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".
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