[Intel-gfx] [PATCH 2/2] drm/i915: Simplify ilk-ivb underrun suppression

[Chris Wilson]

Quoting Ville Syrjälä (2018-05-25 16:43:42)
> On Fri, May 25, 2018 at 04:20:07PM +0100, Chris Wilson wrote:
> > Quoting Ville Syrjala (2018-05-24 20:04:06)
> > > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > 
> > > Let's suppress the underruns around every modeset sequence instead
> > > of trying to avoid it. Planes are disabled at this point anyway so
> > > we don't really gain anything from keeping the underrun reporting
> > > enabled. Also for PCH ports we already suppress all underruns here
> > > anyway so trying avoid it for the CPU eDP doesn't seem all that
> > > important.
> > > 
> > > Maybe this gets rid of some lingering spurious underruns?
> > 
> > I'll bite. Isn't the reason for enabling underrung report for the
> > modeset itself to detect errors in our sequence?
> 
> In theory CPU FIFO underruns shouldn't happen until we have some planes
> enabled. Otherwise we have no data going through the FIFOs and thus
> reporting an underrun isn't particularly sane. That doesn't stop gen2
> from doing it though, but gen3+ seem to follow the more reasonable
> interpretation of what a FIFO underrun means.

Makes sense.
 
> I suppose PCH FIFO underruns are a bit different as there is data flowing
> as soon as the CPU pipe starts running, whether or not any planes have
> been enabled. So those could certainly indicate some kind of programming
> sequence error. Or it could just be totally expected that we start the
> PCH side of the pipe first and there's a small bit of time when the CPU
> pipe isn't yet pushing out pixels, and that's when the PCH side reports
> the underrun.
> 
> > 
> > How certain are we that these are hw limitations vs sw bugs?
> 
> To the best of my knowledge we are reasonably close to the sequence
> listed in bspec. And while it's at least theoretically possible that
> there's some change we could make to eliminate the underruns I don't
> suppose anyone has the time or energy to try out all possible
> variations.
> 
> And as long as the underrun (even if it's real) has vanished by the
> time we enable the planes I think we are reasonably safe wrt. getting
> the correct looking output to the user's display.

Also makes sense. And if glitches during modesetting itself, we hope
nobody complains ;)

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris