Re: [RFC] drm/kms: control display brightness through drm_connector properties

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

Hi Yusuf,

On 8/24/22 04:18, Yusuf Khan wrote:
> Sorry for the necro-bump, I hadnt seen this go by

No problem.

> My main concern with this proposal is the phasing out of /sys/class/backlight/.
> Currently on the user(user, not userland) level its easier for me to just modify
> the file and be done with it. xbacklight doesnt tell me when its failed,
> brightnessctl doesnt make assumptions about what device is what, and
> other brightness setting applications ive seen are much worse than them.
> Someone needs to create a userland application thats less inconvenient
> than `echo`ing into /sys/class/backlight with a name that human beings can
> actually remember before I stop using the sysfs, perhaps "setbrightness"
> could be the binary's name? Also I dont think its wise to disable or make it
> read only though Kconfig as older apps may depend on it, maybe add a
> kernel param that disables the old interface so bigger distros can pressure
> app makers into changing the interface? As a big draw for DDC/CI is that
> many displays support it as a way to change brightness(even if you arent
> doing anything special that would break the old interface) perhaps it could
> be an early adopter to that kernel parameter?

Right, so deprecating the /sys/class/backlight API definitely is the last
step and probably is years away. As you say hiding / making it read-only
should probably be a kernel-parameter at first, with maybe a Kconfig
option to set the default. So the depcration would go like this:

1. Add:
A kernel-parameter to allow hiding or read-only-ing the sysfs interface +
Kconfig to select the default +
dev_warn_once() when the old API is used

2. (much later) Drop the Kconfig option and default to hiding/read-only

3. (even later) Maybe completely remove the sysfs interface?

Note the hiding vs read-only thing is to be decided. ATM I'm rather more
focused on getting the new API in place then on deprecating the old one :)

Anyways I fully agree that we need to do the deprecation carefully and
slowly. This is likely going to take multiple years and then some ...

