Re: trigger problems.

[Takashi Iwai <tiwai@suse.de>]

At Wed, 14 Jul 2004 13:52:45 +0100,
James Courtier-Dutton wrote:
> 
> I am writing a alsa driver for the audigyls and p16v.
> The output on these sound cards can work in 1 of two ways.
> 1) 1 substream with 8 channels, and one interrupt request per period.
> For 5.1 output, we just use alsa.conf to map the user's application 
> channels into the correct channels of the 8 channel substream.
> 2) 4 substreams with 2 channels each.
> For 5.1 output, we just use 3 substreams, front, rear, center_lfe.
> The problem with this is that we then have a separate interrupt per 
> substream, resulting in 3 times more interrupts. The substreams are also 
> not in sample sync with each other, because the trigger for each 
> substream is executed one after the other, instead of in one go.
> 
> I don't know why exactly, maybe some audio experts can help, but (1) 
> sounds a lot better than (2). I would not think that loosing sample sync 
> would make so much difference, but that is the only thing that I see 
> different between (1) and (2).
> 
> So, because of this, I have a feature request for alsa-driver.
> If a user app opens the sound device in such a way that a single stream 
> from the app gets converted into multiple substreams to the same sound 
> card hardware, can we have a single trigger call for all substreams.
> 
> If we could have a single trigger call, I could start all the hardware 
> substreams at the same time with a single outl() call.
> 
> To achieve this, I suggest we change the current:
> static int snd_pcm_trigger_playback(
>          snd_pcm_substream_t *substream,
>          int cmd)
> to
> static int snd_pcm_trigger_playback(
>          snd_pcm_substream_t **substream,
> 	int number_of_substreams,
>          int cmd)
> 
> 
> Any comments ?

We can do it with linked substreams.
For example, see ice1724 driver (snc_ice1724_pcm_trigger).

Basically, the trigger callback checks each substream belonging to the
called substream via snd_pcm_group_for_each().  If it's one of yours,
stamp it via snd_pcm_trigger_done(), so that the trigger callback to
this substream will be skipped.

BTW, both two cases above can coexist.
ice1724 drivers has two PCM devices, one for multi-channel up to 8 and
another for three indepdent 2ch-stereo streams.


Takashi


-------------------------------------------------------
This SF.Net email sponsored by Black Hat Briefings & Training.
Attend Black Hat Briefings & Training, Las Vegas July 24-29 - 
digital self defense, top technical experts, no vendor pitches, 
unmatched networking opportunities. Visit www.blackhat.com