Re: [PATCH 00/14] drm/kms: Stop registering multiple /sys/class/backlight devs for a single display

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

On Wed, 18 May 2022, Hans de Goede <hdegoede@redhat.com> wrote:
> Hi,
>
> On 5/18/22 10:44, Jani Nikula wrote:
>> On Tue, 17 May 2022, Hans de Goede <hdegoede@redhat.com> wrote:
>>> Hi All,
>>>
>>> As mentioned in my RFC titled "drm/kms: control display brightness through
>>> drm_connector properties":
>>> https://lore.kernel.org/dri-devel/0d188965-d809-81b5-74ce-7d30c49fee2d@redhat.com/
>>>
>>> The first step towards this is to deal with some existing technical debt
>>> in backlight handling on x86/ACPI boards, specifically we need to stop
>>> registering multiple /sys/class/backlight devs for a single display.
>> 
>> I guess my question here is, how do you know it's for a *single*
>> display?
>> 
>> There are already designs out there with two flat panels, with
>> independent brightness controls. They're still rare and I don't think we
>> handle them very well. But we've got code to register multiple native
>> backlight interfaces, see e.g. commit 20f85ef89d94 ("drm/i915/backlight:
>> use unique backlight device names").
>> 
>> So imagine a design where one of the panels needs backlight control via
>> ACPI and the other via native backlight control. Granted, I don't know
>> if one exists, but I think it's very much in the realm of possible
>> things the OEMs might do. For example, have an EC PWM for primary panel
>> backlight, and use GPU PWM for secondary. How do you know you actually
>> do need to register two interfaces?
>
> On x86/ACPI devices this is all driven by acpi_video_get_backlight_type() /
> by the drivers/acpi/video_detect.c code. That code already will break on
> systems where there are 2 backlight controls, with one being ACPI based
> and the other a native GPU PWM backlight device.
>
> In this scenario as soon as the native GPU PWM backlight device shows up
> then, if acpi_video_get_backlight_type()==native, video_detect.c will
> currently unregister the acpi_video# backlight device(s) since userspace
> prefers the firmware type over the native type, so for userspace to
> actually honor the acpi_video_get_backlight_type()==native we need to get
> the acpi_video# backlight device "out of the way" which is currently
> handled by unregistering it.
>
> Note in a way we already have a case where userspace sees 2 panels,
> in hybrid laptop setups with a mux and on those systems we may see
> either 2 native backlight devices; or 2 native backlight devices +
> 2 acpi_video backlight devices with userspace preferring the ACPI
> ones.
>
> Also note that userspace already has code to detect if the related
> panel is active (iow which way the mux between the GPU and the panels
> points) and then uses that backlight device. Userspace here very
> much assumes a single panel though.
>
>> I'm fine with dealing with such cases as they arise to avoid
>> over-engineering up front, but I also don't want us to completely paint
>> ourselves in a corner either.
>
> Right. Note that the current code (both with and without this patchset)
> already will work fine from a kernel pov as long as both panels
> are either using native GPU PWM or are both using ACPI. But if we
> ever get a mix then this will need special handling.
>
> Note that all userspace code I know is currently hardcoded
> to assume a single panel. Userspace already picks one preferred
> device under /sys/class/backlight and ignores the rest. Actually
> atm userspace must behave this way, because on x86/ACPI boards we
> do often register multiple backlight devices for a single panel.
>
> So in a way moving to registering only a single backlight device
> prepares for actually having multiple panels work.
>
> Also keep in mind that this is preparation work for making the
> panel brightness a drm_connector property. When the panel uses
> a backlight device other then the native GPU PWM to control the
> brightness then the helper code for this needs to have a way to
> select which backlight_device to use then and the non native
> types are not "linked" to a specific connector so in this case
> we really need there to be only 1 backlight device registered
> so that the code looking up the (non native) backlight device
> for the connector gets the right one and not merely the one
> which happened to get registered first.
>
> And I believe that having the panel brightness be a drm_connector
> property is the way to make it possible for userspace to deal
> with the multiple panels which each have a separate brightness
> control case.

