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]
bool i915_gem_cpu_write_needs_clflush(struct drm_i915_gem_object *obj)
@@ -267,7 +267,7 @@ int i915_gem_object_set_cache_level(struct drm_i915_gem_object *obj,
{
int ret;
- if (obj->cache_level == cache_level)
+ if (i915_gem_object_has_cache_level(obj, cache_level))
return 0;
When userspace calls i915_gem_set_caching_ioctl after having set the PAT index explicitly this will make it silently succeed regardless of the cache level passed in, no? Because of:
+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;
I think we need to let it know it is doing it wrong with an error.
Regards,
Tvrtko
