[RFC,v5,06/11,media] vb2: add explicit fence user API

  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 and 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 a fence to the kernel to be waited on, and
V4L2_BUF_FLAG_OUT_FENCE, to ask the kernel to give back an out-fence.

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       | 15 +++++++++++++++
 drivers/media/usb/cpia2/cpia2_v4l.c           |  2 +-
 drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  4 ++--
 drivers/media/v4l2-core/videobuf2-v4l2.c      |  2 +-
 include/uapi/linux/videodev2.h                |  7 ++++++-
 5 files changed, 25 insertions(+), 5 deletions(-)
  

Em Wed, 15 Nov 2017 15:10:52 -0200
Gustavo Padovan <gustavo@padovan.org> escreveu:

> 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 and 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 a fence to the kernel to be waited on, and
> V4L2_BUF_FLAG_OUT_FENCE, to ask the kernel to give back an out-fence.
> 
> 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       | 15 +++++++++++++++
>  drivers/media/usb/cpia2/cpia2_v4l.c           |  2 +-
>  drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  4 ++--
>  drivers/media/v4l2-core/videobuf2-v4l2.c      |  2 +-
>  include/uapi/linux/videodev2.h                |  7 ++++++-
>  5 files changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index ae6ee73f151c..eeefbd2547e7 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -648,6 +648,21 @@ 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 fence passed in ``fence_fd`` field. The buffer
> +	won't be queued to the driver until the fence signals.
> +
> +    * .. _`V4L2-BUF-FLAG-OUT-FENCE`:
> +
> +      - ``V4L2_BUF_FLAG_OUT_FENCE``
> +      - 0x00400000
> +      - Request a fence to be attached to the buffer. The ``fence_fd``
> +	field on
> +	:ref:`VIDIOC_QBUF` is used as a return argument to send the out-fence
> +	fd to userspace.
>  
>  
>  
> diff --git a/drivers/media/usb/cpia2/cpia2_v4l.c b/drivers/media/usb/cpia2/cpia2_v4l.c
> index 3dedd83f0b19..6cde686bf44c 100644
> --- a/drivers/media/usb/cpia2/cpia2_v4l.c
> +++ b/drivers/media/usb/cpia2/cpia2_v4l.c
> @@ -948,7 +948,7 @@ static int cpia2_dqbuf(struct file *file, void *fh, struct v4l2_buffer *buf)
>  	buf->sequence = cam->buffers[buf->index].seq;
>  	buf->m.offset = cam->buffers[buf->index].data - cam->frame_buffer;
>  	buf->length = cam->frame_size;
> -	buf->reserved2 = 0;
> +	buf->fence_fd = -1;
>  	buf->reserved = 0;
>  	memset(&buf->timecode, 0, sizeof(buf->timecode));
>  
> diff --git a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> index 821f2aa299ae..3a31d318df2a 100644
> --- a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> +++ b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> @@ -370,7 +370,7 @@ struct v4l2_buffer32 {
>  		__s32		fd;
>  	} m;
>  	__u32			length;
> -	__u32			reserved2;
> +	__s32			fence_fd;
>  	__u32			reserved;
>  };
>  
> @@ -533,7 +533,7 @@ static int put_v4l2_buffer32(struct v4l2_buffer *kp, struct v4l2_buffer32 __user
>  		put_user(kp->timestamp.tv_usec, &up->timestamp.tv_usec) ||
>  		copy_to_user(&up->timecode, &kp->timecode, sizeof(struct v4l2_timecode)) ||
>  		put_user(kp->sequence, &up->sequence) ||
> -		put_user(kp->reserved2, &up->reserved2) ||
> +		put_user(kp->fence_fd, &up->fence_fd) ||
>  		put_user(kp->reserved, &up->reserved) ||
>  		put_user(kp->length, &up->length))
>  			return -EFAULT;
> diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> index 0c0669976bdc..110fb45fef6f 100644
> --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> +++ b/drivers/media/v4l2-core/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 = -1;
>  	b->reserved = 0;
>  
>  	if (q->is_multiplanar) {
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index cd6fc1387f47..311c3a331eda 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -925,7 +925,10 @@ struct v4l2_buffer {
>  		__s32		fd;
>  	} m;
>  	__u32			length;
> -	__u32			reserved2;
> +	union {
> +		__s32		fence_fd;

You forgot to document the fence_fd at the uAPI book. In particular,
it should be noticed there that a non-positive value means no fence.

(as userspace should be setting reserved2 to zero, I'm assuming
that we'll likely not accept 0 here).

You should likely also remove the description from reserved2 at
Documentation/media/uapi/v4l/buffer.rst.


