Hi, On 4/7/22 20:58, Carsten Haitzler wrote: > On Thu, 7 Apr 2022 17:38:59 +0200 Hans de Goede <hdegoede@xxxxxxxxxx> said: > > Below you covered our usual /sys/class/backlight device friends... what about > DDC monitor controls? These function similarly but just remotely control a > screen via I2C and also suffer from the same problems of "need root" and "have > to do some fun in mapping them to a given screen". Right, supporting this definitely is part of the plan, this is why my original email had the following footnote: 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@xxxxxxxxx/ :) > Otherwise I welcome this de-uglification of the backlight device and putting it > together with the drm device that controls that monitor. Thx. > Just to make life more fun ... DDC does much more than backlight controls. It > can control essentially anything that is in the OSD for your monitor (contrast, > brightness, backlight, sharpness, color temperatures, input to display (DP vs > HDMI vs DVI etc.), an for extra fun points can even tel you the current > rotation state of your monitor. All of these do make sense to live as drm > connector properties too. Perhaps not a first iteration but something to > consider in this design. One thing which I do want to take into account is to make sure that userspace can still do DDC/CI for all the other features. I know there is demand for adding brightness control over DDC/CI. I'm not aware of any concrete use-cases for the other DDC/CI settings. Also DDC/CI can include some pretty crazy stuff like setting up picture in picture using 2 different inputs of the monitor, which is very vendor specific. So all in all I think that we should just punt most of the DDC/CI stuff to userspace. With that said I agree that it would be good to think about possibly other some of the other settings in case some use-case does show up for those. The biggest problem with doing this is coming up with some prefix to namespace things. I've gone with bl_brightness to avoid a conflict with the existing TV specific properties which have plain "brightness" put if we want to e.g. also add DDC/CI contrast as a property later then it might be good to come up with another more generic prefix which can be shared between laptop-panel-brightness, DDC/CI-brightness and DDC/CI-contrast ... ? So any suggestions for a better prefix? Regards, Hans > >> As discussed already several times in the past: >> https://www.x.org/wiki/Events/XDC2014/XDC2014GoedeBacklight/ >> https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b02c6@xxxxxxxxxxxxxxx/ >> >> 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 >> >> 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@xxxxxxxxx/ >> >> 2) >> https://lore.kernel.org/all/4b17ba08-39f3-57dd-5aad-d37d844b02c6@xxxxxxxxxxxxxxx/ >> 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 An initial >> implementation of this proposal is available here: >> https://cgit.freedesktop.org/~mperes/linux/log/?h=backlight >> > >
