drm/i915/uapi: convert drm_i915_gem_caching to kernel doc

Convert all the drm_i915_gem_caching bits to proper kernel doc.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210705135310.1502437-2-matthew.auld@intel.com

include/uapi/drm/i915_drm.h

@@ -1363,43 +1363,50 @@ struct drm_i915_gem_busy {
 };
 
 /**
- * I915_CACHING_NONE
+ * struct drm_i915_gem_caching - Set or get the caching for given object
+ * handle.
  *
- * GPU access is not coherent with cpu caches. Default for machines without an
- * LLC.
+ * Allow userspace to control the GTT caching bits for a given object when the
+ * object is later mapped through the ppGTT(or GGTT on older platforms lacking
+ * ppGTT support, or if the object is used for scanout). Note that this might
+ * require unbinding the object from the GTT first, if its current caching value
+ * doesn't match.
  */
-#define I915_CACHING_NONE		0
-/**
- * I915_CACHING_CACHED
- *
- * GPU access is coherent with cpu caches and furthermore the data is cached in
- * last-level caches shared between cpu cores and the gpu GT. Default on
- * machines with HAS_LLC.
- */
-#define I915_CACHING_CACHED		1
-/**
- * I915_CACHING_DISPLAY
- *
- * Special GPU caching mode which is coherent with the scanout engines.
- * Transparently falls back to I915_CACHING_NONE on platforms where no special
- * cache mode (like write-through or gfdt flushing) is available. The kernel
- * automatically sets this mode when using a buffer as a scanout target.
- * Userspace can manually set this mode to avoid a costly stall and clflush in
- * the hotpath of drawing the first frame.
- */
-#define I915_CACHING_DISPLAY		2
-
 struct drm_i915_gem_caching {
 	/**
-	 * Handle of the buffer to set/get the caching level of. */
+	 * @handle: Handle of the buffer to set/get the caching level.
+	 */
 	__u32 handle;
 
 	/**
-	 * Cacheing level to apply or return value
+	 * @caching: The GTT caching level to apply or possible return value.
 	 *
-	 * bits0-15 are for generic caching control (i.e. the above defined
-	 * values). bits16-31 are reserved for platform-specific variations
-	 * (e.g. l3$ caching on gen7). */
+	 * The supported @caching values:
+	 *
+	 * I915_CACHING_NONE:
+	 *
+	 * GPU access is not coherent with CPU caches.  Default for machines
+	 * without an LLC. This means manual flushing might be needed, if we
+	 * want GPU access to be coherent.
+	 *
+	 * I915_CACHING_CACHED:
+	 *
+	 * GPU access is coherent with CPU caches and furthermore the data is
+	 * cached in last-level caches shared between CPU cores and the GPU GT.
+	 *
+	 * I915_CACHING_DISPLAY:
+	 *
+	 * Special GPU caching mode which is coherent with the scanout engines.
+	 * Transparently falls back to I915_CACHING_NONE on platforms where no
+	 * special cache mode (like write-through or gfdt flushing) is
+	 * available. The kernel automatically sets this mode when using a
+	 * buffer as a scanout target.  Userspace can manually set this mode to
+	 * avoid a costly stall and clflush in the hotpath of drawing the first
+	 * frame.
+	 */
+#define I915_CACHING_NONE		0
+#define I915_CACHING_CACHED		1
+#define I915_CACHING_DISPLAY		2
 	__u32 caching;
 };