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

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 5/19/22 11:02, Jani Nikula wrote:
> 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;

acpi_video_backlight_use_native() sounds good, I like I will change
this for v2. This also removes churn in all the other
acpi_video_get_backlight_type() callers.

> Perhaps all the drivers/platform/x86/* backlight drivers could use:
> 
> 	if (acpi_video_backlight_use_vendor())
> 		...

Hmm, as part of the ractoring there also will be new apple_gmux
and nvidia_wmi_ec types. I'm not sure about adding seperate functions
for all of those vs get_type() != foo. I like get_type != foo because
it makes clear that there will also be another caller somewhere
where get_type == foo and that that one will rbe the one which
actually gets to register its backlight.

> 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?

Ack.

Regards,

Hans