uapi/drm/i915: Document memory residency and Flat-CCS capability of obj

Capture the impact of memory region preference list of the objects, on
their memory residency and Flat-CCS capability.

v2:
  Fix the Flat-CCS capability of an obj with {lmem, smem} preference
  list [Thomas]
v3:
  Reworded the doc [Matt]
v4:
  Fixed Typos and spelling mistakes [Tvrtko, Joonas]
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
cc: Matthew Auld <matthew.auld@intel.com>
cc: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
cc: Daniel Vetter <daniel.vetter@ffwll.ch>
cc: Jon Bloomfield <jon.bloomfield@intel.com>
cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
cc: Kenneth Graunke <kenneth@whitecape.org>
cc: mesa-dev@lists.freedesktop.org
cc: Jordan Justen <jordan.l.justen@intel.com>
cc: Tony Ye <tony.ye@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220502141508.2327-1-ramalingam.c@intel.com

include/uapi/drm/i915_drm.h

@@ -3443,6 +3443,22 @@ 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.
+ *
+ * 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) have other
+ * memory class in @regions and migrated (by i915, due to memory
+ * constraints) to the non I915_MEMORY_CLASS_DEVICE region, then i915 needs to
+ * decompress the content. But i915 doesn't have the required information to
+ * decompress the userspace compressed objects.
+ *
+ * So i915 supports Flat-CCS, 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. */