[RFC] drm/kms: control display brightness through drm_connector properties
Hans de Goede
hdegoede at redhat.com
Thu Aug 25 08:27:05 UTC 2022
On 8/24/22 04:18, Yusuf Khan wrote:
> Sorry for the necro-bump, I hadnt seen this go by
> 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:
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 ...
> On Thu, Apr 7, 2022 at 10:39 AM Hans de Goede <hdegoede at redhat.com <mailto:hdegoede at 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://firstname.lastname@example.org/ <https://email@example.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
> 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.
> 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://firstname.lastname@example.org/ <https://email@example.com/>
> 2) https://firstname.lastname@example.org/ <https://email@example.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>
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