Merge "Clarify FRONT_BUFFER requirements"

graphics/common/aidl/android/hardware/graphics/common/BufferUsage.aidl

@@ -107,7 +107,24 @@ enum BufferUsage {
     /* Bits 28-31 are reserved for vendor usage */
 
     /**
-    * Buffer is used for front-buffer rendering
+    * Buffer is used for front-buffer rendering.
+    *
+    * To satisfy an allocation with this usage, the resulting buffer
+    * must operate as equivalent to shared memory for all targets.
+    *
+    * For CPU_USAGE_* other than NEVER, this means the buffer must
+    * "lock in place". The buffers must be directly accessible via mapping.
+    *
+    * For GPU_RENDER_TARGET the buffer must behave equivalent to a
+    * single-buffered EGL surface. For example glFlush must perform
+    * a flush, same as if the default framebuffer was single-buffered.
+    *
+    * For COMPOSER_* the HWC must not perform any caching for this buffer
+    * when submitted for composition. HWCs do not need to do any form
+    * of auto-refresh, and they are allowed to cache composition results between
+    * presents from SF (such as for panel self-refresh), but for any given
+    * present the buffer must be composited from even if it otherwise appears
+    * to be the same as a previous composition.
     */
     FRONT_BUFFER                       = 1L << 32,