Re: [PATCH v2 4/4] uapi/drm/i915: Document memory residency and Flat-CCS capability of obj

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On 25/04/2022 17:24, Ramalingam C wrote:
Capture the impact of memory region preference list of an object, on
their memory residency and Flat-CCS capability of the objects.

v2:
   Fix the Flat-CCS capability of an obj with {lmem, smem} preference
   list [Thomas]

Signed-off-by: Ramalingam C <ramalingam.c@xxxxxxxxx>
cc: Matthew Auld <matthew.auld@xxxxxxxxx>
cc: Thomas Hellstrom <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
  include/uapi/drm/i915_drm.h | 18 ++++++++++++++++++
  1 file changed, 18 insertions(+)

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 35ca528803fd..ad191ed6547c 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -3393,6 +3393,24 @@ struct drm_i915_gem_create_ext {
   * At which point we get the object handle in &drm_i915_gem_create_ext.handle,
   * along with the final object size in &drm_i915_gem_create_ext.size, which
   * should account for any rounding up, if required.
+ *
+ * Objects with multiple memory regions in the preference list will be backed
+ * by one of the memory regions mentioned in the preference list. Though I915
+ * tries to honour the order of the memory regions in the preference list,
+ * based on the memory pressure of the regions, objects' backing region
+ * will be selected.
+ *
+ * Userspace has no means of knowing the backing region for such objects.

"Note that userspace has no means of knowing the current backing region for objects where @num_regions is larger than one. The kernel will only ensure that the priority order of the @regions array is honoured, either when initially placing the object, or when moving memory around due to memory pressure."

+ *
+ * On Flat-CCS capable HW, compression is supported for the objects residing
+ * in I915_MEMORY_CLASS_DEVICE. When such objects (compressed) has other
+ * memory class in preference list and migrated (by I915, due to memory
+ * constrain) to the non I915_MEMORY_CLASS_DEVICE region, then I915 needs to
+ * decompress the content. But I915 dont have the required information to

"doesn't", also prefer @regions etc instead of "preference list"

Anyway,
Reviewed-by: Matthew Auld <matthew.auld@xxxxxxxxx>

+ * decompress the userspace compressed objects.
+ *
+ * So I915 supports Flat-CCS, only on the objects which can reside only on
+ * I915_MEMORY_CLASS_DEVICE regions. >    */
  struct drm_i915_gem_create_ext_memory_regions {
  	/** @base: Extension link. See struct i915_user_extension. */



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