Regression of v4.6-rc vs. v4.5 bisected: a98ee79317b4 "drm/i915/fbc: enable FBC by default on HSW and BDW"

[Stefan Richter]

On May 05 Zanoni, Paulo R wrote:
> Em Qui, 2016-05-05 às 19:45 +0200, Stefan Richter escreveu:
> >     Oh, and in case you - the person reading this commit message - found
> >     this commit through git bisect, please do the following:
> >      - Check your dmesg and see if there are error messages mentioning
> >        underruns around the time your problem started happening.
> > 
> > Well, I always had the followings lines in dmesg:
> > [drm:intel_set_cpu_fifo_underrun_reporting] *ERROR* uncleared fifo underrun on pipe A
> > [drm:intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun  
> 
> Oh, well... I had a patch that would just disable FBC in case we saw a
> FIFO underrun, but it was rejected. Maybe this is the time to think
> about it again? Otherwise, I can't think of much besides disabling FBC
> on HSW until all the underruns and watermarks regressions are fixed
> forever.

Just to be clear though, I know that these messages are emitted when the
monitor is switched on, and when sddm is being shut down --- but I do not
know whether there is any sort of underrun when I get the FBC related
freeze (since I just don't get any kernel messages at that point).

Is there a chance that a serial console would fare better than
netconsole?  This board and another PC in its vicinity have got onboard
serial ports but I don't have cables at the moment.

> >      - Download intel-gpu-tools, compile it, and run:
> >        $ sudo ./tests/kms_frontbuffer_tracking --run-subtest '*fbc-*' 2>&1 | tee fbc.txt  
> >        Then send us the fbc.txt file, especially if you get a failure.
> >        This will really maximize your chances of getting the bug fixed
> >        quickly.
> > 
> > Do you need this while FBC is enabled, or can I run it while FBC is
> > disabled?  
> 
> FBC enabled. Considering your description, my hope is that maybe some
> specific subtest will be able to hang your machine, so testing this
> again will require only running the specific subtest instead of waiting
> 18 hours.

The kms_frontbuffer_tracking runs from which I posted output two hours
ago did not trigger a lockup.

(I ran them while X11 was shut down because otherwise
kms_frontbuffer_tracking would skip all tests with "Can't become DRM
master, please check if no other DRM client is running.")

> > PS:
> > I am mentioning the following just in case that it has any relationship
> > with the FBC related kernel freezes.  Maybe it doesn't...  There is
> > another recent regression on this PC, but I have not yet figured out
> > whether it was introduced by any particular kernel version.  The
> > regression is:  When switching from X11 to text console by [Ctrl][Alt][Fx]
> > or by shutting down sddm, I often only get a blank screen.  I suspect
> > that this regression was introduced when I replaced kdm by sddm, but
> > I am not sure about that.  
> 
> Maybe there is some relationship, since this operation involves a mode
> change. You can also try checking dmesg to see if there are underruns
> right when you do the change.

Yes, this is accompanied by
[drm:intel_set_cpu_fifo_underrun_reporting] *ERROR* uncleared fifo underrun on pipe A
[drm:intel_cpu_fifo_underrun_irq_handler] *ERROR* CPU pipe A FIFO underrun  
-- 
Stefan Richter
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