Re: Slow video output from webcam with qemu-xhci but smooth with usb-ehci

[Dr. David Alan Gilbert]

* Diego Viola (diego.viola@gmail.com) wrote:
> On Sat, Dec 12, 2020 at 1:28 PM Diego Viola <diego.viola@gmail.com> wrote:
> >
> > Hi,
> >
> > I'm experiencing a lot of choppiness in the video output when I pass
> > through my USB webcam to the guest using qemu-xhci as follows:
> >
> > qemu-system-x86_64 -enable-kvm -hda arch-zoom.qcow2 -m 4G -vga virtio
> > -device qemu-xhci,id=xhci -device
> > usb-host,bus=xhci.0,hostdevice=/dev/bus/usb/002/004
> >
> > My webcam is:
> >
> > Bus 002 Device 004: ID 04f2:b449 Chicony Electronics Co., Ltd Integrated 
> > Camera
> >
> > I am using mpv /dev/video0 from the guest.
> >
> > It works fine if I use usb-ehci instead, e.g.:
> >
> > qemu-system-x86_64 -enable-kvm -hda arch-zoom.qcow2 -m 4G -vga virtio
> > -device usb-ehci,id=ehci -device
> > usb-host,bus=ehci.0,hostdevice=/dev/bus/usb/002/004
> >
> > In this case, the video output from mpv /dev/video0 is not choppy.
> >
> > My QEMU version is 5.2.0 -- I am running Arch Linux on the host and guest.
> >
> > Any ideas please?
> >
> > Thanks,
> > Diego
> 
> OK, I just noticed that it's mostly at the beginning (when I first run
> mpv) that the video is jerky/choppy, looks like mpv is still buffering
> at that stage, after buffering gets to 99% the video is smoother (with
> xhci).
> 
> Now I don't understand why buffering with ehci happens a lot faster/smoother.

I don't understand it either; but cc'ing in Gerd for his USB wisdom.

Dave

> Nevermind, not an issue after all.
> 
> Diego
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK