On 14/07/2023 06:43, Yang, Fei wrote:
From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
According to the comment in i915_gem_object_can_bypass_llc the
purpose of the function is to return false if the platform/object
has a caching mode where GPU can bypass the LLC.
So far the only platforms which allegedly can do this are Jasperlake
and Elkhartlake, and that via MOCS (not PAT).
Instead of blindly assuming that objects where userspace has set the
PAT index can (bypass the LLC), question is is there a such PAT index
on a platform. Probably starting with Meteorlake since that one is the
only one where set PAT extension can be currently used. Or if there is
a MOCS entry which can achieve the same thing on Meteorlake.
If there is such PAT, now that i915 can be made to understand them
better, we can make the check more fine grained. Or if there is a MOCS
entry then we probably should apply the blanket IS_METEORLAKE condition.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Fixes: 9275277d5324 ("drm/i915: use pat_index instead of cache_level")
Cc: Chris Wilson <chris.p.wilson@xxxxxxxxxxxxxxx>
Cc: Fei Yang <fei.yang@xxxxxxxxx>
Cc: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
---
drivers/gpu/drm/i915/gem/i915_gem_object.c | 6 ------
1 file changed, 6 deletions(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c
index 33a1e97d18b3..1e34171c4162 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
@@ -229,12 +229,6 @@ bool i915_gem_object_can_bypass_llc(struct drm_i915_gem_object *obj)
if (!(obj->flags & I915_BO_ALLOC_USER))
return false;
- /*
- * Always flush cache for UMD objects at creation time.
- */
- if (obj->pat_set_by_user)
I'm afraid this is going to break MESA. Can we run MESA tests with this patch?
I can't, but question is why it would break Mesa which would need a nice
comment here?
For instance should the check be IS_METEORLAKE?
Or should it be "is wb" && "not has 1-way coherent"?
Or both?
Or, given how Meteorlake does not have LLC, how can anything bypass it
there? Or is it about snooping on Meteorlake and how?
Regards,
Tvrtko
/*
* EHL and JSL add the 'Bypass LLC' MOCS entry, which should make it
* possible for userspace to bypass the GTT caching bits set by the
--
2.39.2
