[Intel-gfx] [PATCH 2/3] drm/i915/gt: Stall around xcs invalidations on tgl

[Chris Wilson]

Quoting Mika Kuoppala (2020-07-24 12:59:04)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Whether this is an arbitrary stall or a vital ingredient, neverthess the
> > impact is noticeable. If we do not have the stall around the xcs
> > invalidation before a request, writes within that request sometimes go
> > astray.
> >
> > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2169
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_lrc.c | 18 ++++++++++++------
> >  1 file changed, 12 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > index 353b1717fe84..104bef04498d 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > @@ -4761,10 +4761,12 @@ static int gen12_emit_flush_render(struct i915_request *request,
> >  
> >  static int gen12_emit_flush(struct i915_request *request, u32 mode)
> >  {
> > +#define WA_CNT 32 /* Magic delay */
> 
> Utterly nasty. We need to hunt for an explanation.

Utterly. The cost is about 20us per request. I'm going to see if I can
find an alternative.
-Chris