> +		__u32		reserved2;
> +	};
>  	__u32			reserved;
>  };
>  
> @@ -962,6 +965,8 @@ struct v4l2_buffer {
>  #define V4L2_BUF_FLAG_TSTAMP_SRC_SOE		0x00010000
>  /* mem2mem encoder/decoder */
>  #define V4L2_BUF_FLAG_LAST			0x00100000
> +#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
  

On 15/11/17 18:10, Gustavo Padovan 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 and 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 a fence to the kernel to be waited on, and
> V4L2_BUF_FLAG_OUT_FENCE, to ask the kernel to give back an out-fence.
> 
> 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       | 15 +++++++++++++++
>  drivers/media/usb/cpia2/cpia2_v4l.c           |  2 +-
>  drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  4 ++--
>  drivers/media/v4l2-core/videobuf2-v4l2.c      |  2 +-
>  include/uapi/linux/videodev2.h                |  7 ++++++-
>  5 files changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> index ae6ee73f151c..eeefbd2547e7 100644
> --- a/Documentation/media/uapi/v4l/buffer.rst
> +++ b/Documentation/media/uapi/v4l/buffer.rst
> @@ -648,6 +648,21 @@ 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 fence passed in ``fence_fd`` field. The buffer
> +	won't be queued to the driver until the fence signals.
> +
> +    * .. _`V4L2-BUF-FLAG-OUT-FENCE`:
> +
> +      - ``V4L2_BUF_FLAG_OUT_FENCE``
> +      - 0x00400000
> +      - Request a fence to be attached to the buffer. The ``fence_fd``
> +	field on
> +	:ref:`VIDIOC_QBUF` is used as a return argument to send the out-fence
> +	fd to userspace.

How would userspace know if fences are not supported? E.g. any driver that does
not use vb2 will have no support for it.

While the driver could clear the flag on return, the problem is that it is a bit
late for applications to discover lack of fence support.

Perhaps we do need a capability flag for this? I wonder what others think.

Regards,

	Hans

>  
>  
>  
> diff --git a/drivers/media/usb/cpia2/cpia2_v4l.c b/drivers/media/usb/cpia2/cpia2_v4l.c
> index 3dedd83f0b19..6cde686bf44c 100644
> --- a/drivers/media/usb/cpia2/cpia2_v4l.c
> +++ b/drivers/media/usb/cpia2/cpia2_v4l.c
> @@ -948,7 +948,7 @@ static int cpia2_dqbuf(struct file *file, void *fh, struct v4l2_buffer *buf)
>  	buf->sequence = cam->buffers[buf->index].seq;
>  	buf->m.offset = cam->buffers[buf->index].data - cam->frame_buffer;
>  	buf->length = cam->frame_size;
> -	buf->reserved2 = 0;
> +	buf->fence_fd = -1;
>  	buf->reserved = 0;
>  	memset(&buf->timecode, 0, sizeof(buf->timecode));
>  
> diff --git a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> index 821f2aa299ae..3a31d318df2a 100644
> --- a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> +++ b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> @@ -370,7 +370,7 @@ struct v4l2_buffer32 {
>  		__s32		fd;
>  	} m;
>  	__u32			length;
> -	__u32			reserved2;
> +	__s32			fence_fd;
>  	__u32			reserved;
>  };
>  
> @@ -533,7 +533,7 @@ static int put_v4l2_buffer32(struct v4l2_buffer *kp, struct v4l2_buffer32 __user
>  		put_user(kp->timestamp.tv_usec, &up->timestamp.tv_usec) ||
>  		copy_to_user(&up->timecode, &kp->timecode, sizeof(struct v4l2_timecode)) ||
>  		put_user(kp->sequence, &up->sequence) ||
> -		put_user(kp->reserved2, &up->reserved2) ||
> +		put_user(kp->fence_fd, &up->fence_fd) ||
>  		put_user(kp->reserved, &up->reserved) ||
>  		put_user(kp->length, &up->length))
>  			return -EFAULT;
> diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> index 0c0669976bdc..110fb45fef6f 100644
> --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> +++ b/drivers/media/v4l2-core/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 = -1;
>  	b->reserved = 0;
>  
>  	if (q->is_multiplanar) {
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index cd6fc1387f47..311c3a331eda 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -925,7 +925,10 @@ struct v4l2_buffer {
>  		__s32		fd;
>  	} m;
>  	__u32			length;
> -	__u32			reserved2;
> +	union {
> +		__s32		fence_fd;
> +		__u32		reserved2;
> +	};
>  	__u32			reserved;
>  };
>  
> @@ -962,6 +965,8 @@ struct v4l2_buffer {
>  #define V4L2_BUF_FLAG_TSTAMP_SRC_SOE		0x00010000
>  /* mem2mem encoder/decoder */
>  #define V4L2_BUF_FLAG_LAST			0x00100000
> +#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
>
  

