Re: playback audio overwritten before it is sent to the card

[Takashi Iwai <tiwai@suse.de>]

At Fri, 26 Jul 2002 13:51:01 -0700 (PDT),
Guilhem Tardy wrote:
> 
> 
> I am getting to know all the internals (although I didn't really want to)...
> the ops->pointer() is called only from snd_pcm_update_hw_ptr(), which updates
> the runtime->status->hw_ptr. But it doesn't:
> 
>   playback_copy() on sound dev0,  hwoff:0  pos:0 ->160
> 
>   playback_pointer() on sound dev0,  last:160  pos:160
>      ptr:160  {hw:0, appl:160}
>      hw_ptr_base:0  min_align:1  avail_min:160  avail_max:0
> 
> So far, so good. In snd_pcm_update_hw_ptr(), new_hw_ptr = runtime->hw_ptr_base
> + pos = 160, so the next time around it should read "hw:160"...
> 
>   playback_prepare() on sound dev0,  bsize:320  boundary:1342177280
> 
> Is it normal that boundary is so high?

the boundary is not the buffer size (although it's aligned to the
buffer size).
it's the maximal size of the buffer pointer which is countable by an
(long) integer.  the buffer pointer will be rewound to zero after it
reaches this value.


> 
>   playback_copy() on sound dev0,  hwoff:0  pos:160 ->320
> 
>   playback_pointer() on sound dev0,  last:160  pos:320  ptr:160
>      {hw:0, appl:160}
>      hw_ptr_base:0  min_align:1  avail_min:160  avail_max:320
> 
> Ouch! "hw:0"... Why didn't change? And runtime->control->appl_ptr hasn't
> changed either?

because the prepare() was called before this operation.

> 
>   playback_copy() on sound dev0,  hwoff:160  pos:0 ->160
> 
> Keep copying to the buffer in runtime->dma_area...
> 
>   playback_prepare() on sound dev0,  bsize:320  boundary:1342177280
>   playback_copy() on sound dev0,  hwoff:0  pos:160 ->320
> 
> Ouch! This one hasn't been sent to the card yet.
 
again prepare() was called.  hence, the hwoff was rewound.

but i don't know what happens here:  why prepare() is called.
does your application calls snd_pcm_prepare() so often?

basically, prepare callback is called only from snd_pcm_prepare() in
pcm_native.c, i.e. only via ioctl.  (or, in the case of OSS emulation,
via snd_pcm_oss_prepare(), which can be called multiple times at each
change of configuration).


ciao,

Takashi


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