[Bug 108462] two external screens permanently go blank on HP EliteBook Folio G1

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108462

--- Comment #18 from Imre Deak <imre.deak at intel.com> ---
(In reply to Johannes Berg from comment #17)
> FWIW, I don't actually know of a way of *triggering* this. It seems to just
> happen all by itself, sometimes with lots of screen activity, sometimes with
> none at all.
> 
> I noticed that this message *sometimes* seems to coincide with the issue:
> 
> [drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe C FIFO
> underrun
> 
> but I suppose that's just be a symptom of the issue, rather than the cause,
> since it doesn't *always* happen.

It could be a marker that we program wrong watermark levels (which I'd like to
test with my script).

> Your script doesn't actually work:
> 
> jberg1-mobl2:/sys/kernel/debug/dri/0# for plane in pri cur spr; do
> > cat i915_${plane}_wm_latency
> > echo $wm 1000 1000 1000 1000 > i915_${plane}_wm_latency
> > done

Ah sorry, you have SKL and so 8 watermark levels not 5. The following should
work better, could you try it? Forgot to say, but after running the script you
also have to force a display modeset, for example by making both displays blank
then unblank. Then just leave it running and see if any FIFO underrun message
shows up or if the displays flicker. Thanks.

# for plane in pri cur spr; do
> echo 20 500 500 500 500 500 500 500 > i915_${plane}_wm_latency
> done

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