Re: [PATCH 2/4] drm/i915: Use maximum write flush for pwrite_gtt

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Ville Syrjälä (2019-07-18 19:28:43)
> On Thu, Jul 18, 2019 at 03:54:05PM +0100, Chris Wilson wrote:
> > As recently disovered by forcing big-core (!llc) machines to use the GTT
> > paths, we need our full GTT write flush before manipulating the GTT PTE
> > or else the writes may be directed to the wrong page.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > Cc: Matthew Auld <matthew.william.auld@gmail.com>
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Cc: stable@vger.kernel.org
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index fed0bc421a55..c6ba350e6e4f 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -610,7 +610,8 @@ i915_gem_gtt_pwrite_fast(struct drm_i915_gem_object *obj,
> >               unsigned int page_length = PAGE_SIZE - page_offset;
> >               page_length = remain < page_length ? remain : page_length;
> >               if (node.allocated) {
> > -                     wmb(); /* flush the write before we modify the GGTT */
> > +                     /* flush the write before we modify the GGTT */
> > +                     intel_gt_flush_ggtt_writes(ggtt->vm.gt);
> 
> Matches the story told by intel_gt_flush_ggtt_writes().
> 
> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Ta, pushed to dinq. Hopefully, this may explain some mystery fails!
(Not that any sane userspace does for(;;) { gem_write(); gem_read(); })
-Chris