From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx> >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@xxxxxxxxx> Cc: Dave Airlie <airlied@xxxxxxxx> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx> --- 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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
