[alsa-devel] io-plugin does not call start callback function

Takashi Iwai tiwai at suse.de
Tue Dec 8 12:21:55 CET 2009

At Sun, 06 Dec 2009 23:02:00 +0100,
Stefan Schoenleitner wrote:
> Hi,
> Raul Xiong wrote:
> > Hi Stefan
> > 
> > This is because your pointer callback function is not implemented properly.
> > "pointer" callback function should return the right position of the
> > playback/capture buffer, apps over alsa framework will call
> > snd_pcm_avail_update to get the position.
> > Also, you should call snd_pcm_ioplug_set_param_minmax in your
> > play_hw_constraint function to define the buffer size relevant parameters.
> after a full day of experimenting and trying to find out why the start
> callback function is not called I observed something interesting:
> It seems that for the io plugin there is an application pointer
> io->appl_ptr and the hardware pointer io->hw_ptr.
> Once playback is started, the start callback function is *not* called,
> but instead data transfer is started immediately.
> More precisely, each time the pointer callback function is called to get
> the current position of the hardware pointer and after that the transfer
> function is called to do the actual data transfer.
> After each call of the transfer function the number of transfered frames
> is returned (i.e. the number of frames that have been written to some
> buffer we would like to fill behind a file descriptor).
> However, as data transfers have to be timed precisely, we can not just
> increment the hw_ptr from inside the transfer callback function.
> If the hw_ptr is not touched at all (thus it stays 0 at all times), a
> few more transfers are performed until the application pointer appl_ptr
> has reached a certain (high) amount of "transfered" frames.
> Then, which is most interesting, the start callback function *is* called
> followed by calls to the pointer and polling/revent callback functions.
> As I am not incrementing the hw_ptr at any place right now, the revent
> polling function is called endlessly.
> After a full day of even more experiments I think I know how it is
> supposed to work:
> In the beginning a certain amount of frames (I guess this is the
> "start_threshold") is transfered (or more precisely prefilled in the
> output buffer).
> As soon as this start threshold is reached the start callback function
> is called and alsa waits until the hardware pointer hw_ptr has reached a
> certain position so that the next data transfers can be performed.
> For this reason it seems that the hw_ptr can be seen as the pointer that
> indicates how many frames already have been received (and maybe even
> processed) at the receiving end.
> Besides it also polls the filedescriptor(s) to check if the next data
> transfers can be performed (without blocking actually).
> At the moment I'm not sure if my assumptions above are close enough to
> reality, but at least I think that I understand what is going on now.

Yeah, your guess is correct.
This "two pointers" concept is applied in general to all ALSA PCM core
(including the kernel driver).

> If something like this would have been in the alsa documentation I guess
> it would have saved me like a week of work.

What about contributing a patch for documentation?
It'd save another week of someone else ;)

Seriously, "better documented" can be done only if you know where and
how to improve.  Developers know already what's that and why this.
So, it's really appreciated if anyone else can check and update the



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