GPIOs it can get its ACPI handle and extract this information
>>  from ACPI tables.
>>  
>> +ACPI bus
>> +====================
>> +
>> +Historically some devices not connected to any bus were represented as ACPI
>> +devices, and had to implement ACPI driver. This is not a preferred way for new
>> +drivers. As explained above devices not connected to any bus should implement
>> +platform driver. ACPI device would be created during enumeration nonetheless,
>> +and would be accessible through ACPI_COMPANION() macro, and the ACPI handle would
>> +be accessible through ACPI_HANDLE() macro. ACPI device is meant to describe
>> +information related to ACPI entry e.g. handle of the ACPI entry. Think -
>> +ACPI device interfaces with the FW, and the platform device with the rest of
>> +the system.
>> +
>>  DMA support
>>  ===========
> I rewrote the above entirely, so here's a new patch to replace this one:
>
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Subject: [PATCH v2 2/9] 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@xxxxxxxxx>
> Co-developed-by: Michal Wilczynski <michal.wilczynski@xxxxxxxxx>
> Signed-off-by: Michal Wilczynski <michal.wilczynski@xxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  Documentation/firmware-guide/acpi/enumeration.rst |   43 ++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
>
> Index: linux-pm/Documentation/firmware-guide/acpi/enumeration.rst
> ===================================================================
> --- linux-pm.orig/Documentation/firmware-guide/acpi/enumeration.rst
> +++ linux-pm/Documentation/firmware-guide/acpi/enumeration.rst
> @@ -64,6 +64,49 @@ If the driver needs to perform more comp
>  configuring GPIOs it can get its ACPI handle and extract this information
>  from ACPI tables.
>  
> +ACPI device objects
> +===================
> +
> +Generally speaking, there are two categories of devices in a system in which
> +ACPI is used as an interface between the platform firmware and the OS: Devices
> +that can be discovered and enumerated natively, through a protocol defined for
> +the specific bus that they are on (for example, configuration space in PCI),
> +without the platform firmware assistance, and devices that need to be described
> +by the platform firmware so that they can be discovered.  Still, for any device
> +known to the platform firmware, regardless of which category it falls into,
> +there can be a corresponding ACPI device object in the ACPI Namespace in which
> +case the Linux kernel will create a struct acpi_device object based on it for
> +that device.
> +
> +Those struct acpi_device objects are never used for binding drivers to natively
> +discoverable devices, because they are represented by other types of device
> +objects (for example, struct pci_dev for PCI devices) that are bound to by
> +device drivers (the corresponding struct acpi_device object is then used as
> +an additional source of information on the configuration of the given device).
> +Moreover, the core ACPI device enumeration code creates struct platform_device
> +objects for the majority of devices that are discovered and enumerated with the
> +help of the platform firmware and those platform device objects can be bound to
> +by platform drivers in direct analogy with the natively enumerable devices
> +case.  Therefore it is logically inconsistent and so generally invalid to bind
> +drivers to struct acpi_device objects, including drivers for devices that are
> +discovered with the help of the platform firmware.
> +
> +Historically, ACPI drivers that bound directly to struct acpi_device objects
> +were implemented for some devices enumerated with the help of the platform
> +firmware, but this is not recommended for any new drivers.  As explained above,
> +platform device objects are created for those devices as a rule (with a few
> +exceptions that are not relevant here) and so platform drivers should be used
> +for handling them, even though the corresponding ACPI device objects are the
> +only source of device configuration information in that case.
> +
> +For every device having a corresponding struct acpi_device object, the pointer
> +to it is returned by the ACPI_COMPANION() macro, so it is always possible to
> +get to the device configuration information stored in the ACPI device object
> +this way.  Accordingly, struct acpi_device can be regarded as a part of the
> +interface between the kernel and the ACPI Namespace, whereas device objects of
> +other types (for example, struct pci_dev or struct platform_device) are used
> +for interacting with the rest of the system.
> +
>  DMA support
>  ===========

Thanks a lot !
Looks very good, will include this in next revision.

Michał

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