Hi all,
Based on the discussion on the mailinglist I came to the following conclusions
which I will be implementing on top of the reqv18 patches:
On 08/16/18 12:25, Hans Verkuil wrote:
> Laurent raised a few API issues/questions in his review of the documentation.
>
> I've consolidated those in this RFC. I would like to know what others think
> and if I should make changes.
>
> 1) Should you be allowed to set controls directly if they are also used in
> requests? Right now this is allowed, although we warn in the spec that
> this can lead to undefined behavior.
>
> In my experience being able to do this is very useful while testing,
> and restricting this is not all that easy to implement. I also think it is
> not our job. It is not as if something will break when you do this.
>
> If there really is a good reason why you can't mix this for a specific
> control, then the driver can check this and return -EBUSY.
We allow this. The warning in the spec is sufficient and there are legitimate
use-cases where you want to be able to do this.
>
> 2) If request_fd in QBUF or the control ioctls is not a request fd, then we
> now return ENOENT. Laurent suggests using EBADR ('Invalid request descriptor')
> instead. This seems like a good idea to me. Should I change this?
Mauro wasn't too keen on it and EBADR appears to be arch dependent (different
values for different architectures).
I think using EBADR is a bit too risky.
However, I do agree with Laurent that ENOENT isn't the best error code. We have a
similar situation with vb2 and dmabuf if the fd for a dmabuf is invalid. In that
case vb2 returns -EINVAL, but it also issues a dprintk in that case so it is a
bit easier to discover the reason for EINVAL. I'll do the same.
>
> 3) Calling VIDIOC_G_EXT_CTRLS for a request that has not been queued yet will
> return either the value of the control you set earlier in the request, or
> the current HW control value if it was never set in the request.
>
> I believe it makes sense to return what was set in the request previously
> (if you set it, you should be able to get it), but it is an idea to return
> ENOENT when calling this for controls that are NOT in the request.
>
> I'm inclined to implement that. Opinions?
The consensus is to prohibit this, at least for now. So attempts to get controls
from a request that is not in completed state will return -EACCES.
>
> 4) When queueing a buffer to a request with VIDIOC_QBUF you set V4L2_BUF_FLAG_REQUEST_FD
> to indicate a valid request_fd. For other queue ioctls that take a struct v4l2_buffer
> this flag and the request_fd field are just ignored. Should we return EINVAL
> instead if the flag is set for those ioctls?
>
> The argument for just ignoring it is that older kernels that do not know about
> this flag will ignore it as well. There is no check against unknown flags.
We'll just leave this as-is. I'll add a note to the spec that userspace should clear this
flag for ioctls other than QBUF.
Regards,
Hans
