Re: [PATCH 00/22] Fallback to native backlight

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2022/10/24 20:53, Hans de Goede wrote:
> Hi Akihiko,
> 
> On 10/24/22 13:34, Akihiko Odaki wrote:
>> Commit 2600bfa3df99 ("ACPI: video: Add acpi_video_backlight_use_native()
>> helper") and following commits made native backlight unavailable if
>> CONFIG_ACPI_VIDEO is set and the backlight feature of ACPI video is
>> unavailable, which broke the backlight functionality on Lenovo ThinkPad
>> C13 Yoga Chromebook. Allow to fall back to native backlight in such
>> cases.
> 
> I appreciate your work on this, but what this in essence does is
> it allows 2 backlight drivers (vendor + native) to get registered
> for the same panel again. While the whole goal of the backlight refactor
> series landing in 6.1 was to make it so that there always is only
> *1* backlight device registered instead of (possibly) registering
> multiple and letting userspace figure it out. It is also important
> to only always have 1 backlight device per panel for further
> upcoming changes.
> 
> So nack for this solution, sorry.
> 
> I am aware that this breaks backlight control on some Chromebooks,
> this was already reported and I wrote a long reply explaining why
> things are done the way they are done now and also suggesting
> 2 possible (much simpler) fixes, see:
> https://lore.kernel.org/linux-acpi/42a5f2c9-a1dc-8fc0-7334-fe6c390ecfbb@redhat.com/
> 
> Unfortunately the reported has not followed-up on this and
> I don't have the hardware to test this myself.
> 
> Can you please try implementing 1 of the fixes suggested there
> and then submit that upstream ?
> 
> Regards,
> 
> Hans
> 

Hi Hans,

Thanks for reviewing and letting me know the prior attempt.

In this case, there is only a native backlight device and no vendor 
backlight device so the duplication of backlight devices does not 
happen. I think it is better to handle such a case without quirks.

I understand it is still questionable to cover the case by allowing 
duplication when both of a vendor backlight device and native one. To 
explain my understanding and reasoning for *not* trying to apply the 
de-duplication rule to the vendor/native combination, let me first 
describe that the de-duplication which happens in 
acpi_video_get_backlight_type() is a heuristics and limited.

As the background of acpi_video_get_backlight_type(), there is an 
assumption that it should be common that both of the firmware, 
implementing ACPI, and the kernel have code to drive backlight. In the 
case, the more reliable one should be picked instead of using both, and 
that is what the statements in `if (video_caps & ACPI_VIDEO_BACKLIGHT)` 
does.

However, the method has two limitations:
1. It does not cover the case where two backlight devices with the same 
type exist.
2. The underlying assumption does not apply to vendor/native combination.

Regarding the second limitation, I don't even understand the difference 
between vendor and native. My guess is that a vendor backlight device 
uses vendor-specific ACPI interface, and a native one directly uses 
hardware registers. If my guess is correct, the difference between 
vendor and native does not imply that both of them are likely to exist 
at the same time. As the conclusion, there is no more motivation to try 
to de-duplicate the vendor/native combination than to try to 
de-duplicate combination of devices with a single type.

Of course, it is better if we could also avoid registering two devices 
with one type for one physical device. Possibly we can do so by 
providing a parameter to indicate that it is for the same "internal" 
backlight to devm_backlight_device_register(), and let the function 
check for the duplication. However, this rule may be too restrict, or 
may have problems I missed.

Based on the discussion above, we can deduce three possibilities:
a. There is no reason to distinguish vendor and native in this case, and 
we can stick to my current proposal.
b. There is a valid reason to distinguish vendor and native, and we can 
adopt the same strategy that already adopted for ACPI video/vendor 
combination.
c. We can implement de-duplication in devm_backlight_device_register().
d. The other possible options are not worth, and we can just implement 
quirks specific to Chromebook/coreboot.

In case b, it should be noted that vendor and native backlight device do 
not require ACPI video, and CONFIG_ACPI_VIDEO may not be enabled. In the 
case, the de-duplication needs to be implemented in backlight class device.

Regards,
Akihiko Odaki

> 
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> 
>>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>
>> Akihiko Odaki (22):
>>    drm/i915/opregion: Improve backlight request condition
>>    ACPI: video: Introduce acpi_video_get_backlight_types()
>>    LoongArch: Use acpi_video_get_backlight_types()
>>    platform/x86: acer-wmi: Use acpi_video_get_backlight_types()
>>    platform/x86: asus-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: asus-wmi: Use acpi_video_get_backlight_types()
>>    platform/x86: compal-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: eeepc-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: fujitsu-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: ideapad-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: msi-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: msi-wmi: Use acpi_video_get_backlight_types()
>>    platform/x86: nvidia-wmi-ec-backlight: Use
>>      acpi_video_get_backlight_types()
>>    platform/x86: panasonic-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: samsung-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: sony-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: thinkpad_acpi: Use acpi_video_get_backlight_types()
>>    platform/x86: toshiba_acpi: Use acpi_video_get_backlight_types()
>>    platform/x86: dell-laptop: Use acpi_video_get_backlight_types()
>>    platform/x86: intel_oaktrail: Use acpi_video_get_backlight_types()
>>    ACPI: video: Remove acpi_video_get_backlight_type()
>>    ACPI: video: Fallback to native backlight
>>
>>   Documentation/gpu/todo.rst                    |  8 +--
>>   drivers/acpi/acpi_video.c                     |  2 +-
>>   drivers/acpi/video_detect.c                   | 54 ++++++++++---------
>>   drivers/gpu/drm/i915/display/intel_opregion.c |  3 +-
>>   drivers/platform/loongarch/loongson-laptop.c  |  4 +-
>>   drivers/platform/x86/acer-wmi.c               |  2 +-
>>   drivers/platform/x86/asus-laptop.c            |  2 +-
>>   drivers/platform/x86/asus-wmi.c               |  4 +-
>>   drivers/platform/x86/compal-laptop.c          |  2 +-
>>   drivers/platform/x86/dell/dell-laptop.c       |  2 +-
>>   drivers/platform/x86/eeepc-laptop.c           |  2 +-
>>   drivers/platform/x86/fujitsu-laptop.c         |  4 +-
>>   drivers/platform/x86/ideapad-laptop.c         |  2 +-
>>   drivers/platform/x86/intel/oaktrail.c         |  2 +-
>>   drivers/platform/x86/msi-laptop.c             |  2 +-
>>   drivers/platform/x86/msi-wmi.c                |  2 +-
>>   .../platform/x86/nvidia-wmi-ec-backlight.c    |  2 +-
>>   drivers/platform/x86/panasonic-laptop.c       |  2 +-
>>   drivers/platform/x86/samsung-laptop.c         |  2 +-
>>   drivers/platform/x86/sony-laptop.c            |  2 +-
>>   drivers/platform/x86/thinkpad_acpi.c          |  4 +-
>>   drivers/platform/x86/toshiba_acpi.c           |  2 +-
>>   drivers/video/backlight/backlight.c           | 18 +++++++
>>   include/acpi/video.h                          | 21 ++++----
>>   include/linux/backlight.h                     |  1 +
>>   25 files changed, 85 insertions(+), 66 deletions(-)
>>
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