RE: ALSA PCM DMA questions

[Giuliano Pochini <pochini@shiny.it>]

On 19-May-2005 Derek Jones wrote:

> After the DMA transfer completes, I call snd_pcm_period_elaped(substream).
> I'm guessing that this moves a buffer pointer. What I cannot figure out
> is how to determine where I am in the buffer so that I can start the next
> DMA from the correct position.

Does your hw require restart the DMA after each block ?  This is bad. It's
nearly impossible to do that without a small delay. BTW, the hw should
provide a way to know what it is playing/recording. It it doesn't, you have
to count IRQs. If you have to restart the DMA every time, it's simple: just
store the current block number when you start it for later use.


> When the entire buffer has been DMA'ed, is the buffer overwritten with new
> PCM data from userland

During playback, part of the buffer (one or more periods) is filled with
new data. snd_pcm_period_elapsed() triggers it.


> or does the DMA buffer work like a circular buffer with userland data
> being transferred into the buffer at each PCM period.

Yes.


--
Giuliano.


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