Documentation: add Sync File doc

			      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

process boundaries.

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

drivers.

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

the in-fences.

Creating Sync Files

-------------------

When a driver needs to send an out-fence userspace it creates a sync_file.

Interface:

	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:

	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.

References:

[1] struct sync_file in include/linux/sync_file.h

[2] All interfaces mentioned above defined in include/linux/sync_file.h