[SDL] alsa and DMA..

Stephane Marchesin stephane.marchesin at wanadoo.fr
Mon Jan 31 15:28:54 PST 2005

Dave Airlie wrote:

>>>I see that the ALSA support in SDL doesn't use DMA, but read/write...
>>>I'm wondering has anyone thought about implementing DMA transfers for
>>>Alsa? or know if it would give much advantage.....
>>No idea if it would give a real speed advantage (probably not much). But
>>I do have plans to fix the alsa backend. I run the alsa kernel modules
>>at home (with oss emulation), and since the oss backend got broken from
>>the recent modifications (and alsa never quite worked) I'm stuck now.
>>Actually, I even think the SDL interface could map quite nicely to the
>>alsa dma stuff.
>What problems are therre with the current alsa backend?
The problem seems specific to ac97 cards (I have one, and other ac97 
owners reported sound issues too), but I'm not sure.

> I was going to
>use it for my embedded app but I'm still waiting on a working
>development system with sound... I might get some time to look at this
>soon.... i.e. any CPU usage saving is a win for my current system (its
>getting a bit CPU starved...)

I'd say at most 3-4% cpu on a 500Mhz machine. I have not done such 
measurements for a long time, however so I can't say for a more recent cpu.

>>How old it the alsa dma api ? We have to keep backwards compatibility
>>with alsa < 1.0 too...
>I think the dma api has been stable for a while.. but I'm not quite sure....
It'd be interesting to give it a try. I suspect it could fix the ac97 
problems as a side effect.

Well, ping me on irc or by mail if you do anything.


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