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Gustavo Padovanc784c822016-04-28 10:47:00 -03001 Sync File API Guide
2 ~~~~~~~~~~~~~~~~~~~
3
4 Gustavo Padovan
5 <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
Chris Wilsonf54d1862016-10-25 13:00:45 +01009the fences(struct dma_fence) that are needed to synchronize between drivers or
Javier Martinez Canillasfac84342016-05-14 02:28:36 -040010across process boundaries.
Gustavo Padovanc784c822016-04-28 10:47:00 -030011
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
Chris Wilsonf54d1862016-10-25 13:00:45 +010035the 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.
Gustavo Padovanc784c822016-04-28 10:47:00 -030038
39On the other hand if the driver receives fence(s) through a sync_file from
40userspace we call these fence(s) 'in-fences'. Receiveing 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:
Chris Wilsonf54d1862016-10-25 13:00:45 +010050 struct sync_file *sync_file_create(struct dma_fence *fence);
Gustavo Padovanc784c822016-04-28 10:47:00 -030051
52The caller pass the out-fence and gets back the sync_file. That is just the
53first step, next it needs to install an fd on sync_file->file. So it gets an
54fd:
55
56 fd = get_unused_fd_flags(O_CLOEXEC);
57
58and installs it on sync_file->file:
59
60 fd_install(fd, sync_file->file);
61
62The sync_file fd now can be sent to userspace.
63
64If the creation process fail, or the sync_file needs to be released by any
65other reason fput(sync_file->file) should be used.
66
Gustavo Padovan395dec62016-08-05 10:39:37 -030067Receiving Sync Files from Userspace
68-----------------------------------
69
70When userspace needs to send an in-fence to the driver it passes file descriptor
71of the Sync File to the kernel. The kernel can then retrieve the fences
72from it.
73
74Interface:
Chris Wilsonf54d1862016-10-25 13:00:45 +010075 struct dma_fence *sync_file_get_fence(int fd);
Gustavo Padovan395dec62016-08-05 10:39:37 -030076
77
78The returned reference is owned by the caller and must be disposed of
Chris Wilsonf54d1862016-10-25 13:00:45 +010079afterwards using dma_fence_put(). In case of error, a NULL is returned instead.
Gustavo Padovan395dec62016-08-05 10:39:37 -030080
Gustavo Padovanc784c822016-04-28 10:47:00 -030081References:
82[1] struct sync_file in include/linux/sync_file.h
83[2] All interfaces mentioned above defined in include/linux/sync_file.h