|author||Gustavo Padovan <firstname.lastname@example.org>||2016-04-28 10:47:00 -0300|
|committer||Greg Kroah-Hartman <email@example.com>||2016-04-29 17:37:10 -0700|
Documentation: add Sync File doc
Add sync_file documentation on dma-buf-sync_file.txt Reviewed-by: Daniel Vetter <firstname.lastname@example.org> Signed-off-by: Greg Kroah-Hartman <email@example.com>
1 files changed, 69 insertions, 0 deletions
diff --git a/Documentation/sync_file.txt b/Documentation/sync_file.txt
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+ Sync File API Guide
+ Gustavo Padovan
+ <gustavo at padovan dot org>
+This document serves as a guide for device drivers writers on what the
+sync_file API is, and how drivers can support it. Sync file is the carrier of
+the fences(struct fence) that needs to synchronized between drivers or across
+The sync_file API is meant to be used to send and receive fence information
+to/from userspace. It enables userspace to do explicit fencing, where instead
+of attaching a fence to the buffer a producer driver (such as a GPU or V4L
+driver) sends the fence related to the buffer to userspace via a sync_file.
+The sync_file then can be sent to the consumer (DRM driver for example), that
+will not use the buffer for anything before the fence(s) signals, i.e., the
+driver that issued the fence is not using/processing the buffer anymore, so it
+signals that the buffer is ready to use. And vice-versa for the consumer ->
+producer part of the cycle.
+Sync files allows userspace awareness on buffer sharing synchronization between
+Sync file was originally added in the Android kernel but current Linux Desktop
+can benefit a lot from it.
+in-fences and out-fences
+Sync files can go either to or from userspace. When a sync_file is sent from
+the driver to userspace we call the fences it contains 'out-fences'. They are
+related to a buffer that the driver is processing or is going to process, so
+the driver an create out-fence to be able to notify, through fence_signal(),
+when it has finished using (or processing) that buffer. Out-fences are fences
+that the driver creates.
+On the other hand if the driver receives fence(s) through a sync_file from
+userspace we call these fence(s) 'in-fences'. Receiveing in-fences means that
+we need to wait for the fence(s) to signal before using any buffer related to
+Creating Sync Files
+When a driver needs to send an out-fence userspace it creates a sync_file.
+ struct sync_file *sync_file_create(struct fence *fence);
+The caller pass the out-fence and gets back the sync_file. That is just the
+first step, next it needs to install an fd on sync_file->file. So it gets an
+ fd = get_unused_fd_flags(O_CLOEXEC);
+and installs it on sync_file->file:
+ fd_install(fd, sync_file->file);
+The sync_file fd now can be sent to userspace.
+If the creation process fail, or the sync_file needs to be released by any
+other reason fput(sync_file->file) should be used.
+ struct sync_file in include/linux/sync_file.h
+ All interfaces mentioned above defined in include/linux/sync_file.h