Re: Using snd_async_add_pcm_handler

[Takashi Iwai <tiwai@suse.de>]

At Tue, 12 Oct 2004 22:02:25 -0300 (CLST),
Eric Ross wrote:
> 
> Hello there
> 
> I've using ALSA to develop a little video player for the dillo browser, 
> also using the ffmpeg library.
> 
> I based the audio part in the pcm.c example found in the library source, 
> but I have some questions about how alsa works. I'm using 
> SND_PCM_ACCESS_RW_INTERLEAVED access.
> 
> Currently I'm using snd_async_add_pcm_handler to make ALSA call some 
> function when it needs data to play. I've read somewhere that before 
> calling snd_pcm_start() one need to feed some data into the buffer or I'll 
> get an error. So I have a call to snd_pcm_writei before calling 
> snd_pcm_start. The problem is that I cant figure out how much data I need 
> to write: in the pcm.c example, it does a loop to write 2*period_size 
> frames before calling _start(), so I do the same thing, but I'd like to 
> know the real reason (and make sure that my player will work in any 
> situation).

It depends on the setting your app does.  Usually for the playback (in
so-called "push style" programming), the app feeds data as much as
possible (= buffer_size as consequence) before starting, to be more
robust for the scheduling latency.

BTW, snd_async_add_pcm_handler() isn't recommended so much.
If possible, use a dedicated thread for the audio processing,
instead.


> Also, I've ported the player to FamiliarLinux running in a IPaq, and it 
> runs ok without sound, but when starting to write data to alsa handler, I 
> just get a "Broken pipe" error.

-EPIPE is the buffer xrun.  The DMA is short of data.


Takashi


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