Re: [PATCH] drm/i915: Flush WC GGTT only on required platforms

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Oct 13, 2023 at 02:28:21PM +0200, Nirmoy Das wrote:
> Hi Ville,
> 
> On 10/13/2023 12:50 PM, Ville Syrjälä wrote:
> > On Fri, Oct 13, 2023 at 12:31:40PM +0200, Nirmoy Das wrote:
> > > gen8_ggtt_invalidate() is only needed for limitted set of platforms
> > > where GGTT is mapped as WC
> > I know there is supposed to be some kind hw snooping of the ggtt
> > pte writes to invalidate the tlb, but are we sure GFX_FLSH_CNTL
> > has no other side effects we depend on?
> 
> I spent some time searching through the gfxspec. This GFX_FLSH_CNTL register
> only seems to be for
> 
> invalidating TLB for GUnit  and (from git log ) we started to do that to
> enable WC based GGTT updates.

Might be good to cite the relevant git commits in the commit message to
make this clear.
-Sima

> 
> 
> So if I am not missing anything obvious then this should be safe.
> 
> 
> Regards,
> 
> Nirmoy
> 
> > 
> > > otherwise this can cause unwanted
> > > side-effects on XE_HP platforms where GFX_FLSH_CNTL_GEN6 is not
> > > valid.
> > > 
> > > Fixes: d2eae8e98d59 ("drm/i915/dg2: Drop force_probe requirement")
> > > Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> > > Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > Cc: Jani Nikula <jani.nikula@linux.intel.com>
> > > Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
> > > Cc: John Harrison <john.c.harrison@intel.com>
> > > Cc: Andi Shyti <andi.shyti@linux.intel.com>
> > > Cc: <stable@vger.kernel.org> # v6.2+
> > > Suggested-by: Matt Roper <matthew.d.roper@intel.com>
> > > Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
> > > ---
> > >   drivers/gpu/drm/i915/gt/intel_ggtt.c | 6 +++++-
> > >   1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > > index 4d7d88b92632..c2858d434bce 100644
> > > --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > > +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > > @@ -197,13 +197,17 @@ void gen6_ggtt_invalidate(struct i915_ggtt *ggtt)
> > >   static void gen8_ggtt_invalidate(struct i915_ggtt *ggtt)
> > >   {
> > > +	struct drm_i915_private *i915 = ggtt->vm.i915;
> > >   	struct intel_uncore *uncore = ggtt->vm.gt->uncore;
> > >   	/*
> > >   	 * Note that as an uncached mmio write, this will flush the
> > >   	 * WCB of the writes into the GGTT before it triggers the invalidate.
> > > +	 *
> > > +	 * Only perform this when GGTT is mapped as WC, see ggtt_probe_common().
> > >   	 */
> > > -	intel_uncore_write_fw(uncore, GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
> > > +	if (!IS_GEN9_LP(i915) && GRAPHICS_VER(i915) < 11)
> > > +		intel_uncore_write_fw(uncore, GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
> > >   }
> > >   static void guc_ggtt_invalidate(struct i915_ggtt *ggtt)
> > > -- 
> > > 2.41.0

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch