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I plan to usurp the short name of struct fence for a core kernel struct, and so I need to rename the specialised fence/timeline for DMA operations to make room. A consensus was reached in https://lists.freedesktop.org/archives/dri-devel/2016-July/113083.html that making clear this fence applies to DMA operations was a good thing. Since then the patch has grown a bit as usage increases, so hopefully it remains a good thing! (v2...: rebase, rerun spatch) v3: Compile on msm, spotted a manual fixup that I broke. v4: Try again for msm, sorry Daniel coccinelle script: @@ @@ - struct fence + struct dma_fence @@ @@ - struct fence_ops + struct dma_fence_ops @@ @@ - struct fence_cb + struct dma_fence_cb @@ @@ - struct fence_array + struct dma_fence_array @@ @@ - enum fence_flag_bits + enum dma_fence_flag_bits @@ @@ ( - fence_init + dma_fence_init | - fence_release + dma_fence_release | - fence_free + dma_fence_free | - fence_get + dma_fence_get | - fence_get_rcu + dma_fence_get_rcu | - fence_put + dma_fence_put | - fence_signal + dma_fence_signal | - fence_signal_locked + dma_fence_signal_locked | - fence_default_wait + dma_fence_default_wait | - fence_add_callback + dma_fence_add_callback | - fence_remove_callback + dma_fence_remove_callback | - fence_enable_sw_signaling + dma_fence_enable_sw_signaling | - fence_is_signaled_locked + dma_fence_is_signaled_locked | - fence_is_signaled + dma_fence_is_signaled | - fence_is_later + dma_fence_is_later | - fence_later + dma_fence_later | - fence_wait_timeout + dma_fence_wait_timeout | - fence_wait_any_timeout + dma_fence_wait_any_timeout | - fence_wait + dma_fence_wait | - fence_context_alloc + dma_fence_context_alloc | - fence_array_create + dma_fence_array_create | - to_fence_array + to_dma_fence_array | - fence_is_array + dma_fence_is_array | - trace_fence_emit + trace_dma_fence_emit | - FENCE_TRACE + DMA_FENCE_TRACE | - FENCE_WARN + DMA_FENCE_WARN | - FENCE_ERR + DMA_FENCE_ERR ) ( ... ) Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk> Acked-by: Sumit Semwal <sumit.semwal@linaro.org> Acked-by: Christian König <christian.koenig@amd.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: http://patchwork.freedesktop.org/patch/msgid/20161025120045.28839-1-chris@chris-wilson.co.uk
84 lines
3.1 KiB
Plaintext
84 lines
3.1 KiB
Plaintext
Sync File API Guide
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~~~~~~~~~~~~~~~~~~~
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Gustavo Padovan
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<gustavo at padovan dot org>
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This document serves as a guide for device drivers writers on what the
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sync_file API is, and how drivers can support it. Sync file is the carrier of
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the fences(struct dma_fence) that are needed to synchronize between drivers or
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across process boundaries.
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The sync_file API is meant to be used to send and receive fence information
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to/from userspace. It enables userspace to do explicit fencing, where instead
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of attaching a fence to the buffer a producer driver (such as a GPU or V4L
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driver) sends the fence related to the buffer to userspace via a sync_file.
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The sync_file then can be sent to the consumer (DRM driver for example), that
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will not use the buffer for anything before the fence(s) signals, i.e., the
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driver that issued the fence is not using/processing the buffer anymore, so it
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signals that the buffer is ready to use. And vice-versa for the consumer ->
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producer part of the cycle.
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Sync files allows userspace awareness on buffer sharing synchronization between
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drivers.
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Sync file was originally added in the Android kernel but current Linux Desktop
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can benefit a lot from it.
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in-fences and out-fences
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------------------------
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Sync files can go either to or from userspace. When a sync_file is sent from
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the driver to userspace we call the fences it contains 'out-fences'. They are
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related to a buffer that the driver is processing or is going to process, so
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the driver creates an out-fence to be able to notify, through
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dma_fence_signal(), when it has finished using (or processing) that buffer.
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Out-fences are fences that the driver creates.
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On the other hand if the driver receives fence(s) through a sync_file from
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userspace we call these fence(s) 'in-fences'. Receiveing in-fences means that
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we need to wait for the fence(s) to signal before using any buffer related to
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the in-fences.
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Creating Sync Files
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-------------------
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When a driver needs to send an out-fence userspace it creates a sync_file.
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Interface:
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struct sync_file *sync_file_create(struct dma_fence *fence);
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The caller pass the out-fence and gets back the sync_file. That is just the
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first step, next it needs to install an fd on sync_file->file. So it gets an
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fd:
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fd = get_unused_fd_flags(O_CLOEXEC);
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and installs it on sync_file->file:
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fd_install(fd, sync_file->file);
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The sync_file fd now can be sent to userspace.
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If the creation process fail, or the sync_file needs to be released by any
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other reason fput(sync_file->file) should be used.
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Receiving Sync Files from Userspace
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-----------------------------------
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When userspace needs to send an in-fence to the driver it passes file descriptor
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of the Sync File to the kernel. The kernel can then retrieve the fences
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from it.
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Interface:
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struct dma_fence *sync_file_get_fence(int fd);
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The returned reference is owned by the caller and must be disposed of
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afterwards using dma_fence_put(). In case of error, a NULL is returned instead.
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References:
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[1] struct sync_file in include/linux/sync_file.h
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[2] All interfaces mentioned above defined in include/linux/sync_file.h
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