On 23/04/2023 07:12, Yang, Fei wrote:
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
We are ending the support for set_caching_ioctl.
Not on all platforms.
after having set the PAT index explicitly this will make it silently succeed
regardless of the cache level passed in, no? Because of:
Yes, that's the point. For objects created by userspace with PAT index set,
KMD is not supposed to touch the setting.
Why would that be a reason to lie to it? What would would be the problem
with telling it of the mistake?
+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.
This is not an error, by design userspace should know exactly what it's doing.
IMO when return values can be misleading that means the API is not great.
I don't see a good reason to lie to both in kernel callers and to
userspace (set_caching).
Regards,
Tvrtko
