linux-kernel - Re: [PATCH v3 0/2] media: cedrus: Add H264 decoding support

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Date:   Wed, 13 Feb 2019 12:02:08 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Ezequiel Garcia <ezequiel@...labora.com>
Cc:     Maxime Ripard <maxime.ripard@...tlin.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Pawel Osciak <posciak@...omium.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        jenskuske@...il.com, Jernej Skrabec <jernej.skrabec@...il.com>,
        Jonas Karlman <jonas@...boo.se>, linux-sunxi@...glegroups.com,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v3 0/2] media: cedrus: Add H264 decoding support

On Wed, Feb 13, 2019 at 6:22 AM Ezequiel Garcia <ezequiel@...labora.com> wrote:
>
> Hey Tomasz,
>
> On Tue, 2019-02-12 at 21:50 +0900, Tomasz Figa wrote:
> > Hi Maxime,
> >
> > On Mon, Feb 11, 2019 at 11:39 PM Maxime Ripard
> > <maxime.ripard@...tlin.com> wrote:
> > > Hi,
> > >
> > > Here is a new version of the H264 decoding support in the cedrus
> > > driver.
> >
> > Thanks for working on this. Please see my comments below.
> >
> > > As you might already know, the cedrus driver relies on the Request
> > > API, and is a reverse engineered driver for the video decoding engine
> > > found on the Allwinner SoCs.
> > >
> > > This work has been possible thanks to the work done by the people
> > > behind libvdpau-sunxi found here:
> > > https://github.com/linux-sunxi/libvdpau-sunxi/
> > >
> > > I've tested the various ABI using this gdb script:
> > > http://code.bulix.org/jl4se4-505620?raw
> > >
> > > And this test script:
> > > http://code.bulix.org/8zle4s-505623?raw
> > >
> > > The application compiled is quite trivial:
> > > http://code.bulix.org/e34zp8-505624?raw
> > >
> > > The output is:
> > > arm:    builds/arm-test-v4l2-h264-structures
> > >         SHA1: 88cbf7485ba81831fc3b93772b215599b3b38318
> > > x86:    builds/x86-test-v4l2-h264-structures
> > >         SHA1: 88cbf7485ba81831fc3b93772b215599b3b38318
> > > x64:    builds/x64-test-v4l2-h264-structures
> > >         SHA1: 88cbf7485ba81831fc3b93772b215599b3b38318
> > > arm64:  builds/arm64-test-v4l2-h264-structures
> > >         SHA1: 88cbf7485ba81831fc3b93772b215599b3b38318
> > >
> > > Let me know if there's any flaw using that test setup, or if you have
> > > any comments on the patches.
> > >
> > > Maxime
> > >
> > > Changes from v2:
> > >   - Simplified _cedrus_write_ref_list as suggested by Jernej
> > >   - Set whether the frame is used as reference using nal_ref_idc
> > >   - Respect chroma_format_idc
> > >   - Fixes for the scaling list and prediction tables
> > >   - Wrote the documentation for the flags
> > >   - Added a bunch of defines to the driver bit fields
> > >   - Reworded the controls and data format descriptions as suggested
> > >     by Hans
> > >   - Reworked the controls' structure field size to avoid padding
> > >   - Removed the long term reference flag
> >
> > This and...
> >
>
> Maxime has dropped this because of Ayaka's mail about long term references
> not making much sense in stateless decoders.

I haven't seen any argument confirming that thesis, though. I should
have kicked in earlier, sorry.

>
> I noticed that RK3399 TRM has a field to specify long term refs and
> so was wondering about this item as well.
>
> > >   - Reintroduced the neighbor info buffer
> > >   - Removed the ref_pic_list_p0/b0/b1 arrays that are redundant with the
> > >     one in the DPB
> >
> > these are used in our Rockchip VDEC driver.
> >
> > Could you elaborate on the reasons why they got removed?
> >
>
> If I understood correctly, there are two reference picture lists.
> P-frames will populate ref_pic_list0 and B-frames will populate both.
>
> According to this, v4l2_ctrl_h264_slice_param.ref_pic_list0 and .ref_pic_list1
> should be enough and ref_pic_list_p0/b0/b1 are not needed.
>
> What do you think?

The lists in v4l2_ctrl_h264_slice_param are expected to be past the
per-slice modification stage (which is quite complicated and better
done in userspace), while the ones in v4l2_ctrl_h264_decode_param just
in the original order. Rockchip VPU expects them in the original order
and does the modification in the hardware.

Best regards,
Tomasz