Re: [RFC PATCH v6 02/11] media: v4l2: Extend pixel formats to unify single/multi-planar handling (and more)

[Tomasz Figa]

On Fri, Nov 11, 2022 at 5:43 PM Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Tomasz,
>
> On Fri, Nov 11, 2022 at 02:48:48PM +0900, Tomasz Figa wrote:
> > On Fri, Nov 11, 2022 at 12:04 PM Hsia-Jun Li wrote:
> > > On 11/11/22 01:06, Nicolas Dufresne wrote:
> > > > Le samedi 05 novembre 2022 à 23:19 +0800, Hsia-Jun Li a écrit :
> > > >>>> VIDIOC_ENUM_EXT_PIX_FMT would report NV12 and NV12M, while
> > > >>>> VIDIOC_ENUM_FMT
> > > >>>> would just report NV12M.
> > > >>>
> > > >>> If NV12 and NV12M are equivalent in Ext API, I don't see why we would
> > > >>> report both (unless I'm missing something, which is probably the case).
> > > >>>
> > > >>> The idea was to deprecate the M-variants one day.
> > > >> I was thinking the way in DRM API is better, always assuming it would
> > > >> always in a multiple planes. The only problem is we don't have a way to
> > > >> let the allocator that allocate contiguous memory for planes when we
> > > >> need to do that.
> > > >
> > > > Its not too late to allow this to be negotiated, but I would move this out of
> > > > the pixel format definition to stop the explosion of duplicate pixel formats,
> > > > which is a nightmare to deal with.
> > >
> > > I wonder whether we need to keep the pixel formats in videodev2.h
> > > anymore. If we would like to use the modifiers from drm_fourcc.h, why
> > > don't we use their pixel formats, they should be the same values of
> > > non-M variant pixel formats of v4l2.
> > >
> > > Let videodev2.h only maintain the those codecs or motion based
> > > compressed (pixel) formats.
> > >
> > > If I simplify the discussion, we want to
> > >
> > > > negotiate contiguity with the driver. The new FMT structure should have a
> > > > CONTIGUOUS flag. So if userpace sets:
> > > >
> > > >    S_FMT(NV12, CONTIGUOUS)
> > >
> > > I wonder whether we would allow some planes being contiguous while some
> > > would not. For example, the graphics planes could be in a contiguous
> > > memory address while its compression metadata are not.
> > > Although that is not the case of our platform. I believe it sounds like
> > > reasonable case for improving the performance, two meta planes could
> > > resident in a different memory bank.
> >
> > I feel like this would be only useful in the MMAP mode. Looking at how
> > the other UAPIs are evolving, things are going towards
> > userspace-managed allocations, using, for example, DMA-buf heaps. I
> > think we should follow the trend and keep the MMAP mode just at the
> > same level of functionality as is today and focus on improvements and
> > new functionality for the DMABUF mode.
>
> I agree, but we will need an API to expose the memory constraints of the
> device, or userspace won't be able to allocate memory compatible with
> the hardware or driver requirements.

Yes, I fully agree and that's why I think we should rather focus our
efforts in that direction rather than expanding the existing MMAP
capabilities.

>
> > > That lead to another question which I forgot whether I mention it before.
> > >
> > > There are four modifiers in DRM while we would only one in these patches.
> > >  From the EGL
> > > https://registry.khronos.org/EGL/extensions/EXT/EGL_EXT_image_dma_buf_import_modifiers.txt
> > >
> > > The modifier for echo plane could be different. I wish it would be
> > > better to create a framebuffer being aware of which planes are graphics
> > > or metadata.
> >
> > What's an echo plane?
> >
> > That said, it indeed looks like we may want to be consistent with DRM
> > here and allow per-plane modifiers.
> >
> > > I wonder whether it would be better that convincing the DRM maintainer
> > > adding a non vendor flag for contiguous memory allocation here(DRM
> > > itself don't need it).
> > > While whether the memory could be contiguous for these vendor pixel
> > > formats, it is complex vendor defined.
> >
> > Memory allocation doesn't sound to me like it is related to formats or
> > modifiers in any way. I agree with Nicolas that if we want to allow
> > the userspace to specify if the memory should be contiguous or not,
> > that should be a separate flag and actually I'd probably see it in
> > REQBUF_EXT and CREATE_BUFS_EXT, rather than as a part of the format.
>
> I like how DRM decouples allocation of buffer objects and creation of
> frame buffers.

Exactly why I proposed so rather than coupling it with S_FMT. (But
then it's moot if we decide to focus on DMABUF mode.)

>
> > > > The driver can accepts, and return the unmodified structure, or may drop the
> > > > CONTIGUOUS flag, which would mean its not supported. Could be the other way
> > > > around too. As for allocation, if you have CONTIGUOUS flag set, userspace does
> > > > not have to export or map memory for each planes, as they are the same. We
> > > > simply need to define the offset as relative to their allocation, which I think
> > > > is the most sensible thing.
>
> --
> Regards,
>
> Laurent Pinchart