drm/i915: Describe the bottom of stack in processing a batchbuffer

Now that "DOC: User command execution" of i915_gem_execbuffer.c is included
in the i915.rst, it is benecifial (for new developers) to read what happens
at the bottom of the driver stack (in terms of bytes written to be read
by the GPU) when processing a user-space batchbuffer.

v5:
  Typo correction of lacking double tick.
Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
[Joonas: correcting the patch title]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523001957-6427-4-git-send-email-kevin.rogovin@intel.com

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -81,6 +81,35 @@ enum {
  * but this remains just a hint as the kernel may choose a new location for
  * any object in the future.
  *
+ * At the level of talking to the hardware, submitting a batchbuffer for the
+ * GPU to execute is to add content to a buffer from which the HW
+ * command streamer is reading.
+ *
+ * 1. Add a command to load the HW context. For Logical Ring Contexts, i.e.
+ *    Execlists, this command is not placed on the same buffer as the
+ *    remaining items.
+ *
+ * 2. Add a command to invalidate caches to the buffer.
+ *
+ * 3. Add a batchbuffer start command to the buffer; the start command is
+ *    essentially a token together with the GPU address of the batchbuffer
+ *    to be executed.
+ *
+ * 4. Add a pipeline flush to the buffer.
+ *
+ * 5. Add a memory write command to the buffer to record when the GPU
+ *    is done executing the batchbuffer. The memory write writes the
+ *    global sequence number of the request, ``i915_request::global_seqno``;
+ *    the i915 driver uses the current value in the register to determine
+ *    if the GPU has completed the batchbuffer.
+ *
+ * 6. Add a user interrupt command to the buffer. This command instructs
+ *    the GPU to issue an interrupt when the command, pipeline flush and
+ *    memory write are completed.
+ *
+ * 7. Inform the hardware of the additional commands added to the buffer
+ *    (by updating the tail pointer).
+ *
  * Processing an execbuf ioctl is conceptually split up into a few phases.
  *
  * 1. Validation - Ensure all the pointers, handles and flags are valid.