[Intel-gfx] i915 PCH backlight vs. Dell XPS13

[Daniel Vetter]

You might want to try the latest drm-intel-nightly git branch from
http://cgit.freedesktop.org/~danvet/drm-intel This has a few more
tricks which helped on some similar platforsm to yours (and similar
resume issues).

And please always include a mailing list when reporting issues, since
your dear maintainer might be on vacation (or simply swamped with
other stuff).

Cheers, Daniel

On Fri, Feb 8, 2013 at 8:36 PM, Kamal Mostafa <kamal at canonical.com> wrote:
> Hi Paulo and Daniel-
>
> I've been tracking Daniel's work on i915 backlight for quite some time
> now, in particular to support the Dell XPS13 (Sandybridge-mobile,
> CougarPoint PCH) in the Ubuntu kernels.
>
> Daniel's backlight fixes which landed in 3.6 did enable working
> backlight controls on the XPS13, but it appears that this commit from
> Paolo broke it again:
>
>         a4f32fc drm/i915: don't forget the PCH backlight registers
>
> More specifically, the behavior on the XPS13 is this:
>
> With the current 3.8-rc6 i915 stack (including a4f32fc), this machine
> gets both of: /sys/class/backlight/{acpi_video0,intel_backlight}.
> Changes to acpi_video0/brightness don't have any effect on the
> backlight, unless I first "echo 0 > intel_backlight/brightness", then
> acpi_video0 works properly.  If I stuff a low non-zero number into
> intel_backlight/brightness, then the backlight pulsates and flickers
> continuously.
>
> If I revert a4f32fc (or just omit the setting of BLM_PCH_PWM_ENABLE in
> intel_panel_enable_backlight) then my backlight controls work fine:
> acpi_video0/brightness works at boot, and intel_backlight/brightness no
> longer induces the pulsate/flicker effect (or any effect at all).
>
> So... before I go shooting my mouth off on intel-gfx, I wonder if either
> of you can shed any (back)light on the situation (sorry ;-).
>
> I don't really have any reason to think that this issue is specific to
> CougarPoint PCH, but since the XPS13 is PCH_CPT this patch fixes it for
> me, for instance:
>
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -347,7 +347,9 @@ void intel_panel_enable_backlight(struct drm_device *dev,
>                 POSTING_READ(reg);
>                 I915_WRITE(reg, tmp | BLM_PWM_ENABLE);
>
> -               if (HAS_PCH_SPLIT(dev)) {
> +               /* Note setting BLM_PCH_PWM_ENABLE breaks backlight on
> +                * Dell XPS13 (sandybridge_m, CougarPoint PCH). */
> +               if (HAS_PCH_SPLIT(dev) && !HAS_PCH_CPT(dev)) {
>                         tmp = I915_READ(BLC_PWM_PCH_CTL1);
>                         tmp |= BLM_PCH_PWM_ENABLE;
>                         tmp &= ~BLM_PCH_OVERRIDE_ENABLE;
>
>
> I don't know if switching this based on HAS_PCH_CPT() is really relevant
> -- it just works for the Dell XPS.  (And please forgive my obvious
> cluelessness about what PCH_CPT even means!)
>
> Any advice?
>
>  -Kamal



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch