1===================
2Sync File API Guide
3===================
4
5:Author: Gustavo Padovan <gustavo at padovan dot org>
6
7This document serves as a guide for device drivers writers on what the
8sync_file API is, and how drivers can support it. Sync file is the carrier of
9the fences(struct dma_fence) that are needed to synchronize between drivers or
10across process boundaries.
11
12The sync_file API is meant to be used to send and receive fence information
13to/from userspace. It enables userspace to do explicit fencing, where instead
14of attaching a fence to the buffer a producer driver (such as a GPU or V4L
15driver) sends the fence related to the buffer to userspace via a sync_file.
16
17The sync_file then can be sent to the consumer (DRM driver for example), that
18will not use the buffer for anything before the fence(s) signals, i.e., the
19driver that issued the fence is not using/processing the buffer anymore, so it
20signals that the buffer is ready to use. And vice-versa for the consumer ->
21producer part of the cycle.
22
23Sync files allows userspace awareness on buffer sharing synchronization between
24drivers.
25
26Sync file was originally added in the Android kernel but current Linux Desktop
27can benefit a lot from it.
28
29in-fences and out-fences
30------------------------
31
32Sync files can go either to or from userspace. When a sync_file is sent from
33the driver to userspace we call the fences it contains 'out-fences'. They are
34related to a buffer that the driver is processing or is going to process, so
35the driver creates an out-fence to be able to notify, through
36dma_fence_signal(), when it has finished using (or processing) that buffer.
37Out-fences are fences that the driver creates.
38
39On the other hand if the driver receives fence(s) through a sync_file from
40userspace we call these fence(s) 'in-fences'. Receiving in-fences means that
41we need to wait for the fence(s) to signal before using any buffer related to
42the in-fences.
43
44Creating Sync Files
45-------------------
46
47When a driver needs to send an out-fence userspace it creates a sync_file.
48
49Interface::
50
51	struct sync_file *sync_file_create(struct dma_fence *fence);
52
53The caller pass the out-fence and gets back the sync_file. That is just the
54first step, next it needs to install an fd on sync_file->file. So it gets an
55fd::
56
57	fd = get_unused_fd_flags(O_CLOEXEC);
58
59and installs it on sync_file->file::
60
61	fd_install(fd, sync_file->file);
62
63The sync_file fd now can be sent to userspace.
64
65If the creation process fail, or the sync_file needs to be released by any
66other reason fput(sync_file->file) should be used.
67
68Receiving Sync Files from Userspace
69-----------------------------------
70
71When userspace needs to send an in-fence to the driver it passes file descriptor
72of the Sync File to the kernel. The kernel can then retrieve the fences
73from it.
74
75Interface::
76
77	struct dma_fence *sync_file_get_fence(int fd);
78
79
80The returned reference is owned by the caller and must be disposed of
81afterwards using dma_fence_put(). In case of error, a NULL is returned instead.
82
83References:
84
851. struct sync_file in include/linux/sync_file.h
862. All interfaces mentioned above defined in include/linux/sync_file.h
87