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