[Intel-gfx] [PATCH 1/3] drm/i915: Suppress spurious CPU FIFO underruns on ILK-IVB

[Ville Syrjälä]

On Sat, Nov 21, 2015 at 10:51:50AM +0000, Chris Wilson wrote:
> On Fri, Nov 20, 2015 at 10:09:18PM +0200, ville.syrjala at linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > We still get spurious pipe underruns on ILK/SNB/IVB under two
> > circumstances when dealing with PCH ports:
> > * When the pipe has been disabled, but FDI RX/TX is still enabled
> > * During FDI link training
> > 
> > Both cases seem to happen at least when we do VGA+HDMI cloning
> > from the same pipe. I don't think I've seen them when not cloning,
> > but can't be 100% sure.
> > 
> > Disable underrun reporting around those places to eliminate the
> > dmesg errors.
> > 
> > Testcase: igt/kms_setmode/basic-clone-single-crtc
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> I wondered if logging the suppressed errors would be of any use?

Hmm. Maybe just to confirm that they still happen, and thus suppressing
is still neded.

> Does
> the check_cpu_fifo work if the reporting is disabled? Could we do a
> manual check and DRM_DEBUG_KMS() if the enable did generate a failure.
> If the check_cpu_fifo does work, won't we still get the error from the
> added check in atomic_commit()?

The check only looks at pipes that have underrun reporting enabled. I
suppose it might be possible to have it check all the pipes. At the
point where we call it no explicit suppression should be happening, so
the only reason why underrun reporting would be disabled on any pipe is
due to detecting a previous underrun via the interrupt.

-- 
Ville Syrjälä
Intel OTC