[Intel-gfx] [PATCH v12 3/3] drm/i915: Add option to support dynamic backlight via DPCD

[David Weinehall]

On Tue, Jun 27, 2017 at 10:29:33PM +0000, Pandiyan, Dhinakaran wrote:
> 
> 
> 
> On Tue, 2017-06-27 at 16:23 +0300, David Weinehall wrote:
> > On Mon, Jun 26, 2017 at 05:18:19PM +0300, David Weinehall wrote:
> > > On Thu, Jun 22, 2017 at 12:03:39PM -0700, Puthikorn Voravootivat wrote:
> > > > This patch adds option to enable dynamic backlight for eDP
> > > > panel that supports this feature via DPCD register and
> > > > set minimum / maximum brightness to 0% and 100% of the
> > > > normal brightness.
> > > 
> > > This patch causes a regression on my ThinkPad X1 Carbon Gen 4;
> > > with enable_dbc = true the backlight stays off; setting i915.enable_dbc=0
> > > on boot makes things work properly again.
> > 
> > Some more testing indicates that while i915.enable_dbc=0 solves the
> > initial issue of backlight not coming on, it's not enough--after
> > suspend/resume the backlight stays off.
> > 
> > Passing i915.enable_dpcd_backlight=0 seems to work both for the initial
> > case of backlight not coming on at boot, and the issue with backlight
> > not coming on after suspend/resume.
> > 
> > So:
> > 
> > DBC:1, DPCD: -1: blank screen on boot
> > DBC:0, DPCD: -1: screen OK on boot, blank after suspend/resume
> > DBC:1, DPCD:  0: screen OK on boot, screen OK after suspend/resume
> > DBC:0, DPCD:  0: screen OK on boot, screen OK after suspend/resume
> > 
> > So it seems that at least for the ThinkPad X1 Carbon Gen 4 (possibly
> > limited to the 2560x1440 model) DPCD backlight isn't supported.
> > 
> 
> Hi David, 
> 
> Thanks for the bug report. It's come a bit sooner than I expected
> though.
> 
> We shouldn't be enabling DPCD backlight control if the panel does not
> support it. I'd like to look at the dmesg with drm.debug=0xE. We debug
> log the panel capabilities early during boot and that will be useful.
> Can you also provide the output of i915_dpcd in debugfs for the eDP
> connector?
> 
> I guess it makes sense to file an FDO and attach the logs there. 

https://bugs.freedesktop.org/show_bug.cgi?id=101619


Kind regards, David