Em Fri, 17 Nov 2017 14:29:23 +0100
Hans Verkuil <hverkuil@xs4all.nl> escreveu:

> On 15/11/17 18:10, Gustavo Padovan 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 and 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 a fence to the kernel to be waited on, and
> > V4L2_BUF_FLAG_OUT_FENCE, to ask the kernel to give back an out-fence.
> > 
> > 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       | 15 +++++++++++++++
> >  drivers/media/usb/cpia2/cpia2_v4l.c           |  2 +-
> >  drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  4 ++--
> >  drivers/media/v4l2-core/videobuf2-v4l2.c      |  2 +-
> >  include/uapi/linux/videodev2.h                |  7 ++++++-
> >  5 files changed, 25 insertions(+), 5 deletions(-)
> > 
> > diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
> > index ae6ee73f151c..eeefbd2547e7 100644
> > --- a/Documentation/media/uapi/v4l/buffer.rst
> > +++ b/Documentation/media/uapi/v4l/buffer.rst
> > @@ -648,6 +648,21 @@ 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 fence passed in ``fence_fd`` field. The buffer
> > +	won't be queued to the driver until the fence signals.
> > +
> > +    * .. _`V4L2-BUF-FLAG-OUT-FENCE`:
> > +
> > +      - ``V4L2_BUF_FLAG_OUT_FENCE``
> > +      - 0x00400000
> > +      - Request a fence to be attached to the buffer. The ``fence_fd``
> > +	field on
> > +	:ref:`VIDIOC_QBUF` is used as a return argument to send the out-fence
> > +	fd to userspace.  
> 
> How would userspace know if fences are not supported? E.g. any driver that does
> not use vb2 will have no support for it.
> 
> While the driver could clear the flag on return, the problem is that it is a bit
> late for applications to discover lack of fence support.
> 
> Perhaps we do need a capability flag for this? I wonder what others think.

We're almost running out of flags at v4l2 caps (and at struct v4l2_buffer).

So, I would prefer to not add more flags on those structs if there is
another way.

As the fences out of order flags should go to ENUM_FMT (and, currently
there's just one flag defined there), I wander if it would make sense
to also add CAN_IN_FENCES/CAN_OUT_FENCES flags there, as maybe we
would want to disable/enable fences based on the format.

> 
> Regards,
> 
> 	Hans
> 
> >  
> >  
> >  
> > diff --git a/drivers/media/usb/cpia2/cpia2_v4l.c b/drivers/media/usb/cpia2/cpia2_v4l.c
> > index 3dedd83f0b19..6cde686bf44c 100644
> > --- a/drivers/media/usb/cpia2/cpia2_v4l.c
> > +++ b/drivers/media/usb/cpia2/cpia2_v4l.c
> > @@ -948,7 +948,7 @@ static int cpia2_dqbuf(struct file *file, void *fh, struct v4l2_buffer *buf)
> >  	buf->sequence = cam->buffers[buf->index].seq;
> >  	buf->m.offset = cam->buffers[buf->index].data - cam->frame_buffer;
> >  	buf->length = cam->frame_size;
> > -	buf->reserved2 = 0;
> > +	buf->fence_fd = -1;
> >  	buf->reserved = 0;
> >  	memset(&buf->timecode, 0, sizeof(buf->timecode));
> >  
> > diff --git a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> > index 821f2aa299ae..3a31d318df2a 100644
> > --- a/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> > +++ b/drivers/media/v4l2-core/v4l2-compat-ioctl32.c
> > @@ -370,7 +370,7 @@ struct v4l2_buffer32 {
> >  		__s32		fd;
> >  	} m;
> >  	__u32			length;
> > -	__u32			reserved2;
> > +	__s32			fence_fd;
> >  	__u32			reserved;
> >  };
> >  
> > @@ -533,7 +533,7 @@ static int put_v4l2_buffer32(struct v4l2_buffer *kp, struct v4l2_buffer32 __user
> >  		put_user(kp->timestamp.tv_usec, &up->timestamp.tv_usec) ||
> >  		copy_to_user(&up->timecode, &kp->timecode, sizeof(struct v4l2_timecode)) ||
> >  		put_user(kp->sequence, &up->sequence) ||
> > -		put_user(kp->reserved2, &up->reserved2) ||
> > +		put_user(kp->fence_fd, &up->fence_fd) ||
> >  		put_user(kp->reserved, &up->reserved) ||
> >  		put_user(kp->length, &up->length))
> >  			return -EFAULT;
> > diff --git a/drivers/media/v4l2-core/videobuf2-v4l2.c b/drivers/media/v4l2-core/videobuf2-v4l2.c
> > index 0c0669976bdc..110fb45fef6f 100644
> > --- a/drivers/media/v4l2-core/videobuf2-v4l2.c
> > +++ b/drivers/media/v4l2-core/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 = -1;
> >  	b->reserved = 0;
> >  
> >  	if (q->is_multiplanar) {
> > diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> > index cd6fc1387f47..311c3a331eda 100644
> > --- a/include/uapi/linux/videodev2.h
> > +++ b/include/uapi/linux/videodev2.h
> > @@ -925,7 +925,10 @@ struct v4l2_buffer {
> >  		__s32		fd;
> >  	} m;
> >  	__u32			length;
> > -	__u32			reserved2;
> > +	union {
> > +		__s32		fence_fd;
> > +		__u32		reserved2;
> > +	};
> >  	__u32			reserved;
> >  };
> >  
> > @@ -962,6 +965,8 @@ struct v4l2_buffer {
> >  #define V4L2_BUF_FLAG_TSTAMP_SRC_SOE		0x00010000
> >  /* mem2mem encoder/decoder */
> >  #define V4L2_BUF_FLAG_LAST			0x00100000
> > +#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
> >   
>
  

