Fixing the kernels backlight API (original) (raw)

David Herrmann dh.herrmann at gmail.com
Wed Feb 12 14:26:34 PST 2014

Hi

On Wed, Feb 12, 2014 at 9:43 PM, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:

On Thu, Feb 13, 2014 at 06:14:04AM +1000, Dave Airlie wrote:

> > The biggest remaining stumbling block is the backlight API, because opening the > sysfs files requires root rights. I'll very likely write a little helper for this > for now, but in the long run it would be good to have a better solution. > > While discussion this in the graphics devroom at Fosdem, the general consensus > seemed to be that the current backlight API is in need of an overhaul anyways. > > There are several issues with the current API: > -there is no reliable way to determine the relation between a backlight > control in sysfs and the display it controls the backlight off > -on many laptops we end up with multiple providers of backlight control > all battling over control of the same backlight controller through various > firmware interfaces > -and there is no way to do acl management on it because of sysfs usage > > At Fosdem it was suggested to "simply" make the backlight a property of > the connector in drm/kms and let users control it that way. From an acl pov > this makes a ton of sense, if a user controls the other display-panel settings, > then he should be able to control the backlight too. > > This also nicely solves the issue of userspace having to figure out which backlight > control to use for a certain output. > > Last this makes it the kernels responsibility to figure out which firmware interface > (if any) to use and tie that to the connector, rather then just exporting all of > them, including conflicting ones, and just hoping that userspace will figure things out. > > Note that wrt the last point, the kernel is the one which should have all the hardware > knowledge to do this properly, after all hardware abstraction is one of the tasks of > the kernel. > > I realize moving this more into the kernel, and tying things into drm is in no means > easy, but it is about time we clean up this mess. > > Note that although I'm kicking of this discussion, my focus within the graphics team is > mostly on input devices, so I'm hoping that someone else will pick things up once we've > a better idea of how we would like to solve this. >

I hate to respond with yeah no, but yeah no, http://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&showhtml=true&highlightnames=&date=2014-02-04 http://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&showhtml=true&highlightnames=&date=2014-02-05 read down that until you see me and tagr talking, read it a few times, and the follow on chat with dvdhrm. The biggest problem with leaving the kernel to pick the correct one, is the kernel simply never knows which is the correct one, That could be solved by still allowing userspace to change the connection between the property and the actual backlight device. With the prop approach + atomic modeset you could also do some slightly fancier things like changing the backlight at the same time as doing a modeset or just adjusting some other display related properties.

The "attach" stuff actually sounds doable, but who decides which one to attach? You still need some user-space script during device-plug for that. But to be honest, the simplest way would be a "backlightd" bus-activatable daemon. SetBacklight() then takes a DRM-connector and brightness-value, which the daemon looks up in /sys and sets.. This has the advantage that we can do any fancy matching in user-space. We can provide quirks (maybe even via udev-hwdb) and other helpers for weird setups.

Cheers David

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