linux-kernel - [PATCH v2 13/13] Documentation: add Sync File doc

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Message-Id: <1461851220-1583-14-git-send-email-gustavo@padovan.org>
Date:	Thu, 28 Apr 2016 10:47:00 -0300
From:	Gustavo Padovan <gustavo@...ovan.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	dri-devel@...ts.freedesktop.org,
	Daniel Stone <daniels@...labora.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Riley Andrews <riandrews@...roid.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	John Harrison <John.C.Harrison@...el.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>
Subject: [PATCH v2 13/13] Documentation: add Sync File doc

From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>

Add sync_file documentation on dma-buf-sync_file.txt
Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
---
 Documentation/sync_file.txt | 69 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 Documentation/sync_file.txt

diff --git a/Documentation/sync_file.txt b/Documentation/sync_file.txt
new file mode 100644
index 0000000..eaf8297
--- /dev/null
+++ b/Documentation/sync_file.txt
@@ -0,0 +1,69 @@
+			      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
-- 
2.5.5