On 20/04/2023 00:00, fei.yang@xxxxxxxxx wrote:
From: Fei Yang <fei.yang@xxxxxxxxx>
Currently the KMD is using enum i915_cache_level to set caching policy for
buffer objects. This is flaky because the PAT index which really controls
the caching behavior in PTE has far more levels than what's defined in the
enum. In addition, the PAT index is platform dependent, having to translate
between i915_cache_level and PAT index is not reliable, and makes the code
more complicated.
From UMD's perspective there is also a necessity to set caching policy for
performance fine tuning. It's much easier for the UMD to directly use PAT
index because the behavior of each PAT index is clearly defined in Bspec.
Having the abstracted i915_cache_level sitting in between would only cause
more ambiguity.
For these reasons this patch replaces i915_cache_level with PAT index. Also
note, the cache_level is not completely removed yet, because the KMD still
has the need of creating buffer objects with simple cache settings such as
cached, uncached, or writethrough. For such simple cases, using cache_level
would help simplify the code.
Cc: Chris Wilson <chris.p.wilson@xxxxxxxxxxxxxxx>
Cc: Matt Roper <matthew.d.roper@xxxxxxxxx>
Signed-off-by: Fei Yang <fei.yang@xxxxxxxxx>
Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
[snip]
@@ -306,20 +304,13 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
goto out;
}
- switch (obj->cache_level) {
- case I915_CACHE_LLC:
- case I915_CACHE_L3_LLC:
+ if (i915_gem_object_has_cache_level(obj, I915_CACHE_LLC) ||
+ i915_gem_object_has_cache_level(obj, I915_CACHE_L3_LLC))
args->caching = I915_CACHING_CACHED;
- break;
-
- case I915_CACHE_WT:
+ else if (i915_gem_object_has_cache_level(obj, I915_CACHE_WT))
args->caching = I915_CACHING_DISPLAY;
- break;
-
- default:
+ else
args->caching = I915_CACHING_NONE;
- break;
- }
out:
rcu_read_unlock();
return err;
[snip]
+bool i915_gem_object_has_cache_level(const struct drm_i915_gem_object *obj,
+ enum i915_cache_level lvl)
+{
+ /*
+ * cache_level == I915_CACHE_INVAL indicates the UMD's have set the
+ * caching policy through pat_index, in which case the KMD should
+ * leave the coherency to be managed by user space, simply return
+ * true here.
+ */
+ if (obj->cache_level == I915_CACHE_INVAL)
+ return true;
+
+ /*
+ * Otherwise the pat_index should have been converted from cache_level
+ * so that the following comparison is valid.
+ */
+ return obj->pat_index == i915_gem_get_pat_index(obj_to_i915(obj), lvl);
+}
+
Isn't i915_gem_get_caching_ioctl always going to report
I915_CACHING_CACHED if any PAT index has been set?
Not sure if that is okay or not, or if it only needs mentioning in the
commit, I am still reading through it all.
Regards,
Tvrtko
