Re: [PATCH v3 02/15] [media] vb2: add explicit fence user API

[Hans Verkuil]

On 09/11/2017 12:55 PM, Hans Verkuil wrote:
> On 09/07/2017 08:42 PM, Gustavo Padovan 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 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.
>>
>> v2: add documentation
>>
>> Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxx>
>> ---
>>  Documentation/media/uapi/v4l/buffer.rst       | 19 +++++++++++++++++++
>>  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                |  4 +++-
>>  5 files changed, 26 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/media/uapi/v4l/buffer.rst b/Documentation/media/uapi/v4l/buffer.rst
>> index ae6ee73f151c..664507ad06c6 100644
>> --- a/Documentation/media/uapi/v4l/buffer.rst
>> +++ b/Documentation/media/uapi/v4l/buffer.rst
>> @@ -648,6 +648,25 @@ 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 for the next buffer to be queued to V4L2 driver.
>> +	The fence received back through the ``fence_fd`` field  doesn't
>> +	necessarily relate to the current buffer in the
>> +	:ref:`VIDIOC_QBUF <VIDIOC_QBUF>` ioctl. Although, most of the time
>> +	the fence will relate to the current buffer it can't be guaranteed.
>> +	So to tell userspace which buffer is associated to the out_fence,
>> +	one should listen for the ``V4L2_EVENT_BUF_QUEUED`` event that
>> +	provide the id of the buffer when it is queued to the V4L2 driver.
> 
> As mentioned in the previous patch, this flag should just signal that userspace
> wants to receive a BUF_QUEUED event with the out-fence for this buffer. It's up
> to the driver to ensure the correct ordering of the events.

I'll wait for v4 before continuing my review.

	Hans

> 
> I'm missing documentation for the new fence_fd field.
> 
> It should mention that fence_fd is ignored if the IN_FENCE isn't set. Applications
> will set fence_fd (aka reserved2) to 0, which should be fine as long as IN_FENCE
> isn't set.
> 
>>  
>>  
>>  
>> 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..d624fb5df130 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;
>>  };
>>  
>> @@ -533,8 +533,8 @@ 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->reserved, &up->reserved) ||
>> +		put_user(kp->fence_fd, &up->fence_fd) ||
> 
> Could you move this up one line? It's easier to parse this diff if it is clear
> that fence_fd replaced reserved2.
> 
>>  		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 185d6a0acc06..e5abab9a908c 100644
>> --- a/include/uapi/linux/videodev2.h
>> +++ b/include/uapi/linux/videodev2.h
>> @@ -924,7 +924,7 @@ struct v4l2_buffer {
>>  		__s32		fd;
>>  	} m;
>>  	__u32			length;
>> -	__u32			reserved2;
>> +	__s32			fence_fd;
>>  	__u32			reserved;
>>  };
>>  
>> @@ -961,6 +961,8 @@ 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
>>
> 
> Regards,
> 
> 	Hans
>