Re: [PATCH v4 0/8] media: add virtio-media driver

[Albert Esteve <aesteve@redhat.com>]

On Sun, Jul 12, 2026 at 9:10 AM Mauro Carvalho Chehab
<mchehab+huawei@kernel.org> wrote:
>
> On Mon, 22 Jun 2026 16:43:35 -0400
> Brian Daniels <briandaniels@google.com> wrote:
>
> > From: Alexandre Courbot <gnurou@gmail.com>
> >
> > Add the first version of the virtio-media driver.
> >
> > This driver acts roughly as a V4L2 relay between user-space and the
> > virtio virtual device on the host, so it is relatively simple, yet
> > unconventional. It doesn't use VB2 or other frameworks typically used in
> > a V4L2 driver, and most of its complexity resides in correctly and
> > efficiently building the virtio descriptor chain to pass to the host,
> > avoiding copies whenever possible. This is done by
> > scatterlist_builder.[ch].
> >
> > This version supports MMAP buffers, while USERPTR buffers can also be
> > enabled through a driver option. DMABUF support is still pending.
>
> In practice, USERPTR was used on several drivers that wanted to
> share buffers between V4L2 and GPU (so, a previous approach before
> DMABUF implementation).
>
> On my tests with this driver, I was unable use a 1080p camera with
> V4L2 and GPU on crossvm. Lower resolutions worked. No idea if this
> was a limitation of crossvm (I only used it to test this driver)
> or if it is due to a poor MMAP implementation.
>
>
>
> > Compliance Testing
> >
> > This was tested using v4l2-compliance. Since virtio-media serves as
> > a proxy to host devices for the guest VMs, we expect the guest
> > compliance test to essentially match the host compliance test for the
> > same device.
> >
> > NOTE: v4l2-compliance changes its test behavior depending on the driver
> > name. In the guest, the driver name for virtio-media proxied-devices is
> > always "virtio-media", even if the actual host device has a driver name
> > of e.g. "uvcvideo". To ensure the test is consistent between the host
> > and the guest, I created a patch for the v4l2-compliance tool that
> > allows you to override the driver name. All test results that follow use
> > this patch:
> > https://lore.kernel.org/r/20260528163448.4031965-1-briandaniels@google.com/
>
> As mentioned before, please submit this with their rationale in
> separate as a [PATCH v4l-utils] to linux-media ML.
>
> >
> > All tests used a Logitech USB Webcam C925e.
>
> Please test it displaying inside crossvm - or even better to QEMU if
> you manage to add virtio-media support to it.
>
> Being at QEMU makes a lot easier for everyone to test it.

Hi,

Regarding the QEMU support mention, I created this series in QEMU to
add the virtio-media PCI device:
https://lore.kernel.org/all/20260630112310.552606-1-aesteve@redhat.com/

Testing was done using an older driver version at
https://github.com/chromeos/virtio-media/tree/main/driver as described
in the cover letter. But I can try testing it with this series. Either
way, the procedure for using QEMU is in the cover letter, so anyone
can try it.

BR,
Albert.

>
>
> Thanks,
> Mauro
>