linux-kernel - Re: [PATCH v1 0/2] NVIDIA Tegra20 video decoder driver

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Date:   Tue, 26 Sep 2017 23:35:49 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     devel@...verdev.osuosl.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH v1 0/2] NVIDIA Tegra20 video decoder driver

On Tue, Sep 26, 2017 at 03:32:23PM +0300, Dmitry Osipenko wrote:
> On 26.09.2017 09:54, Greg Kroah-Hartman wrote:
> > On Tue, Sep 26, 2017 at 01:15:41AM +0300, Dmitry Osipenko wrote:
> >> This driver provides accelerated video decoding to NVIDIA Tegra20 SoC's,
> >> it is a result of reverse-engineering efforts. Driver has been tested on
> >> Toshiba AC100 and Acer A500, it should work on any Tegra20 device.
> >>
> >> In userspace this driver is utilized by libvdpau-tegra [0] that implements
> >> VDPAU interface, so any video player that supports VDPAU can provide
> >> accelerated video decoding on Tegra20 on Linux.
> > 
> > Why not use the v4l2 api instead?  Doesn't that provide the same needed
> > user/kernel api here instead of creating yet-another-custom ioctl?
> > 
> 
> 1) The HW doesn't generalize for the common API. Like for example, it isn't
> capable of unpacking bitstream encoded with CABAC (Context-adaptive binary
> arithmetic coding), so unpacking should be done in software and then VDE HW
> isn't capable of decoding such a stream in a fully-automated manner, software
> would have to feed engine with a chunks of macroblocks untill the whole frame is
> decoded. That lameness is partially hidden in the BLOB's firmware, that firmware
> actually is just a driver BTW.
> 
> 2) We want to have decoding integrated with the presentation of the decoded
> video frame. So having v4l interface for decoding would be just an extra
> unnecessary shim, increasing CPU / memory resources usage and complexity of the
> code.
> 
> 3) The decoding and presentation are already implemented using VDPAU API and
> proven to work decently in that way.

This sounds like something you should be talking over with the media
driver developers, why are they not even cc:ed on this submission?

I need their ack on this new api before I can take this.

thanks,

greg k-h