accessing sw data buffers in card driver?

accessing sw data buffers in card driver?

[John Shepherd]

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hi,

um ....how do i extract the audio buffers from memory inside say a card driver, like the snd-dummy.  i am just experimenting with ALSA to see if it is possible to stream the audio data to the 1394 bus...
so i just need to know how to get the data out of the buffer...

i assume a call to this gets the size out:

snd_pcm_lib_buffer_bytes(substream);


what pcm_function will return the *buffer?  do i access it through runtime->dma_area?

thanx,
john.

================================
John Shepherd 

Dept. of Computer Science
Rhodes University
Grahamstown
South Africa
6140.
================================

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Re: accessing sw data buffers in card driver?

[Takashi Iwai]

Hi,

At Tue, 5 Nov 2002 18:02:46 +0200,
John Shepherd wrote:
> 
> hi,
>  
> um ....how do i extract the audio buffers from memory inside say a card driver, like
> the snd-dummy.  i am just experimenting with ALSA to see if it is possible to stream
> the audio data to the 1394 bus...

for accessing from the user space, usually you need to call mmap from
the application.
and, perhaps you misunderstand about snd-dummy.  snd-dummy does
nothing.  it works just like /dev/null, simply ignores the data.


> so i just need to know how to get the data out of the buffer...
>  
> i assume a call to this gets the size out:
>  
> snd_pcm_lib_buffer_bytes(substream);
> 
> what pcm_function will return the *buffer?  do i access it through runtime->dma_area?

snd_pcm_lib_buffer_bytes() returns the size of the buffer in bytes.
in general, the sizes are handled in frames on ALSA.  note that
	bytes per frame = # channels * sample-width-in-bytes

unfortuantely, there are really no documents about the ALSA kernel
API, although the API is fairly easy to understand...

you can find my post with a very brief explanation of pcm API in past
on this ML from the archive.  it would help a bit for understanding.


Takashi


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