Sound card DMA questions.

Sound card DMA questions.

[James Courtier-Dutton]

With DMA accesses, one has a sound card connected to a PCI bus, which is 
then connected to the RAM.

The sound card will schedule a DMA transfer.

My questions are:
1) What size is the DMA transfer?
2) Are the min/max limits on how many bytes can be passed on a transfer?
3) When does the transfer happen. At period boundarys or when ?
4) What triggers a user land poll() to return. An irq based 
snd_period_elapsed() call, or something else?

And finally,
5) if I am reverse engineering a card, how can I discover these min/max 
values?

James


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RE: Sound card DMA questions.

[Giuliano Pochini]

On 20-Jul-2004 James Courtier-Dutton wrote:
> With DMA accesses, one has a sound card connected to a PCI bus, which
> is then connected to the RAM.
>
> The sound card will schedule a DMA transfer.
>
> My questions are:
> 1) What size is the DMA transfer?

It's defined by the driver.


> 2) Are the min/max limits on how many bytes can be passed on a transfer?

Usually, no.  There may be limits defined by DMA-able region and
card's capabilities.


> 3) When does the transfer happen. At period boundarys or when ?

DMA transfers are managed entirely by the card once the driver
starts them. You should ask the manufacturer for these infos. Btw
they're not important from the driver's viewpoint.


> 4) What triggers a user land poll() to return. An irq based
> snd_period_elapsed() call, or something else?

_elapsed() doesn't require to be called from inside irq handler.


> 5) if I am reverse engineering a card, how can I discover these
> min/max values?

There isn't a standard procedure to reverse engeneer something.


--
Giuliano.


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