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authorOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>2017-01-27 10:04:25 +0300
committerDaniel Vetter <daniel.vetter@ffwll.ch>2017-01-27 17:00:26 +0300
commitfa4c1de4a1aeeb0ef4dca692c779abbcc6c4960f (patch)
tree21c239cacb6f16cfbc0c56de44c06278eb228b2f /drivers/gpu/drm/drm_prime.c
parent239ac65fa5ffab71adf66e642750f940e7241d99 (diff)
downloadlinux-fa4c1de4a1aeeb0ef4dca692c779abbcc6c4960f.tar.xz
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
Diffstat (limited to 'drivers/gpu/drm/drm_prime.c')
-rw-r--r--drivers/gpu/drm/drm_prime.c17
1 files changed, 15 insertions, 2 deletions
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 485e558d6661..25aa4558f1b5 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 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
*