Re: Re: [Alsa-user] AD1985 full-duplex(?)

[Jaroslav Kysela <perex@suse.cz>]

On Mon, 6 Sep 2004, Adam Tla/lka wrote:

> IMHO DMA mmaped mode in ALSA is not as easy to use as OSS DMA mode.
> All we need is theoretically only the hw pointer position in DMA ring buffer 
> to properly mix data from different events. But in ALSA we must
> calculate it doing additional calls. Some functions like snd_pcm_delay
> in normal DMA mode seems to be not needed. An app needs only ring buffer
> hw position. Doing normal system call it can obtain current system time
> so it can calculate the delay and correct sample position to mix.

There is always a drift between system and card's timing.

> In ALSA we should move appl pointer using mmap_begin and then mmap_commit

The appl_ptr is not moved in mmap_begin. The mmap_begin function only 
returns the actual position to ring buffer based on appl_ptr.

> functions to avoid buffer underrun and calculate hw pointer manually
> to know where mix the new data. So I think there is more to do then in
> OSS case and more to do in the wrong way. I will send my sources
> after more testing.

We need the intelligent mmap transport for additional functionality 
(conversions, resampling etc.). I think it's the proper way and nobody 
(except OSS emulation) has the useability problem. You can still move 
backwards (although it is problematic when you have a mixing on the path).

> I have some questions. If I get delay < 0 and then do
> err = snd_pcm_forward(pcm,-delay) what err returned value really means
> (I am using dmix and of course rate plugins)?
> How interpret err <= 0 value?

err < 0 -> error code
err == 0 -> no action (appl_ptr was unchanged in case when you're already 
behind hw_ptr)

> Also what is the meaning of snd_pcm_available > alsa.buffer_size?

You're too late, so you need to fill (or better skip via forward) some 
more samples to synchronize appl_ptr.

						Jaroslav

-----
Jaroslav Kysela <perex@suse.cz>
Linux Kernel Sound Maintainer
ALSA Project, SUSE Labs


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