Hi, On Fri 12 Jul 19, 12:26, wens Tsai wrote: > On Thu, Jul 11, 2019 at 8:19 PM Paul Kocialkowski > <paul.kocialkow...@bootlin.com> wrote: > > > > On Thu 11 Jul 19, 19:51, wens Tsai wrote: > > > On Thu, Jul 11, 2019 at 5:39 PM Paul Kocialkowski > > > <paul.kocialkow...@bootlin.com> wrote: > > > > > > > > Hi, > > > > > > > > On Thu 11 Jul 19, 17:21, wens Tsai wrote: > > > > > I noticed that recent codec driver additions, such as hantro, > > > > > meson/vdec, > > > > > mtk, used the multi-planar API (mplane) instead of the single-planar > > > > > API. > > > > > > > > > > Is there a preference for moving towards mplane? > > > > > > > > Well, there is strict technical requirement for using the single-planar > > > > API > > > > instead of the multi-planar one in cedrus. > > > > > > > > Historically, we started with mplane because we can definitely pass > > > > different addresses for luma and chroma to our decoder. However, we > > > > found > > > > out that there are some internal limitations of the decoder block and > > > > the > > > > addresses cannot be "too far apart". This means that if we allocate luma > > > > at the begging of our pool and chroma at the end, the gap was too big > > > > and the resulting decoded frame was not stored at the right address. > > > > > > > > We found out about this purely by accident, where one particular > > > > sequence of > > > > events lead to such a gap and showed the issue. One possible explanation > > > > would be that the decoder uses an offset from the luma address for > > > > chroma > > > > internally, on a limited number of bits. > > > > > > > > So unfortunately, this means we're stuck with having our multi-planar > > > > (in the YUV sense) buffers allocated in a single chunk (the single-plane > > > > V4L2 API). I don't have any better answer than "userspace should cope > > > > with both > > > > APIs as it reflects a hardware constraint", which is indeed what ffmpeg > > > > is > > > > already doing. > > > > > > If I understand the API correctly, using the multi-planar API doesn't mean > > > you have to actually support multi-planar formats such as NVxyM and > > > YUVxyzM. > > > AFAICT these are the only two families that have non contiguous planes. > > > > That is indeed quite right and maybe switching away from mplane was > > unnecessary. > > In practice, we can only use single-planar (non-M formats) but we could > > totally > > support the mplane semantics for these formats as well. > > > > What I was thinking probably boiled down to the idea that without mplane > > supported, userspace wouldn't be able to allocate multiple planes and then > > try to use them with a single-planar format. I think this is what I had > > implemented before and something that "worked" although being totally wrong > > regarding the API. It looks like the core doesn't check that single-planar > > formats are only given one plane by userspace and, as long as the driver > > goes > > with it, it can still operate like this. Could be nice to have this kind of > > checking around. > > > > So in the end, it looks like we can safely bring back mplane support :) > > This brings me to another question: is there anything (policy or technical) > preventing us from supporting both APIs? I haven't seen any drivers do this, > and with the compatibility plugin in libv4l2, I suppose it isn't necessary.
So according to Hans, this makes drivers particuarly more complex to write, so we might want to fully switch over to mplane for the time being. I'm not sure there are common cases where that would really be beneficial, and as you said, it's probably not necessary. Cheers, Paul > ChenYu > > > > So for all the other existing formats, the buffers will still be > > > contiguous, > > > which means cedrus can actually support the multi-planar API, right? > > > > Thinking twice about it, I think you're right yes. > > > > Cheers, > > > > Paul > > > > > ChenYu > > > > > > > Cheers, > > > > > > > > Paul > > > > > > > > > Also I noticed that v4l-utils has a plugin that seamlessly adapts > > > > > mplane > > > > > devices to work with single-planar API calls, but there isn't one the > > > > > other way around. Would that be something worthwhile working on? To me > > > > > it seems having one central adapter is better than having applications > > > > > try to use both APIs, though some such as FFmpeg already do so. > > > > > > > > > > My interest in this is trying to get Chromium's v4l2 video decode > > > > > accelerator to work on the Raspberry Pi 3 with their downstream > > > > > kernel, > > > > > which has the bcm2835-v4l2 driver. It would also benefit other > > > > > platforms > > > > > that have a stateful VPU, such as Amlogic (meson) or Mediatek SoCs. > > > > > Chromium's code uses MPLANE exclusively, and it's geared towards > > > > > ChromeOS, > > > > > not standard Linux, but still it might be interesting to get it to > > > > > work. > > > > > > > > > > There's also the v4l2 slice video decode accelerator which uses the > > > > > request API which has the same issue, but lets not get ahead of > > > > > ourselves. > > > > > > > > > > Regards > > > > > ChenYu > > > > > > > > -- > > > > Paul Kocialkowski, Bootlin > > > > Embedded Linux and kernel engineering > > > > https://bootlin.com > > > > -- > > Paul Kocialkowski, Bootlin > > Embedded Linux and kernel engineering > > https://bootlin.com -- Paul Kocialkowski, Bootlin Embedded Linux and kernel engineering https://bootlin.com
