linux/Documentation/sync_file.txt
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   1                              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
   9the fences(struct 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 fence_signal(),
  36when it has finished using (or processing) that buffer. Out-fences are fences
  37that 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'. 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:
  50        struct sync_file *sync_file_create(struct fence *fence);
  51
  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
  67Receiving 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:
  75        struct fence *sync_file_get_fence(int fd);
  76
  77
  78The returned reference is owned by the caller and must be disposed of
  79afterwards using fence_put(). In case of error, a NULL is returned instead.
  80
  81References:
  82[1] struct sync_file in include/linux/sync_file.h
  83[2] All interfaces mentioned above defined in include/linux/sync_file.h
  84