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

[Wilczynski, Michal]

On 10/6/2023 5:36 PM, Rafael J. Wysocki wrote:
> On Thu, Oct 5, 2023 at 10:39 PM Wilczynski, Michal
> <michal.wilczynski@xxxxxxxxx> wrote:
>>
>>
>> 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 :-)
> So I went ahead and queued up my versions of patches [1-2/9].  They
> are present in the acpi-bus branch in linux-pm.git (based on 6.6-rc4)
> and in the bleeding-edge branch (I'll merge acpi-bus into linux-next
> next week if all goes well).

Thanks, great !
Will re-send the rest of the patchset.

Michał

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