[PATCH v4 31/31] drm/todo: Add entry about dealing with brightness control on devices with > 1 panel

[Lyude Paul]

Reviewed-by: Lyude Paul <lyude at redhat.com>

On Wed, 2022-08-24 at 14:15 +0200, Hans de Goede wrote:
> Add an entry summarizing the discussion about dealing with brightness
> control on devices with more then 1 internal panel.
> 
> The original discussion can be found here:
> https://lore.kernel.org/dri-devel/20220517152331.16217-1-hdegoede@redhat.com/
> 
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
>  Documentation/gpu/todo.rst | 68 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> index 7634c27ac562..393d218e4a0c 100644
> --- a/Documentation/gpu/todo.rst
> +++ b/Documentation/gpu/todo.rst
> @@ -679,6 +679,74 @@ Contact: Sam Ravnborg
>  
>  Level: Advanced
>  
> +Brightness handling on devices with multiple internal panels
> +============================================================
> +
> +On x86/ACPI devices there can be multiple backlight firmware interfaces:
> +(ACPI) video, vendor specific and others. As well as direct/native (PWM)
> +register programming by the KMS driver.
> +
> +To deal with this backlight drivers used on x86/ACPI call
> +acpi_video_get_backlight_type() which has heuristics (+quirks) to select
> +which backlight interface to use; and backlight drivers which do not match
> +the returned type will not register themselves, so that only one backlight
> +device gets registered (in a single GPU setup, see below).
> +
> +At the moment this more or less assumes that there will only
> +be 1 (internal) panel on a system.
> +
> +On systems with 2 panels this may be a problem, depending on
> +what interface acpi_video_get_backlight_type() selects:
> +
> +1. native: in this case the KMS driver is expected to know which backlight
> +   device belongs to which output so everything should just work.
> +2. video: this does support controlling multiple backlights, but some work
> +   will need to be done to get the output <-> backlight device mapping
> +
> +The above assumes both panels will require the same backlight interface type.
> +Things will break on systems with multiple panels where the 2 panels need
> +a different type of control. E.g. one panel needs ACPI video backlight control,
> +where as the other is using native backlight control. Currently in this case
> +only one of the 2 required backlight devices will get registered, based on
> +the acpi_video_get_backlight_type() return value.
> +
> +If this (theoretical) case ever shows up, then supporting this will need some
> +work. A possible solution here would be to pass a device and connector-name
> +to acpi_video_get_backlight_type() so that it can deal with this.
> +
> +Note in a way we already have a case where userspace sees 2 panels,
> +in dual GPU laptop setups with a mux. On those systems we may see
> +either 2 native backlight devices; or 2 native backlight devices.
> +
> +Userspace already has code to deal with this by detecting 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. It picks only 1 of the 2 backlight devices
> +and then only uses that one.
> +
> +Note that all userspace code (that I know off) is currently hardcoded
> +to assume a single panel.
> +
> +Before the recent changes to not register multiple (e.g. video + native)
> +/sys/class/backlight devices for a single panel (on a single GPU laptop),
> +userspace would see multiple backlight devices all controlling the same
> +backlight.
> +
> +To deal with this userspace had to always picks one preferred device under
> +/sys/class/backlight and will ignore the others. So to support brightness
> +control on multiple panels userspace will need to be updated too.
> +
> +There are plans to allow brightness control through the KMS API by adding
> +a "display brightness" property to drm_connector objects for panels. This
> +solves a number of issues with the /sys/class/backlight API, including not
> +being able to map a sysfs backlight device to a specific connector. Any
> +userspace changes to add support for brightness control on devices with
> +multiple panels really should build on top of this new KMS property.
> +
> +Contact: Hans de Goede
> +
> +Level: Advanced
> +
>  Outside DRM
>  ===========
>  

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat