[Openchrome-devel] Hardware video decoding performance

[Xavier Bachelot]

Bart Hartgers wrote:
> Hi,
> 
> Since a lot more documentation for the VIA chips was released recently, I
> decided it was a good moment to see if I
> could get MPEG2/MPEG4 acceleration working for the CX700 chip on my EPIA
> EX15000. I made some progress in studying and
> disassembling the original VIA XFree86 drivers on
> (CX700XF40064-kernel-src_20060222d) viaarena.com. I've done linux driver by
> reverse engineering
> before, although for less complex hardware. Then again, there actually are
> quite some pieces in the provided source, the remaining binary-only part
> is relatively small.
> 
> At the moment I have good hope that I could get it to work, if I spend
> enough (probably considerable) time on it. However, this brings
> me to the point of this mail: how relevant is this decoding-ability? This
> depends on the decoding performance of the chips,
> relative to the decoding performance of the CPU itself. And I cannot find
> hard information about the first .
> 
Standard resolution DVDs are around 10% CPU usage on a CLE266 with an
1Ghz C3 (around 80-90% without acceleration iirc). The same ratio is
probably more or less correct for a CX700 with an 1 Ghz C7.

> With proper hardware decoding performance, can I expect a (say) 1GHz C7 to
> handle MPEG2-HD on 1080i or 720P?
For the chipsets that are HD capable, I'd say yes. The CX700 is HD
capable. The CLE266 is unfortunately not so I can't tell for sure. The
CN400 is also HD capable.
See http://www.openchrome.org/trac/wiki/SupportedHardware.

> And what about MPEG4? All info I could find just says 'MPEG4' is
> accelerated, but what bitrates are feasible, and is this MPEG4-ASP, or
> MPEG4-AVC (aka H.264)?
> I am guessing decoding blueray would be out of the question, but some hard
> data would be appreciated.
> 
See http://www.openchrome.org/trac/wiki/Configuration#XvMC for a more
accurate list of accelerated format per chipset.
CX700 should also have h.264 support but this is broken according to
some VIA guys, so not activated in their driver. It may or may not worth
looking at it, depending on the level of brokenness.

MPEG4 anyway is a lots more work than adding only CX700 MPEG2, alot of
requireed pieces are needed. However, XvMC VLD is already implemented
for mpeg engines prior to the CX700 engine, so most of the work for
CX700 MPEG2 should already be done. see the libxvmc dir in openchrome
sources.

> I understand that VIA cannot give us proper documentation for the video
> decoders, but I suppose they could tell more about the possible performance.
> 
You can probably use the binary only VIA driver to get an idea of the
MPEG 4 performances. Ivor made some experiments some time ago, the CX700
H.263 wasn't that interesting and H.263 is less useful than H.264 anyway.

> Or is there already information available on this, that google doesn't want
> me to have? ;-)
> 
> Thanks in advance for any thoughts or information on this.
> 
> Groeten,
> Bart
> 
Hope that helps,
Xavier