linux-kernel - Re: [PATCH v3 1/2] staging: Introduce NVIDIA Tegra20 video decoder driver

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Date:   Wed, 11 Oct 2017 16:47:18 -0400
From:   Nicolas Dufresne <nicolas@...fresne.ca>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Stephen Warren <swarren@...dotorg.org>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        linux-media@...r.kernel.org, linux-tegra@...r.kernel.org,
        devel@...verdev.osuosl.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] staging: Introduce NVIDIA Tegra20 video decoder
 driver

Le mercredi 11 octobre 2017 à 23:08 +0300, Dmitry Osipenko a écrit :
> diff --git a/drivers/staging/tegra-vde/TODO b/drivers/staging/tegra-
> vde/TODO
> new file mode 100644
> index 000000000000..e98bbc7b3c19
> --- /dev/null
> +++ b/drivers/staging/tegra-vde/TODO
> @@ -0,0 +1,5 @@
> +TODO:
> +       - Figure out how generic V4L2 API could be utilized by this
> driver,
> +         implement it.
> +

That is a very interesting effort, I think it's the first time someone
is proposing an upstream driver for a Tegra platform. When I look
tegra_vde_h264_decoder_ctx, it looks like the only thing that the HW is
not parsing is the media header (pps/sps). Is that correct ?

I wonder how acceptable it would be to parse this inside the driver. It
is no more complex then parsing an EDID. If that was possible, wrapping
this driver as a v4l2 mem2mem should be rather simple. As a side
effect, you'll automatically get some userspace working, notably
GStreamer and FFmpeg.

For the case even parsing the headers is too much from a kernel point
of view, then I think you should have a look at the following effort.
It's a proposal base on yet to be merged Request API. Hugues is also
propose a libv4l2 adapter that makes the driver looks like a normal
v4l2 m2m, hiding all the userspace parsing and table filling. This
though, is long term plan to integrate state-less or parser-less
encoders into linux-media. It seems rather overkill for state-full
driver that requires parsed headers like PPS/SPS.

https://lwn.net/Articles/720797/

regards,
Nicolas
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