Regards,

Hans



> 
> On Thu, Apr 7, 2022 at 10:39 AM Hans de Goede <hdegoede@redhat.com <mailto:hdegoede@redhat.com>> wrote:
> 
>     As discussed already several times in the past:
>      https://www.x.org/wiki/Events/XDC2014/XDC2014GoedeBacklight/ <https://www.x.org/wiki/Events/XDC2014/XDC2014GoedeBacklight/>
>      https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b02c6@linux.intel.com/ <https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b02c6@linux.intel.com/>
> 
>     The current userspace API for brightness control offered by
>     /sys/class/backlight devices has various issues, the biggest 2 being:
> 
>     1. There is no way to map the backlight device to a specific
>        display-output / panel (1)
>     2. Controlling the brightness requires root-rights requiring
>        desktop-environments to use suid-root helpers for this.
> 
>     As already discussed on various conference's hallway tracks
>     and as has been proposed on the dri-devel list once before (2),
>     it seems that there is consensus that the best way to to solve these
>     2 issues is to add support for controlling a video-output's brightness
>     through properties on the drm_connector.
> 
>     This RFC outlines my plan to try and actually implement this,
>     which has 3 phases:
> 
> 
>     Phase 1: Stop registering multiple /sys/class/backlight devs for a single display
>     =================================================================================
> 
>     On x86 there can be multiple firmware + direct-hw-access methods
>     for controlling the backlight and in some cases the kernel registers
>     multiple backlight-devices for a single internal laptop LCD panel:
> 
>     a) i915 and nouveau unconditionally register their "native" backlight dev
>        even on devices where /sys/class/backlight/acpi_video0 must be used
>        to control the backlight, relying on userspace to prefer the "firmware"
>        acpi_video0 device over "native" devices.
>     b) amdgpu and nouveau rely on the acpi_video driver initializing before
>        them, which currently causes /sys/class/backlight/acpi_video0 to usually
>        show up and then they register their own native backlight driver after
>        which the drivers/acpi/video_detect.c code unregisters the acpi_video0
>        device. This means that userspace briefly sees 2 devices and the
>        disappearing of acpi_video0 after a brief time confuses the systemd
>        backlight level save/restore code, see e.g.:
>        https://bbs.archlinux.org/viewtopic.php?id=269920 <https://bbs.archlinux.org/viewtopic.php?id=269920>
> 
>     I already have a pretty detailed plan to tackle this, which I will
>     post in a separate RFC email. I plan to start working on this right
>     away, as it will be good to have this fixed regardless.
> 
> 
>     Phase 2: Add drm_connector properties mirroring the matching backlight device
>     =============================================================================
> 
>     The plan is to add a drm_connector helper function, which optionally takes
>     a pointer to the backlight device for the GPU's native backlight device,
>     which will then mirror the backlight settings from the backlight device
>     in a set of read/write brightness* properties on the connector.
> 
>     This function can then be called by GPU drivers for the drm_connector for
>     the internal panel and it will then take care of everything. When there
>     is no native GPU backlight device, or when it should not be used then
>     (on x86) the helper will use the acpi_video_get_backlight_type() to
>     determine which backlight-device should be used instead and it will find
>     + mirror that one.
> 
> 
>     Phase 3: Deprecate /sys/class/backlight uAPI
>     ============================================
> 
>     Once most userspace has moved over to using the new drm_connector
>     brightness props, a Kconfig option can be added to stop exporting
>     the backlight-devices under /sys/class/backlight. The plan is to
>     just disable the sysfs interface and keep the existing backlight-device
>     internal kernel abstraction as is, since some abstraction for (non GPU
>     native) backlight devices will be necessary regardless.
> 
>     An alternative to disabling the sysfs class entirely, would be
>     to allow setting it to read-only through Kconfig.
> 
> 
>     What scale to use for the drm_connector bl_brightness property?
>     ===============================================================
> 
>     The tricky part of this plan is phase 2 and then esp. defining what the
>     new brightness properties will look like and how they will work.
> 
>     The biggest challenge here is to decide on a fixed scale for the main
>     brightness property, say 0-65535, using scaling where the actual hw scale
>     is different, or if this should simply be a 1:1 mirror of the current
>     backlight interface, with the actual hw scale / brightness_max value
>     exposed as a drm_connector property.
> 
>     1:1 advantages / 0-65535 disadvantages
>     - Userspace will likely move over to the connector-props quite slowly and
>       we can expect various userspace bits, esp. also custom user scripts, to
>       keep using the old uAPI for a long time. Using the 2 APIs are intermixed
>       is fine when using a 1:1 brightness scale mapping. But if we end up doing
>       a scaling round-trip all the time then eventually the brightness is going
>       do drift. This can even happen if the user never changes the brightness
>       when userspace saves it over suspend/resume or reboots.
>     - Almost all laptops have brightness up/down hotkeys. E.g GNOME decides
>       on a step size for the hotkeys by doing min(brightness_max/20, 1).
>       Some of the vendor specific backlight fw APIs (e.g. dell-laptop) have
