[alsa-devel] dmix without mmap support in soundcard driver

Takashi Iwai tiwai at suse.de
Mon Jan 7 12:00:28 CET 2008

At Sun, 06 Jan 2008 11:15:35 +0100,
Joachim Foerster wrote:
> Hi Cliff, 
> On Wed, 2008-01-02 at 06:26 +0000, cailinhua wrote:
> > I want to enable dmix for our sound card.Unfortunately for some reason the sound card driver don't support 
> > 'mmap' way,it only support 'copy' callback to copy data from upper level to DMA.So ,I think maybe I can modify 
> > the alsa-lib dmix source code to let it use 'copy' way to send data to the driver. Is it possible?
> > Could you please give me some ideas?
> I don't know how difficult it would be to change alsa-lib's dmix
> sources, but my suggestion - in general - would be to alter the driver
> of your sound card to use the "intermediate buffer" technique (using
> pcm-indirect.h): Setup a buffer in RAM, which can be mmap'ed by
> applications and copy the contents of this buffer to the hardware in a
> background process for example.
> (See Takashi's "Writing an ALSA Driver" guide.)
> I know, perhaps this is not the easiest solution ... and it's not a
> "direct" answer to your question - just a thought.

Yes, currently it's the only "solution".  The dmix requires mmap
because the shared mmap among several processes is the key technique
of dmix.


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