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

[Ramalingam C]

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 at intel.com>
cc: Matthew Auld <matthew.auld at intel.com>
cc: Thomas Hellstrom <thomas.hellstrom at linux.intel.com>
---
 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.
+ *
+ * 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
+ * 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. */
-- 
2.20.1