linux-kernel - Re: [PATCH v7 6/6] arm64: dts: rockchip: Add VPU121 support for RK3588

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Date: Thu, 27 Jun 2024 09:53:01 -0400
From: Nicolas Dufresne <nicolas.dufresne@...labora.com>
To: Jianfeng Liu <liujianfeng1994@...il.com>
Cc: conor+dt@...nel.org, detlev.casanova@...labora.com, 
	devicetree@...r.kernel.org, ezequiel@...guardiasur.com.ar, 
	frattaroli.nicolas@...il.com, heiko@...ech.de, kernel@...labora.com, 
	krzk+dt@...nel.org, linkmauve@...kmauve.fr, linux-kernel@...r.kernel.org, 
	linux-media@...r.kernel.org, linux-rockchip@...ts.infradead.org, 
	p.zabel@...gutronix.de, robh@...nel.org, sebastian.reichel@...labora.com, 
	sigmaris@...il.com
Subject: Re: [PATCH v7 6/6] arm64: dts: rockchip: Add VPU121 support for
 RK3588

Le jeudi 27 juin 2024 à 16:13 +0800, Jianfeng Liu a écrit :
> Hi Nicolas,
> 
> On Wed, 26 Jun 2024 13:46:03 -0400, Nicolas Dufresne wrote:
> > Just to clarify, since you are right that it won't work well with GStreamer. It
> > does work with multiple decoders (it exposes them all), it is simply that it
> > will randomly pick one when decoding, and it may not pick the best one.
> 
> I have tested rkvdec2 and vpu121 with gstreamer 1.24.2 on rk356x to decode
> a 4K video, and gstreamer always fall with error:
> "v4l2slh264dec0: Failed to configure H264 decoder".
> I guess that's because 1080p vpu is at fdea0000 which is always
> initialized earlier than rkvdec2 at fdf80200, so gstreamer will always
> choose the 1080p decoder.

I've never done any research, but that is plausible.

- Probe happen in address order, since DT are in address order
- Media notes are assigned in probe order
- GStreamer register the element in the same order in its registry
- In adsence of a rank or capabilities to differentiate, the probe order is
maintained.

> 
> > In the long term, I'd like to stop having to do "like downstream" and expose
> > them all. I believe the fix is fairly straightforward in GStreamer. We need to
> > expose in the generated element the width/height ranges, and for H.264 the
> > supported profiles and level. With that, we at least won't randomly fail at
> > decoding 4K, and it should be good enough.
> 
> Not only gstreamer, chromium also has similar issue. Chromium will only
> check video resolution globally before starting to use one decoder: if
> there is a 4K decoder detected before, it will mark 4K resolution as
> supported. But when decoding videos, it will choose the first decoder
> supporting profile like H264. So chromium may use a 1080p decoder to
> decode a 4K video.
> 
> Chromium's code about v4l2 is complicated for me. I may create a bug about
> it. But chrome os doesn't support devices with multi v4l2 decoders like
> rockchip's socs, I don't know if they have the motion to fix it quickly.

That's an interesting bug, which makes its more of less equal to GStreamer
"unimplemented behaviour". Filing a bug is best indeed, ChromeOS team, who
maintains this, is probably unaware as they don't have any SoC with multiple
decoders. Even on PC side, their Chromebooks only ever have a single GPU, I
haven't heard about any eGPU support either.

> 
> > For RK3588, which is a new SoC, its not a problem to upstream something that
> > does not work with existing userspace. It would only be a regression if we where
> > to enable VDPU121 on RK3399, as now updating linux would cause bugs with
> > existing userspace.
> 
> There is an old soc just like RK3399: RK3328, which also has a 1080p
> hantro h264 decoder and a 4K rkvdec h264 decoder. I guess less people care
> about its mainline decoding with gstreamer/chromium so it still has 1080p
> decoder enabled.

What I meant by new/old, is supported mainline or not. But yes, on timeline,
there is many older SoC with dual decoders in the Rockchip line.

> 
> > For users, it would be best if we get this sorted out in GStreamer by the time
> > we have a second decoder. Note that I have some vacation coming up this month,
> > so there might be extra delays. Yet, its logical to merge this (the "worst"
> > decoder) first, since then randomly picking a better one won't be a regression.
> 
> Happy vacation days! I will also take a look at chromium's code to see if
> I can fix it.

Great, let's keep everyone on sync, I'm sure we can come up with something
better then disabling the possibly useful hardware.

Nicolas