Re: [PATCH 01/14] ACPI: video: Add a native function parameter to acpi_video_get_backlight_type()

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 18 May 2022, Hans de Goede <hdegoede@redhat.com> wrote:
> Hi,
>
> On 5/18/22 10:55, Jani Nikula wrote:
>> On Tue, 17 May 2022, Hans de Goede <hdegoede@redhat.com> wrote:
>>> ATM on x86 laptops where we want userspace to use the acpi_video backlight
>>> device we often register both the GPU's native backlight device and
>>> acpi_video's firmware acpi_video# backlight device. This relies on
>>> userspace preferring firmware type backlight devices over native ones, but
>>> registering 2 backlight devices for a single display really is undesirable.
>>>
>>> On x86 laptops where the native GPU backlight device should be used,
>>> the registering of other backlight devices is avoided by their drivers
>>> using acpi_video_get_backlight_type() and only registering their backlight
>>> if the return value matches their type.
>>>
>>> acpi_video_get_backlight_type() uses
>>> backlight_device_get_by_type(BACKLIGHT_RAW) to determine if a native
>>> driver is available and will never return native if this returns
>>> false. This means that the GPU's native backlight registering code
>>> cannot just call acpi_video_get_backlight_type() to determine if it
>>> should register its backlight, since acpi_video_get_backlight_type() will
>>> never return native until the native backlight has already registered.
>>>
>>> To fix this add a native function parameter to
>>> acpi_video_get_backlight_type(), which when set to true will make
>>> acpi_video_get_backlight_type() behave as if a native backlight has
>>> already been registered.

Regarding the question below, this is the part that throws me off.

>>>
>>> Note that all current callers are updated to pass false for the new
>>> parameter, so this change in itself causes no functional changes.
>> 
>> 
>>> diff --git a/drivers/acpi/video_detect.c b/drivers/acpi/video_detect.c
>>> index becc198e4c22..0a06f0edd298 100644
>>> --- a/drivers/acpi/video_detect.c
>>> +++ b/drivers/acpi/video_detect.c
>>> @@ -17,12 +17,14 @@
>>>   * Otherwise vendor specific drivers like thinkpad_acpi, asus-laptop,
>>>   * sony_acpi,... can take care about backlight brightness.
>>>   *
>>> - * Backlight drivers can use acpi_video_get_backlight_type() to determine
>>> - * which driver should handle the backlight.
>>> + * Backlight drivers can use acpi_video_get_backlight_type() to determine which
>>> + * driver should handle the backlight. RAW/GPU-driver backlight drivers must
>>> + * pass true for the native function argument, other drivers must pass false.
>>>   *
>>>   * If CONFIG_ACPI_VIDEO is neither set as "compiled in" (y) nor as a module (m)
>>>   * this file will not be compiled and acpi_video_get_backlight_type() will
>>> - * always return acpi_backlight_vendor.
>>> + * return acpi_backlight_native when its native argument is true and
>>> + * acpi_backlight_vendor when it is false.
>>>   */
>> 
>> Frankly, I think the boolean native parameter here, and at the call
>> sites, is confusing, and the slightly different explanations in the
>> commit message and comment here aren't helping.
>
> Can you elaborate the "slightly different explanations in the
> commit message and comment" part a bit (so that I can fix this) ?
>
>> I suggest adding a separate function that the native backlight drivers
>> should use. I think it's more obvious all around, and easier to document
>> too.
>
> Code wise I think this would mean renaming the original and
> then adding 2 wrappers, but that is fine with me. I've no real
> preference either way and I'm happy with adding a new variant of
> acpi_video_get_backlight_type() for the native backlight drivers
> any suggestion for a name ?

Alternatively, do the native backlight drivers have any need for the
actual backlight type information from acpi? They only need to be able
to ask if they should register themselves, right?

I understand this sounds like bikeshedding, but I'm trying to avoid
duplicating the conditions in the drivers where a single predicate
function call could be sufficient, and the complexity could be hidden in
acpi.

	if (!acpi_video_backlight_use_native())
		return;

Perhaps all the drivers/platform/x86/* backlight drivers could use:

	if (acpi_video_backlight_use_vendor())
		...

You can still use the native parameter etc. internally, but just hide
the details from everyone else, and, hopefully, make it harder for them
to do silly things?

BR,
Jani.


>
> Regards,
>
> Hans
>

-- 
Jani Nikula, Intel Open Source Graphics Center