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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Fri, 11 Nov 2022 17:54:01 +0900
Message-ID: <CAAFQd5DZG9QDp-+e8Xs7gKL=ZLkm3hA1-=eJZe8hjoB42xE5qw@mail.gmail.com>
Subject: Re: [RFC PATCH v6 02/11] media: v4l2: Extend pixel formats to unify
 single/multi-planar handling (and more)
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Hsia-Jun Li <Randy.Li@synaptics.com>,
        Nicolas Dufresne <nicolas@ndufresne.ca>, mchehab@kernel.org,
        hans.verkuil@cisco.com, sakari.ailus@iki.fi,
        boris.brezillon@collabora.com, hiroh@chromium.org,
        Brian.Starkey@arm.com, kernel@collabora.com,
        narmstrong@baylibre.com, linux-kernel@vger.kernel.org,
        frkoenig@chromium.org, stanimir.varbanov@linaro.org,
        Hans Verkuil <hverkuil@xs4all.nl>, linux-media@vger.kernel.org
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On Fri, Nov 11, 2022 at 5:43 PM Laurent Pinchart
<laurent.pinchart@ideasonboard.com> 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 =C3=A0 23:19 +0800, Hsia-Jun Li a =C3=A9=
crit :
> > > >>>> 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 w=
ould
> > > >>> 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 wo=
uld
> > > >> always in a multiple planes. The only problem is we don't have a w=
ay 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 t=
his out of
> > > > the pixel format definition to stop the explosion of duplicate pixe=
l 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 so=
me
> > > 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 li=
ke
> > > 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 bef=
ore.
> > >
> > > There are four modifiers in DRM while we would only one in these patc=
hes.
> > >  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 graphi=
cs
> > > 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 maintaine=
r
> > > 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 o=
ther way
> > > > around too. As for allocation, if you have CONTIGUOUS flag set, use=
rspace does
> > > > not have to export or map memory for each planes, as they are the s=
ame. We
> > > > simply need to define the offset as relative to their allocation, w=
hich I think
> > > > is the most sensible thing.
>
> --
> Regards,
>
> Laurent Pinchart