Perhaps the Kconfig modifications could be postponed to stage 2 since for people running distros that suddenly decide to disable /sys/class/backlight/ it may be impractical for them to recompile their kernels and such. Also stage 2 should probably take ~2 decades until it comes into being, for reference fbdev SPECIFIC drivers were removed from fedora just recently and because of that there were some issues with some user's systems. I understand it's much easier to change from the /sys/class/backlight/ interface to the one you have proposed than to change from fbdev to KMS though. On Thu, Aug 25, 2022 at 3:27 AM Hans de Goede wrote: > 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 > 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= and > > PROPERTY= 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> > > > >