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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