On 17/11/17 14:53, Mauro Carvalho Chehab wrote:
> Em Fri, 17 Nov 2017 14:29:23 +0100
> Hans Verkuil <hverkuil@xs4all.nl> escreveu:
> 
>> On 15/11/17 18:10, Gustavo Padovan 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 and 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 a fence to the kernel to be waited on, and
>>> V4L2_BUF_FLAG_OUT_FENCE, to ask the kernel to give back an out-fence.
>>>
>>> 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       | 15 +++++++++++++++
>>>  drivers/media/usb/cpia2/cpia2_v4l.c           |  2 +-
>>>  drivers/media/v4l2-core/v4l2-compat-ioctl32.c |  4 ++--
>>>  drivers/media/v4l2-core/videobuf2-v4l2.c      |  2 +-
>>>  include/uapi/linux/videodev2.h                |  7 ++++++-
>>>  5 files changed, 25 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
>>> index ae6ee73f151c..eeefbd2547e7 100644
>>> --- a/Documentation/media/uapi/v4l/buffer.rst
>>> +++ b/Documentation/media/uapi/v4l/buffer.rst
>>> @@ -648,6 +648,21 @@ 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 fence passed in ``fence_fd`` field. The buffer
>>> +	won't be queued to the driver until the fence signals.
>>> +
>>> +    * .. _`V4L2-BUF-FLAG-OUT-FENCE`:
>>> +
>>> +      - ``V4L2_BUF_FLAG_OUT_FENCE``
>>> +      - 0x00400000
>>> +      - Request a fence to be attached to the buffer. The ``fence_fd``
>>> +	field on
>>> +	:ref:`VIDIOC_QBUF` is used as a return argument to send the out-fence
>>> +	fd to userspace.  
>>
>> How would userspace know if fences are not supported? E.g. any driver that does
>> not use vb2 will have no support for it.
>>
>> While the driver could clear the flag on return, the problem is that it is a bit
>> late for applications to discover lack of fence support.
>>
>> Perhaps we do need a capability flag for this? I wonder what others think.
> 
> We're almost running out of flags at v4l2 caps (and at struct v4l2_buffer).

struct v4l2_capability has more than enough room to add a new device_caps2 field.
So I see no problem there, and it is very useful for applications to know what
features are supported up front and not when you start to use them.

Think about it: you're setting up complete fence support in your application, only
to discover when you queue the first buffer that there is no fence support! That
doesn't work.

The reserved[] array wasn't added for nothing to v4l2_capability.

struct v4l2_buffer is indeed very full. But I posted an RFC on October 26 introducing
a struct v4l2_ext_buffer, designed from scratch. We can switch to a u64 flags there.

See here: https://www.mail-archive.com/linux-media@vger.kernel.org/msg121215.html

I am waiting for fences and the request API to go in before continuing with that
RFC series, unless we think it is better to only support fences/request API with
this redesign. Let me know and I can pick up development of that RFC.

> 
> So, I would prefer to not add more flags on those structs if there is
> another way.
> 
> As the fences out of order flags should go to ENUM_FMT (and, currently
> there's just one flag defined there), I wander if it would make sense
> to also add CAN_IN_FENCES/CAN_OUT_FENCES flags there, as maybe we
> would want to disable/enable fences based on the format.

I don't see a reason for that. There is no relationship between formats
and fences. Fences apply to buffers, not formats. Whereas the 'ordered'
value can be specific to a format.

Regards,

	Hans