>       only 8 steps. When giving userspace the actual max_brightness value, then
>       this will all work just fine. When hardcode brightness_max to 65535 OTOH
>       then in this case GNOME will still give the user 20 steps where only 1
>       in every 2-3 steps actually changes the brightness which IMHO is
>       an unacceptably bad user experience.
> 
>     0-65535 advantages / 1:1 disadvantages
>     - Without a fixed scale for the brightness property the brightness_max
>       value may change after an userspace application's initial enumeration
>       of the drm_connector. This can happen when neither the native GPU nor
>       the acpi_video backlight devices are present/usable in this case
>       acpi_video_get_backlight_type() will _assume_ a vendor specific fw API
>       will be used for backlight control and the driver proving the "vendor"
>       backlight device will show up much later and may even never show-up,
>       so waiting for it is not an option. With a fixed 0-65535 scale userspace
>       can just always assume this and the drm_connector backlight props helper
>       code can even cache writes and send it to the actual backlight device
>       when it shows up. With a 1:1 mapping userspace needs to listen for
>       a uevent and then update the brightness range on such an event.
> 
>     I believe that the 1:1 mapping advantages out way the disadvantages
>     here. Also note that current userspace already blindly assumes that
>     all relevant drivers are loaded before the graphical-environment
>     starts and all the desktop environments as such already only do
>     a single scan of /sys/class/backlight when they start. So when
>     userspace forgets to add code to listen for the uevent when switching
>     to the new API nothing changes; and with the uevent userspace actually
>     gets a good mechanism to detect backlight drivers loading after
>     the graphical-environment has already started.
> 
>     So based on this here is my proposal for a set of new brightness
>     properties on the drm_connector object. Note these are all prefixed with
>     bl which stands for backlight, which is technically not correct for OLED.
>     But we need a prefix to avoid a name collision with the "brightness"
>     attribute which is part of the existing TV specific properties and IMHO
>     it is good to have a common prefix to make it clear that these all
>     belong together.
> 
> 
>     The drm_connector brightness properties
>     =======================================
> 
>     bl_brightness: rw 0-int32_max property controlling the brightness setting
>     of the connected display. The actual maximum of this will be less then
>     int32_max and is given in bl_brightness_max.
> 
>     bl_brightness_max: ro 0-int32_max property giving the actual maximum
>     of the display's brightness setting. This will report 0 when brightness
>     control is not available (yet).
> 
>     bl_brightness_0_is_min_brightness: ro, boolean
>     When this is set to true then it is safe to set brightness to 0
>     without worrying that this completely turns the backlight off causing
>     the screen to become unreadable. When this is false setting brightness
>     to 0 may turn the backlight off, but this is _not_ guaranteed.
>     This will e.g. be true when directly driving a PWM and the video-BIOS
>     has provided a minimum (non 0) duty-cycle below which the driver will
>     never go.
> 
>     bl_brightness_control_method: ro, enum, possible values:
>     none:     The GPU driver expects brightness control to be provided by another
>               driver and that driver has not loaded yet.
>     unknown:  The underlying control mechanism is unknown.
>     pwm:      The brightness property directly controls the duty-cycle of a PWM
>               output.
>     firmware: The brightness is controlled through firmware calls.
>     DDC/CI:   The brightness is controlled through the DDC/CI protocol.
>     gmux:     The brightness is controlled by the GMUX.
>     Note this enum may be extended in the future, so other values may
>     be read, these should be treated as "unknown".
> 
>     When brightness control becomes available after being reported
>     as not available before (bl_brightness_control_method=="none")
>     a uevent with CONNECTOR=<connector-id> and
>     PROPERTY=<bl_brightness_control_method-id> will be generated
>     at this point all the properties must be re-read.
> 
>     When/once brightness control is available then all the read-only
>     properties are fixed and will never change.
> 
>     Besides the "none" value for no driver having loaded yet,
>     the different bl_brightness_control_method values are intended for
>     (userspace) heuristics for such things as the brightness setting
>     linearly controlling electrical power or setting perceived brightness.
> 
>     Regards,
> 
>     Hans
> 
> 
>     1) The need to be able to map the backlight device to a specific display
>     has become clear once more with the recent proposal to add DDCDI support:
>     https://lore.kernel.org/lkml/20220403230850.2986-1-yusisamerican@gmail.com/ <https://lore.kernel.org/lkml/20220403230850.2986-1-yusisamerican@gmail.com/>
> 
>     2) https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b02c6@linux.intel.com/ <https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b02c6@linux.intel.com/>
>     Note this proposal included a method for userspace to be able to tell the
>     kernel if the native/acpi_video/vendor backlight device should be used,
>     but this has been solved in the kernel for years now:
>      https://www.spinics.net/lists/linux-acpi/msg50526.html <https://www.spinics.net/lists/linux-acpi/msg50526.html>
>     An initial implementation of this proposal is available here:
>      https://cgit.freedesktop.org/~mperes/linux/log/?h=backlight <https://cgit.freedesktop.org/~mperes/linux/log/?h=backlight>
>