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

Hans Verkuil
2018-05-07T11:31:03Z

On 04/05/18 22:06, Ezequiel Garcia wrote:
> 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

Type is now __s32.

> - - ``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

descriptors -> descriptor

> + 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.

What happens once the out fence signals? Is this flag cleared? I think that would
make sense.

> +
> + If the creation of the out-fence fails ``VIDIOC_QBUF`` returns an
> + error.
>
>

I would like to see a mention here that it is valid to set both FENCE flags.

>
> 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
>

Regards,

Hans