Skylake graphics regression: projector failure with 4.8-rc3

[James Bottomley]

On Wed, 2016-08-31 at 11:23 -0700, James Bottomley wrote:
> On Fri, 2016-08-26 at 09:10 -0400, James Bottomley wrote:
> > We seem to have an xrandr regression with skylake now.  What's
> > happening is that I can get output on to a projector, but the 
> > system is losing video when I change the xrandr sessions (like 
> > going from a --above b to a --same-as b).  The main screen goes 
> > blank, which is basically a reboot situation.  Unfortunately, I 
> > can't seem to get the logs out of systemd to see if there was a 
> > dump to dmesg (the system was definitely responding).
> > 
> > I fell back to 4.6.2 which worked perfectly, so this is definitely 
> > some sort of regression.  I'll be able to debug more fully when I 
> > get back home from the Linux Security Summit.
> 
> I'm home now.  Unfortunately, my monitor isn't as problematic as the
> projector, but by flipping between various modes and separating and
> overlaying the panels with --above and --same-as (xrandr), I can
> eventually get it to the point where the main LCD panel goes black 
> and can only be restarted by specifying a different mode.
> 
> This seems to be associated with these lines in the X
> 
> [ 14714.389] (EE) intel(0): failed to set mode: Invalid argument [22]
> 
> But the curious thing is that even if this fails with the error 
> message once, it may succeed a second time, so it looks to be a 
> transient error translation problem from the kernel driver.
> 
> I've attached the full log below.
> 
> This is only with a VGA output.  I currently don't have a HDMI 
> dongle, but I'm in the process of acquiring one.

After more playing around, I'm getting thousands of these in the kernel
log (possibly millions: the log wraps very fast):

[23504.873606] [drm:intel_dp_start_link_train [i915]] *ERROR* failed to train DP, aborting

And then finally it gives up with 

[25023.770951] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe B FIFO underrun
[25561.926075] [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe A FIFO underrun

And the crtc for the VGA output becomes non-responsive to any
configuration command.  This requires a reboot and sometimes a UEFI
variable reset before it comes back.

James