Re: DMA, substream, frame, channel - how is this all related

[Giuliano Pochini <pochini@shiny.it>]

On Fri, 02 Mar 2007 22:57:39 +0100
Matthias Nyffenegger <matthias.nyffenegger@bluewin.ch> wrote:

> Hi,
> 
> I am trying to write an alsa driver for my audiowerk8 soundcard.
> The aw8 uses a Philips saa7146 chipset. There are 2 capture and
> 8 playback channels mapped on the saa7146's DMA as follows:
> 
> DMA1: 2xcapture
> DMA2: 4xplayback
> DMA3: 4xplayback
> 
> Sample format is 16bit.
> 
> Frame format is as follows:
> (A number corresponds to a channel, a stereo channel-pair is embraced
> by [] and 1 digit corresponds to one sample byte)
> 
> capture channels [0,1] on DMA1:        00--11--
> playback channels [2,3],[4,5] on DMA2: 22443355
> playback channels [6,7],[8,9] on DMA3: 66887799
> 
> I guess this is interleaved format.

Not exactly. The interleaved format used by ALSA and most hardware is that
the data of each audio frame is written requentially [0,1,2,3,...]. In you
case the data is interleaved, but the channel order is different [0,2,1,3].
It means that if you try to play a stereo sound through DMA2 the sound is
sent to the left channels of the two stereo pairs. You can solve this
problem later.


> Since I don't really understand the relation between DMA, substreams,
> frames and channels I wonder how a alsa substreams map best to this
> configuration:

DMA channels depend on the hardware. A substream is a logical thing. You
have to decide their relation.


> 1) 1 substream per DMA

The simplest way. The downside is that who opens the device monopolyzes all
4 channels regardless how may it really uses.


>    -> can alsa handle frames with more than 2 channels, and if yes, how?

You have to set it, just like any other parameter, constraint, capability:

substream->runtime->hw.channels_max = n;


> 2) 1 substream per stereo channel-pair

It's possible only if a DMA channel can handle two independent substreams.
For example, can only one of the spereo pair play something (while the other
is stopped/paused) ?  Other hw limitations may apply (eg. the sample rate
and the playback volume may be the same for all 4 channels).


> 3) 1 substream per channel

Same as above.


--
Giuliano.

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