[linux-dvb] Re: Adding V4L interface to budget cards

[Michael Hunold <hunold-ml@web.de>]

Hello Holger,

> Have you ever thought about the overhead of kernel/userspace copies, the 
> impossibility to use DMA for memory transfers and how hard it is getting 
> usually to use hardware acceleration units where available? Modern STB 
> processors are different from the PC processors you know, have you ever 
> tried to use an onchip DSP to support decoding?

I'm still living in the "normal" PC world -- I agree that this differs 
from the world where you use STBs with helper DSPs onboard...

But at least DMA-from/to userspace is not impossible any more, but of 
course you'll need a DMA-capable device with in-device memory for example.

>> So you could basically put the "ts2ps" and ps-to-yuv/rgb conversion 
>> into  a fusd-driver and provide a Video4Linux-compatible /dev/video 
>> device like this, if you really want to.
> 
> 
> This would introduce a significand overhead for kernel/userspace 
> communication and context switches. Remember the amounts of data for 
> digital video and that today's STB processors are still running at 
> speeds between 66MHz and a little above 200MHz...

Sure, but if you talk about doing ts to yuv conversion in software, you 
don't talk about CPUs < 1Ghz any more. Well, but we don't need to argue 
about this, in the STB world you'll need spezialized DSPs and a good API 
to encapsulate the functionality.

> Holger

CU
Michael.




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