87 lines
3.1 KiB
ReStructuredText
87 lines
3.1 KiB
ReStructuredText
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===================
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Sync File API Guide
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===================
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:Author: Gustavo Padovan <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'. Receiving 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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