Re: [PATCH v2 1/1] video_video: Add the Virtio Video V4L2 driver

Dmitry Sepp
2020-03-13T10:27:08Z

Hi Tomasz,

On Freitag, 13. März 2020 11:05:35 CET Tomasz Figa wrote:
> On Thu, Mar 12, 2020 at 12:48 PM Dmitry Sepp
>
> <dmitry.sepp@opensynergy.com> wrote:
> > Hi Hans,
> >
> > One more thing:
> > > GFP_DMA? That's unusual. I'd expect GFP_DMA32. All V4L2 drivers use
> > > that.
> >
> > GFP_DMA32 had no effect for me on arm64. Probably I need to recheck.
>
> What's the reason to use any specific GFP flags at all? GFP_DMA(32)
> memory in the guest would typically correspond to host pages without
> any specific location guarantee.
>

Typically, but not always, especially for non x86. Say, some platforms don't
have IOMMUs for codec devices and those devices require physically contig low
memory. We had to find a way to handle that.

Best regards,
Dmitry.

> Best regards,
> Tomasz
>
> > Best regards,
> > Dmitry.
> >
> > On Donnerstag, 12. März 2020 11:18:26 CET Hans Verkuil wrote:
> > > On 3/12/20 11:15 AM, Dmitry Sepp wrote:
> > > > Hi Hans,
> > > >
> > > > Thank you for your great detailed review!
> > > >
> > > > I won't provide inline answers as your comments totally make sense.
> > > > There
> > > > is>
> > > >
> > > > only one thing I want to mention:
> > > >>> + struct video_plane_format plane_format[VIRTIO_VIDEO_MAX_PLANES];
> > > >>
> > > >> Why is this virtio specific? Any reason for not using
> > > >> VIDEO_MAX_PLANES?
> > > >
> > > > I'd say this is because VIDEO_MAX_PLANES does not exist outside of the
> > > > Linux OS, so for whatever other system we need a virtio specific
> > > > definition.
> > >
> > > OK, good reason :-)
> > >
> > > It's probably a good thing to add a comment where
> > > VIRTIO_VIDEO_MAX_PLANES is defined that explains this.
> > >
> > > Regards,
> > >
> > > Hans