[alsa-devel] playback write error (DMA or IRQ trouble)

[Takashi Iwai]

At Mon, 2 Jun 2008 14:33:14 +0530,
Harsha priya gupta wrote:
> 
> Quick question
> 
> From my copy function after I pass the buffer to HW, what would happen if i
> call snd_pcm_period_elapsed.

It's invalid and a misdesign.

I guess you are misunderstanding about when to
callsnd_pcm_period_elapsed().  snd_pcm_period_elapsed() is called when
one period of samples on the hardware is *processed*.  It doesn't mean
that the samples are transferred to the hardware.

Suppose that you have period_size = 48000 (frames) for 48kHz samples.
Then, the first snd_pcm_period_epased() shall be called just one
second after starting the PCM stream.  The second call be another one
second later, and so on.  It doesn't matter how quick the copy to h/w
is done (via copy callback).


Takashi

> 
> On Mon, Jun 2, 2008 at 1:37 PM, Takashi Iwai <tiwai at suse.de> wrote:
> 
>     At Mon, 2 Jun 2008 13:26:01 +0530,
>     Harsha priya gupta wrote:
>     >
>     > I implemented the copy function and immediately transfered the user block
>     data
>     > to the hardware.
>     >
>     > Correct me if am wrong;
>     > .pointer implementation - passes the current buffer pointer. When the
>     .pointer
>     > function returns the size of the buffer = user buffer size logically I
>     need to
>     > expect the hardware to send an interrupt because buffer is consumed and I
>     > should call snd_pcm_period_elapsed after that.
>     >
>     > what would happen if i call the snd_pcm_period_elapsed from the pointer
>     > function once the buffer is consumed from hardware. Would that be right?
>      This
>     > is what i am trying to do
>    
>     The logic is reversed.
>     The pointer callback is a passive one that does nothing but returning
>     the current h/w buffer position.  This is called either from
>     snd_pcm_period_elapsed() or at the PCM status update.
>    
>     You must call snd_pcm_period_elapsed() somewhere in your driver
>     *explicitly* at the timing that one period is finished.  Usually, this
>     is done in an IRQ handler the h/w generates at the period ("fragment",
>     "half-buffer", or whatever) boundary.
>    
>     And note that the valid value from the pointer callback is between 0
>     and buffer_size-1 as it handles the buffer as a ring-buffer.  The
>     value buffer_size is invalid.
> 
>     Takashi
>    
>     > On Mon, Jun 2, 2008 at 1:02 PM, Takashi Iwai <tiwai at suse.de> wrote:
>     >
>     >     At Mon, 2 Jun 2008 12:39:31 +0530,
>     >     Harsha priya gupta wrote:
>     >     >
>     >     > Can anyone give me a clue as to when i would get such an error?
>     >
>     >     ... only if you give more clue what exactly you did.
>     >
>     >     In general, it implies that an interrupt isn't issued properly at PCM
>     >     period boundary.
>     >
>     >     Takashi
>     >
>     > --
>     > -Harsha
>     >
>     >
> 
> --
> -Harsha
> 
>