Re: [Qemu-devel] Webcams under KVM and Linux

[Gerd Hoffmann <kraxel@redhat.com>]

   Hi,

>>> Takes a frame from ANY available V4L2 device (/dev/video0),
>>> caches it, and sends it completely to the guest before requesting
>>> any other frame.
>>
>> I think you can double-buffer (i.e. let the host driver fill one
>> buffer while sending the other one to the guest).  Probably gives a
>> slightly higher frame rate, but maybe at cost of added latencies.
>
> Indeed you can infinite-buffer it, but there is really no gain, the
> added latency makes an effective lower frame rate (total number of
> real frames seen by the guest in percentage)

Just two buffers, so capture and sending to the guest can be 
parallelized.  More buffers don't really help and just increase latency.

>> Nice.  Patches are waiting for EHCI being merged I guess?
>
> Patches are on RFC on June 2010 ML messages. There are some updates I
> did to the emulation (internal conversion from YUV2 to MJPEG, gives
> twice the framerate when the host webcam is YUV2 only) that I have
> not sent to RFC yet.

Can you repost the latest version for review so we can get the bits merged?

> There are also some things that can be enhanced (conversion of more
> strange RAWs like OV511's, show the guest the controls of the real
> webcam) easily but I won't do that until a legal problem about the
> usage of my emulation code along with all the rest of QEMU by a
> commercial vendor in violation of GPL is solved.

Thats fine, we can always add it later once the legal issues are sorted.

> It works really well with USB 1.1 (up to 24fps with KVM, up to 10fps
> with TCG), but your when EHCI is merged it will allow bigger
> resolutions easily

Great.

> The most curious and interesting thing is that, while the
> specification says there can be webcams using bulk transfers (that's
> what mine is doing) I've seen NONE in wild. All do ISO.

Using bulk certainly makes sense for a virtual device.

cheers,
   Gerd