The quirk was added as what I'd say was a stopgap measure in
commit e85843bec6
Author: Kamal Mostafa <kamal@canonical.com>
Date: Fri Jul 19 15:02:01 2013 -0700
drm/i915: quirk no PCH_PWM_ENABLE for Dell XPS13 backlight
without really digging into what was going on.
Also, as mentioned in the related bug [1], having the quirk regressed
some of the machines it was supposed to fix to begin with, and there
were patches posted to disable the quirk on such machines [2]!
The fact is, we do need the BLM_PCH_PWM_ENABLE bit set to have
backlight. With the quirk, we've relied on BIOS to have set it, and our
save/restore code to retain it. With the full backlight setup at enable,
we have no place for things that rely on previous state.
With the per platform hooks, we've also made a change in the PCH
platform enable order: setting the backlight duty cycle between CPU and
PCH PWM enable. Some experimenting and
commit 770c12312a
Author: Takashi Iwai <tiwai@suse.de>
Date: Sat Aug 11 08:56:42 2012 +0200
drm/i915: Fix blank panel at reopening lid
indicate that we can't set the backlight before enabling CPU PWM; the
value just won't stick. But AFAICT we should do it before enabling the
PCH PWM.
Finally, any fallout we should fix properly, preferrably without quirks,
and absolutely without quirks that rely on existing state. With the per
platform hooks have much more flexibility to adjust the sequence as
required by platforms.
[1] https://bugzilla.kernel.org/show_bug.cgi?id=47941
[2] http://lkml.kernel.org/r/1378229848-29113-1-git-send-email-kamal@canonical.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
************************************************************
* For the very latest on DRI development, please see: *
* http://dri.freedesktop.org/ *
************************************************************
The Direct Rendering Manager (drm) is a device-independent kernel-level
device driver that provides support for the XFree86 Direct Rendering
Infrastructure (DRI).
The DRM supports the Direct Rendering Infrastructure (DRI) in four major
ways:
1. The DRM provides synchronized access to the graphics hardware via
the use of an optimized two-tiered lock.
2. The DRM enforces the DRI security policy for access to the graphics
hardware by only allowing authenticated X11 clients access to
restricted regions of memory.
3. The DRM provides a generic DMA engine, complete with multiple
queues and the ability to detect the need for an OpenGL context
switch.
4. The DRM is extensible via the use of small device-specific modules
that rely extensively on the API exported by the DRM module.
Documentation on the DRI is available from:
http://dri.freedesktop.org/wiki/Documentation
http://sourceforge.net/project/showfiles.php?group_id=387
http://dri.sourceforge.net/doc/
For specific information about kernel-level support, see:
The Direct Rendering Manager, Kernel Support for the Direct Rendering
Infrastructure
http://dri.sourceforge.net/doc/drm_low_level.html
Hardware Locking for the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/hardware_locking_low_level.html
A Security Analysis of the Direct Rendering Infrastructure
http://dri.sourceforge.net/doc/security_low_level.html
bc0bb9fd1c