staging/android: add Doc for SW_SYNC ioctl interface

This interface is hidden from kernel headers and it is intended for use
only for testing. So testers would have to add the ioctl information
internally. This is to prevent misuse of this feature.

v2: take in Eric suggestions for the Documentation

v3: really take in Eric suggestions

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/sw_sync.c

@@ -25,6 +25,36 @@
 #define CREATE_TRACE_POINTS
 #include "sync_trace.h"
 
+/*
+ * SW SYNC validation framework
+ *
+ * A sync object driver that uses a 32bit counter to coordinate
+ * synchronization.  Useful when there is no hardware primitive backing
+ * the synchronization.
+ *
+ * To start the framework just open:
+ *
+ * <debugfs>/sync/sw_sync
+ *
+ * That will create a sync timeline, all fences created under this timeline
+ * file descriptor will belong to the this timeline.
+ *
+ * The 'sw_sync' file can be opened many times as to create different
+ * timelines.
+ *
+ * Fences can be created with SW_SYNC_IOC_CREATE_FENCE ioctl with struct
+ * sw_sync_ioctl_create_fence as parameter.
+ *
+ * To increment the timeline counter, SW_SYNC_IOC_INC ioctl should be used
+ * with the increment as u32. This will update the last signaled value
+ * from the timeline and signal any fence that has a seqno smaller or equal
+ * to it.
+ *
+ * struct sw_sync_ioctl_create_fence
+ * @value:	the seqno to initialise the fence with
+ * @name:	the name of the new sync point
+ * @fence:	return the fd of the new sync_file with the created fence
+ */
 struct sw_sync_create_fence_data {
 	__u32	value;
 	char	name[32];
@@ -35,6 +65,7 @@ struct sw_sync_create_fence_data {
 
 #define SW_SYNC_IOC_CREATE_FENCE	_IOWR(SW_SYNC_IOC_MAGIC, 0,\
 		struct sw_sync_create_fence_data)
+
 #define SW_SYNC_IOC_INC			_IOW(SW_SYNC_IOC_MAGIC, 1, __u32)
 
 static const struct fence_ops timeline_fence_ops;