KMS backlight ABI proposition

[Martin Peres]

Hey everyone,

We have been working towards exposing the backlight as a KMS property 
instead of relying on the backlight drivers. We have CC:ed the people we 
have found to be the more likely to be interested in the discussion but 
please add everyone you think would have some experience with this issue.

== Introduction ==

We are trying to bring the same level of support for the backlight on 
both the xf86-video-intel and -modesetting DDX.

Looking into the situation of the backlight, we identified these 
problems which are almost show-stoppers for -modesetting and wayland 
compositors:

  - There is no mapping between the backlight driver and DRM-connectors. 
This means that, in case there are multiple backlight drivers, the 
userspace has to have knowledge of the machine to know which driver 
should be used. See the priority list for the intel driver [0].

  - The luminance curve of the backlight drivers is not specified, which 
can lead to a bad user experience: Little changes in the highest levels 
but drastic changes in the low levels.

  - Writing to the backlight driver still requires root rights. Given 
that the xserver and wayland compositors are now running root-less, this 
means we would need a complex dance involving a setuid helper [1].

Hans de Goede has already given a presentation about these issues at 
XDC2014. The slides are a good read[2].

[0] 
https://cgit.freedesktop.org/xorg/driver/xf86-video-intel/tree/src/backlight.c#n259

[1] 
https://cgit.freedesktop.org/xorg/driver/xf86-video-intel/tree/src/backlight.c#n348

[2] 
https://www.x.org/wiki/Events/XDC2014/XDC2014GoedeBacklight/backlight.pdf

== Proposal ==

Since David Hermann already worked on this and proposed what I consider 
being greats foundations for building towards a solution addressing the 
issues above, I will just ask you to read his original words:

https://lists.freedesktop.org/archives/dri-devel/2014-September/067984.html

== Open issues ==

Here are the open issues we have identified with the solution proposed 
by David:

   1) Backlight device interoperability: How far should we support
      mixing the backlight device and brightness property? Should it be
      unidirectional or bi-directional? What about the start-up value
      exposed by the brightness property?

   2) How many steps should be exposed: fixed or driver-dependent?

   3) Expected output curve: power? luminance? Simply monotonically
      increasing?

   4) Should the userspace be able to turn off the backlight? If so, how
      should it do it? What can we do to let the userspace distinguish
      between backlight off or on?

   5) Should we expose to the userspace what is the current backlight
      power?

Here is our current point of view on the matter:

=== 1) Backlight device interoperability ===

Since we need to keep backward compatibility of the backlight, we have 
to keep the current backlight drivers.

Here are possible options:

  - Exclusive access: Unregister a backlight device when the drm 
brightness property is requested/used;
  - Unidirectional access: When writing to the backlight property, 
update the backlight device;
  - Bi-directional access: Propagate back changes from the backlight 
device to the property's value.

Being bi-directional would be of course the best, but this requires that 
both drivers have the same number of steps, otherwise, we may write a 
value to the property, but get another one when reading it right after, 
due to the non-bijective nature of the transformation.

Uni-directional would work in all cases, with the caveat that mixing 
calls to the KMS property and the backlight device will not be supported 
(changes mades through the sysfs interface of the backlight driver will 
not be reflected in the KMS property). At boot time, we should however 
initialize the value of the backlight property with a value close to 
what is currently set in the backlight driver.

Giving exclusive access does not sound very good to me, as it would be 
hard for the userspace to deal with disappearing drivers...

=== 2) How many steps should be exposed ===

If the KMS property exposes the same number of steps as the backlight 
driver, it allows us to get a bijective function between the two 
interfaces, and allow a bi-directional communication. The downside of 
this is that it forces the userspace to deal with a variable number of 
steps which can range from 4 to 1k+. Also, the userspace would be able 
to handle the case where there are less steps than it would like to expose.

If the KMS property exposes a fixed number of steps (say 100), it 
becomes easy for the userspace to express the wanted brightness. 
However, on drivers exposing less than these 100 steps, we cannot 
guarantee that any change in the value will produce any change. If there 
is only one possible value (on or off), the user may be trying the 
change the brightness, a GUI would show what is the expected backlight 
state, but no change in the luminance would be seen, which is pretty bad.

=== 3) and 4) ===

These issues are not handled at all by the backlight device sysfs interface.

But since David already had to add an in-kernel interface to access the 
backlight devices [0], we could add capabilities to the drivers while 
keeping the backward compatibility.

 From the in-kernel interface, it is already possible to turn on and off 
the backlight for sure (when supported, but this is also reported 
properly). However, what is not supported is to know what the value 0 
means (lowest setting possible but not turned off, or no power at all).

It was brought up that we could simply not allow the backlight to be 
turned off, and just request DPMS to reach this state. However, I do not 
think it is a good idea as some panels (like the one from the OLPC) 
switch to e-paper mode when the backlight is set to 0 and are perfectly 
readable.

I would suggest we design an interface that will allow good drivers to 
expose as many features as possible, but yet gracefully degrade if 
information is not present.

Over time, drivers will improve to expose information about the 
platform, and the user experience will improve as a result.

[0] 
https://lists.freedesktop.org/archives/dri-devel/2014-September/067987.html

===  5) Exposing the current backlight power?  ===

The backlight_current interface in the backlight devices is meant to 
expose the currently-used backlight value, regardless of the wanted 
value that should be used when the backlight is not off.

My current stance on this is that this should not be needed. The 
userspace should describe the intent of the user (wanted backlight 
level) and trust the KMS property to turn off the backlight when 
entering DPMS.

== Current KMS ABI proposal ==

The current ABI proposal has mostly been proposed by Jani Nikula, as a 
result of his experience and our discussions.

It takes the following approach:

  - Fixed number of steps (I think we should change it to expose the 
same number of steps)
  - Uni-directional: KMS -> backlight
  - Do not deal yet with 3) and 4): I have ideas, but I have been 
procrastinating long-enough to send this email and we already have much 
to discuss!
  - Does not expose the current backlight power as we want to let the 
kernel deal with DPMS on its own

=== ABI proposal ===

The brightness property MUST have values 0...100 inclusive.

The display brightness MUST be a monotonically increasing function of
the brightness property.

Brightness property value 1 MUST mean the minimum supported visible
brightness.

Brightness property value 100 MUST mean the maximum supported
brightness.

Brightness property value 0 SHOULD mean backlight off or equivalent for
non-backlight brightness adjustment, typically completely
black. Brightness property value 0 MUST NOT switch the display or pipe
off [1].

If the hardware is not capable of supporting zero brightness, and the
driver knows this, value 0 MUST be equal to value 1.

If the driver does not know whether the hardware is capable of
supporting zero brightness, the driver SHOULD err on the side of 0 not
being off rather than 1 meaning off. In this case, value 0 is likely
different from value 1, and the minimum brightness can only be reached
via property value 0 [2].

If the brightness gets changed outside of the property interface,
reading the property value MAY be out of sync with the actual brightness
[3].

[1] Must be able to support displays which are visible even with the
backlight switched off.

[2] The main downside corner case with this is that if the driver
doesn't know whether it can switch off the backlight, 0 might end up
meaning the minimum visible, and 1 is the second lowest visible, and
with a userspace that avoids black display, the user can't use the
lowest brightness setting.

[3] This is not unlike the "brightness" property in the backlight class
sysfs interface. The intention is that the drm interface does not have
an equivalent of "actual_brightness".