On Wed, 2018-05-09 at 11:33 +0100, Brian Starkey wrote: > Hi Ezequiel, > > On Fri, May 04, 2018 at 05:06:07PM -0300, Ezequiel Garcia wrote: > > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx> > > > > 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@xxxxxxxxxxxxx> > > --- > > 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. > > > > I commented similarly on some of the old patch-sets, and it's a minor > thing, but I still think the ordering of this series is off. It's > strange/wrong to me document all this behaviour, and expose the flags > to userspace, when the functionality isn't implemented yet. > > If I apply this patch to the kernel, then the kernel doesn't do what > the (newly added) kernel-doc says it will. > This has never been a problem, and it has always been the canonical way of doing things. First the required macros, stubs, documentation and interfaces are added, and then they are implemented. I see no reason to go berserk here, unless you see an actual problem? Or something actually broken? The only thing I can think of is that we should return fence_fd -1 if the flags are set. We could do it on this patch, and be consistent with userspace. Regards, Eze
