[PATCH v1] drm/prime: Clarify DMA-BUF/GEM Object lifetime

[Oleksandr Andrushchenko]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>

>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 at gmail.com>
Cc: Dave Airlie <airlied at linux.ie>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>
---
 drivers/gpu/drm/drm_prime.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 8d77b2462594..893a1a6311f0 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/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 refernce 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 +54,15 @@
  * 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, e.g. when
+ * userspace closes both handles to the imported (fd_to_handle_ioctl)
+ * and exported (handle_to_fd_ioctl) dma_buf and closes the corresponding
+ * file descriptor (handle_to_fd), then dma_buf gets destroyed.
+ * This can also happen as a part of the clean up procedure in the
+ * driver .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
  *
-- 
2.7.4