Re: [Linux-uvc-devel] Frame Rate control?

[Laurent P. <lau...@sk...>]

Hi Phil,

> The V4L2 buffer will end up being the same size regardless of frame rate
> (at least for this implementation), right?  So even if the application
> chooses the frame rate after choosing the image size, the buffer size as
> returned by VIDIOC_S_FMT would still be correct.

It should, but that's not required. Frame rate and compression parameters are 
negociated together in the UVC model, and the webcam can request a different 
buffer size based on all those parameters.

> UVC would still have to renegotiate the size of its isochronous USB
> buffers after changing the frame rate, but those buffers are independent
> from the V4L2 queue if I understand correctly.

That's right. USB isochronous buffers are negociated separately, and that's 
already handled at the moment.

> I agree that the V4L2 api is deficient in this respect, but in the case
> of UVC, it seems to me like the current API is still workable, at least
> in the short term.  What do you think?

I could hack the driver, but that would require a-priori knowledge of the 
supported resolutions/frame rates combinations in the user space application. 
That's definitely not a viable long-term solution.

Laurent Pinchart