Re: 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 08 Stefan Richter wrote:
> On May 05 Zanoni, Paulo R wrote:
> > If you don't want to keep carrying a manual revert, you can just boot
> > with i915.enable_fbc=0 for now (or write a /etc/modprobe.d file). Also,
> > it would be good to know in case you still somehow see the machine
> > hangs even with FBC disabled.  
> 
> As expected, i915.enable_fbc=0 works fine.
> No freeze within 2.5 days uptime; tested on v4.6-rc6.

Furthermore, I checked out drm-intel.git (v4.6-rc6-962-g91567024d358
"drm-intel-nightly: 2016y-05m-06d-14h-29m-58s UTC integration manifest")
and applied "git diff v4.6-rc6..." on top of v4.6-rc6.

I booted the result once with default i915.enable_fbc, i.e. FBC enabled,
performed the test which Daniel asked for (I will post the results in
another message), then started X11.
  - The good news:  I was able to switch back and forth between the sddm
    greeter screen on tty7, the text consoles at tty1...6, and the logger
    at tty12 --- without getting any FIFO underrun messages and without
    getting stuck with a blank screen.
  - The bad news:  Less than a minute after login into sddm, just after
    having started openbox + lxpanel + konsole, the kernel froze again
    without netconsole output.

I am now on 4.6.0-rc6+intel-drm-nightly with i915.enable_fbc=0.  This is
running fine so far.  (uptime is just 30 minutes now though, so that
doesn't say a lot.)  Again, switching between ttys works without FIFO
underruns, unlike plain v4.6-rc6.  Not sure if it is coincidence or if
this is because somebody fixed something.

Like v4.6-rc6 and older,4.6.0-rc6+intel-drm-nightly still exhibits the
following behaviour:  If I switch the displayport connected monitor off
and on again, the following messages are logged when the monitor comes on:
    [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
Other than these messages, there is nothing extraordinary going on.
-- 
Stefan Richter
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