[linux-dvb] DVB API update

Manu Abraham abraham.manu at gmail.com
Tue Sep 18 13:33:25 CEST 2007

Georg Acher wrote:
> On Tue, Sep 18, 2007 at 02:50:09AM +0400, Manu Abraham wrote:
>>> The recording filters are exactly the piece from V4 which has the
>>> "mmap DMA buffers" zero copy API. But to be honest, I don't think
>>> it's important on a PC which can copy > 1GByte/s in RAM. More
>>> interesting would be the ability to have multiple independant filtered
>>> TS outputs instead of just one dvr device.
>> Currently have you tried playing back a High Bit rate H.264 stream
>> default of a DVB-S2 stream ? I guess not.
>> If you have had, you will see my reasons why i am trying to optimize the
>> overheads.
>> BTW: it is not RAM that matters here, but CPU horsepower
> I have h.264 playback running on a really slow Geode with 300MHz. For a
> 15Mbit/s stream, the TS path from the DMA buffers into user space needs
> about 5% CPU with traditional memcpy(). I wouldn't call that optimization
> worthy... What really counts is the postprocessing in user space (remuxing,
> repacking, etc.). The API may support this with single PIDs per
> read filedescriptor, but I don't think it makes a difference where the data
> is actually filtered...

You are looking at a single DVB-S2 demod. There are dual S2 demods in a
single chip config.
Consider that with multiple adapters, even if you ignore post processing.

If you have a larger number of adapters and you have to do post
processing, there is quite a large unnecessary overhead, even if you
don't do software decoding.

Are there only a very few people having multiple DVB adapters in a PC ?
(I guess people having dual and quad demods (single demod/chip) on an
adapter and having multiple adapters can be quite a small fraction)

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