[PATCH v9 10/15] vb2: add explicit fence user API

[Ezequiel Garcia <ezequiel@collabora.com>]

From: Gustavo Padovan <gustavo.padovan@collabora.com>

Turn the reserved2 field into fence_fd that we will use to send
an in-fence to the kernel or return an out-fence from the kernel to
userspace.

Two new flags were added, V4L2_BUF_FLAG_IN_FENCE, that should be used
when sending an in-fence to the kernel to be waited on, and
V4L2_BUF_FLAG_OUT_FENCE, to ask the kernel to give back an out-fence.

v7: minor fixes on the Documentation (Hans Verkuil)

v6: big improvement on doc (Hans Verkuil)

v5: - keep using reserved2 field for cpia2
    - set fence_fd to 0 for now, for compat with userspace(Mauro)

v4: make it a union with reserved2 and fence_fd (Hans Verkuil)

v3: make the out_fence refer to the current buffer (Hans Verkuil)

v2: add documentation

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
---
 Documentation/media/uapi/v4l/buffer.rst         | 45 +++++++++++++++++++++++--
 drivers/media/common/videobuf2/videobuf2-v4l2.c |  2 +-
 drivers/media/v4l2-core/v4l2-compat-ioctl32.c   |  4 +--
 include/uapi/linux/videodev2.h                  |  8 ++++-
 4 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
index e2c85ddc990b..be9719cf5745 100644
--- a/Documentation/media/uapi/v4l/buffer.rst
+++ b/Documentation/media/uapi/v4l/buffer.rst
@@ -301,10 +301,22 @@ struct v4l2_buffer
 	elements in the ``planes`` array. The driver will fill in the
 	actual number of valid elements in that array.
     * - __u32
-      - ``reserved2``
+      - ``fence_fd``
       -
-      - A place holder for future extensions. Drivers and applications
-	must set this to 0.
+      - Used to communicate a fence file descriptors from userspace to kernel
+	and vice-versa. On :ref:`VIDIOC_QBUF <VIDIOC_QBUF>` when sending
+	an in-fence for V4L2 to wait on, the ``V4L2_BUF_FLAG_IN_FENCE`` flag must
+	be used and this field set to the fence file descriptor of the in-fence.
+	If the in-fence is not valid ` VIDIOC_QBUF`` returns an error.
+
+        To get an out-fence back from V4L2 the ``V4L2_BUF_FLAG_OUT_FENCE``
+	must be set, the kernel will return the out-fence file descriptor in
+	this field. If it fails to create the out-fence ``VIDIOC_QBUF` returns
+        an error.
+
+	For all other ioctls V4L2 sets this field to -1 if
+	``V4L2_BUF_FLAG_IN_FENCE`` and/or ``V4L2_BUF_FLAG_OUT_FENCE`` are set,
+	otherwise this field is set to 0 for backward compatibility.
     * - __u32
       - ``reserved``
       -
@@ -648,6 +660,33 @@ Buffer Flags
       - Start Of Exposure. The buffer timestamp has been taken when the
 	exposure of the frame has begun. This is only valid for the
 	``V4L2_BUF_TYPE_VIDEO_CAPTURE`` buffer type.
+    * .. _`V4L2-BUF-FLAG-IN-FENCE`:
+
+      - ``V4L2_BUF_FLAG_IN_FENCE``
+      - 0x00200000
+      - Ask V4L2 to wait on the fence passed in the ``fence_fd`` field. The
+	buffer won't be queued to the driver until the fence signals. The order
+	in which buffers are queued is guaranteed to be preserved, so any
+	buffers queued after this buffer will also be blocked until this fence
+	signals. This flag must be set before calling ``VIDIOC_QBUF``. For
+	other ioctls the driver just reports the value of the flag.
+
+        If the fence signals the flag is cleared and not reported anymore.
+	If the fence is not valid ``VIDIOC_QBUF`` returns an error.
+
+
+    * .. _`V4L2-BUF-FLAG-OUT-FENCE`:
+
+      - ``V4L2_BUF_FLAG_OUT_FENCE``
+      - 0x00400000
+      - Request for a fence to be attached to the buffer. The driver will fill
+	in the out-fence fd in the ``fence_fd`` field when :ref:`VIDIOC_QBUF
+	<VIDIOC_QBUF>` returns. This flag must be set before calling
+	``VIDIOC_QBUF``. For other ioctls the driver just reports the value of
+	the flag.
+
+        If the creation of the out-fence fails ``VIDIOC_QBUF`` returns an
+	error.
 
 
 
diff --git a/drivers/media/common/videobuf2/videobuf2-v4l2.c b/drivers/media/common/videobuf2/videobuf2-v4l2.c
index 64503615d00b..b1c0fa2b0b88 100644
--- a/drivers/media/common/videobuf2/videobuf2-v4l2.c
+++ b/drivers/media/common/videobuf2/videobuf2-v4l2.c
@@ -203,7 +203,7 @@ static void __fill_v4l2_buffer(struct vb2_buffer *vb, void *pb)
 	b->timestamp = ns_to_timeval(vb->timestamp);
 	b->timecode = vbuf->timecode;
 	b->sequence = vbuf->sequence;
-	b->reserved2 = 0;
+	b->fence_fd = 0;
 	b->reserved = 0;
 
 	if (q->is_multiplanar) {
diff --git a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
index 4312935f1dfc..93c752459aec 100644
--- a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
+++ b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
@@ -388,7 +388,7 @@ struct v4l2_buffer32 {
 		__s32		fd;
 	} m;
 	__u32			length;
-	__u32			reserved2;
+	__s32			fence_fd;
 	__u32			reserved;
 };
 
@@ -606,7 +606,7 @@ static int put_v4l2_buffer32(struct v4l2_buffer __user *kp,
 	    assign_in_user(&up->timestamp.tv_usec, &kp->timestamp.tv_usec) ||
 	    copy_in_user(&up->timecode, &kp->timecode, sizeof(kp->timecode)) ||
 	    assign_in_user(&up->sequence, &kp->sequence) ||
-	    assign_in_user(&up->reserved2, &kp->reserved2) ||
+	    assign_in_user(&up->fence_fd, &kp->fence_fd) ||
 	    assign_in_user(&up->reserved, &kp->reserved) ||
 	    get_user(length, &kp->length) ||
 	    put_user(length, &up->length))
diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
index a8842a5ca636..1f18dc68ecab 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -934,7 +934,10 @@ struct v4l2_buffer {
 		__s32		fd;
 	} m;
 	__u32			length;
-	__u32			reserved2;
+	union {
+		__s32		fence_fd;
+		__u32		reserved2;
+	};
 	__u32			reserved;
 };
 
@@ -971,6 +974,9 @@ struct v4l2_buffer {
 #define V4L2_BUF_FLAG_TSTAMP_SRC_SOE		0x00010000
 /* mem2mem encoder/decoder */
 #define V4L2_BUF_FLAG_LAST			0x00100000
+/* Explicit synchronization */
+#define V4L2_BUF_FLAG_IN_FENCE			0x00200000
+#define V4L2_BUF_FLAG_OUT_FENCE			0x00400000
 
 /**
  * struct v4l2_exportbuffer - export of video buffer as DMABUF file descriptor
-- 
2.16.3