drm/prime: Clarify DMA-BUF/GEM Object lifetime

From the description of the "DMA-BUF/GEM Object references
and lifetime overview" it is not clear when exactly
dma_buf gets destroyed and memory freed: only driver
.release function mentioned which makes confusion on the
real buffer's lifetime.

Add more description so all the paths are covered.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Dave Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
[danvet: Minor spelling fixes, and some clarification of the 2nd
paragraph.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1485500665-27690-1-git-send-email-andr2000@gmail.com

drivers/gpu/drm/drm_prime.c

@@ -40,8 +40,11 @@
  * On the export the dma_buf holds a reference to the exporting GEM
  * object. It takes this reference in handle_to_fd_ioctl, when it
  * first calls .prime_export and stores the exporting GEM object in
- * the dma_buf priv. This reference is released when the dma_buf
- * object goes away in the driver .release function.
+ * the dma_buf priv. This reference needs to be released when the
+ * final reference to the &dma_buf itself is dropped and its
+ * &dma_buf_ops.release function is called. For GEM-based drivers,
+ * the dma_buf should be exported using drm_gem_dmabuf_export() and
+ * then released by drm_gem_dmabuf_release().
  *
  * On the import the importing GEM object holds a reference to the
  * dma_buf (which in turn holds a ref to the exporting GEM object).
@@ -51,6 +54,16 @@
  * when the imported object is destroyed, we remove the attachment
  * and drop the reference to the dma_buf.
  *
+ * When all the references to the &dma_buf are dropped, i.e. when
+ * userspace has closed both handles to the imported GEM object (through the
+ * FD_TO_HANDLE IOCTL) and closed the file descriptor of the exported
+ * (through the HANDLE_TO_FD IOCTL) dma_buf, and all kernel-internal references
+ * are also gone, then the dma_buf gets destroyed.  This can also happen as a
+ * part of the clean up procedure in the drm_release() function if userspace
+ * fails to properly clean up.  Note that both the kernel and userspace (by
+ * keeeping the PRIME file descriptors open) can hold references onto a
+ * &dma_buf.
+ *
  * Thus the chain of references always flows in one direction
  * (avoiding loops): importing_gem -> dmabuf -> exporting_gem
  *