Re: [RFCv4,19/21] media: vim2m: add request support

[Paul Kocialkowski]

Hi,

On Mon, 2018-03-12 at 17:15 +0900, Tomasz Figa wrote:
> Hi Paul, Dmitry,
> 
> On Mon, Mar 12, 2018 at 5:10 PM, Paul Kocialkowski
> <paul.kocialkowski@xxxxxxxxxxx> wrote:
> > Hi,
> > 
> > On Sun, 2018-03-11 at 22:42 +0300, Dmitry Osipenko wrote:
> > > Hello,
> > > 
> > > On 07.03.2018 19:37, Paul Kocialkowski wrote:
> > > > Hi,
> > > > 
> > > > First off, I'd like to take the occasion to say thank-you for
> > > > your
> > > > work.
> > > > This is a major piece of plumbing that is required for me to add
> > > > support
> > > > for the Allwinner CedarX VPU hardware in upstream Linux. Other
> > > > drivers,
> > > > such as tegra-vde (that was recently merged in staging) are also
> > > > badly
> > > > in need of this API.
> > > 
> > > Certainly it would be good to have a common UAPI. Yet I haven't
> > > got my
> > > hands on
> > > trying to implement the V4L interface for the tegra-vde driver,
> > > but
> > > I've taken a
> > > look at Cedrus driver and for now I've one question:
> > > 
> > > Would it be possible (or maybe already is) to have a single IOCTL
> > > that
> > > takes input/output buffers with codec parameters, processes the
> > > request(s) and returns to userspace when everything is done?
> > > Having 5
> > > context switches for a single frame decode (like Cedrus VAAPI
> > > driver
> > > does) looks like a bit of overhead.
> > 
> > The V4L2 interface exposes ioctls for differents actions and I don't
> > think there's a combined ioctl for this. The request API was
> > introduced
> > precisely because we need to have consistency between the various
> > ioctls
> > needed for each frame. Maybe one single (atomic) ioctl would have
> > worked
> > too, but that's apparently not how the V4L2 API was designed.
> > 
> > I don't think there is any particular overhead caused by having n
> > ioctls
> > instead of a single one. At least that would be very surprising
> > IMHO.
> 
> Well, there is small syscall overhead, which normally shouldn't be
> very painful, although with all the speculative execution hardening,
> can't be sure of anything anymore. :)

Oh, my mistake then, I had it in mind that it is not really something
noticeable. Hopefully, it won't be a limiting factor in our cases.

> Hans and Alex can correct me if I'm wrong, but I believe there is a
> more atomic-like API being planned, which would only need one IOCTL to
> do everything. However, that would be a more serious change to the
> V4L2 interfaces, so should be decoupled from Request API itself.
> 
> Best regards,
> Tomasz
-- 
Paul Kocialkowski, Bootlin (formerly Free Electrons)
Embedded Linux and kernel engineering
https://bootlin.com

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