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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, May 25, 2018 at 04:55:36PM +0100, Chris Wilson wrote:
> 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@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@chris-wilson.co.uk>

Thanks. Pushed.

-- 
Ville Syrjälä
Intel
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