Agreed.

Thanks for the explanations and recording them here.

BR,
Jani.

>
> Regards,
>
> Hans
>
>
>
>
>
>> 
>> BR,
>> Jani.
>> 
>> 
>>>
>>> This series implements my RFC describing my plan for these cleanups:
>>> https://lore.kernel.org/dri-devel/98519ba0-7f18-201a-ea34-652f50343158@redhat.com/
>>>
>>> Specifically patches 1-6 implement the "Fixing kms driver unconditionally
>>> register their "native" backlight dev" part.
>>>
>>> And patches 7-14 implement the "Fixing acpi_video0 getting registered for
>>> a brief time" time.
>>>
>>> Note this series does not deal yet with the "Other issues" part, I plan
>>> to do a follow up series for that.
>>>
>>> The changes in this series are good to have regardless of the further
>>> "drm/kms: control display brightness through drm_connector properties"
>>> plans. So I plan to push these upstream once they are ready (once
>>> reviewed). Since this crosses various subsystems / touches multiple
>>> kms drivers my plan is to provide an immutable branch based on say
>>> 5.19-rc1 and then have that get merged into all the relevant trees.
>>>
>>> Please review.
>>>
>>> Regards,
>>>
>>> Hans
>>>
>>>
>>> Hans de Goede (14):
>>>   ACPI: video: Add a native function parameter to
>>>     acpi_video_get_backlight_type()
>>>   drm/i915: Don't register backlight when another backlight should be
>>>     used
>>>   drm/amdgpu: Don't register backlight when another backlight should be
>>>     used
>>>   drm/radeon: Don't register backlight when another backlight should be
>>>     used
>>>   drm/nouveau: Don't register backlight when another backlight should be
>>>     used
>>>   ACPI: video: Drop backlight_device_get_by_type() call from
>>>     acpi_video_get_backlight_type()
>>>   ACPI: video: Remove acpi_video_bus from list before tearing it down
>>>   ACPI: video: Simplify acpi_video_unregister_backlight()
>>>   ACPI: video: Make backlight class device registration a separate step
>>>   ACPI: video: Remove code to unregister acpi_video backlight when a
>>>     native backlight registers
>>>   drm/i915: Call acpi_video_register_backlight()
>>>   drm/nouveau: Register ACPI video backlight when nv_backlight
>>>     registration fails
>>>   drm/amdgpu: Register ACPI video backlight when skipping amdgpu
>>>     backlight registration
>>>   drm/radeon: Register ACPI video backlight when skipping radeon
>>>     backlight registration
>>>
>>>  drivers/acpi/acpi_video.c                     | 69 ++++++++++++++-----
>>>  drivers/acpi/video_detect.c                   | 53 +++-----------
>>>  drivers/gpu/drm/Kconfig                       |  2 +
>>>  .../gpu/drm/amd/amdgpu/atombios_encoders.c    | 14 +++-
>>>  .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |  9 +++
>>>  .../gpu/drm/i915/display/intel_backlight.c    |  7 ++
>>>  drivers/gpu/drm/i915/display/intel_display.c  |  1 +
>>>  drivers/gpu/drm/i915/display/intel_opregion.c |  2 +-
>>>  drivers/gpu/drm/nouveau/nouveau_backlight.c   | 14 ++++
>>>  drivers/gpu/drm/radeon/atombios_encoders.c    |  7 ++
>>>  drivers/gpu/drm/radeon/radeon_encoders.c      | 11 ++-
>>>  .../gpu/drm/radeon/radeon_legacy_encoders.c   |  7 ++
>>>  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 +-
>>>  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 +-
>>>  include/acpi/video.h                          |  8 ++-
>>>  28 files changed, 156 insertions(+), 84 deletions(-)
>> 
>

-- 
Jani Nikula, Intel Open Source Graphics Center