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

[Wilczynski, Michal]

On 10/5/2023 8:28 PM, Wilczynski, Michal wrote:
>
> 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

Aww, forgot that you can also just apply it yourself, so I can just fetch and
rebase. Whichever version you prefer is fine with me :-)


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