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

2 Jun 2008

      Say if my hardware is such that it shall interrupt only after it has
processed entire sample and not ever period or sample. What will ensure that
i get my next buffer down? Will calling the snd_pcm_period_elapsed in the
interrupt function help?
On Mon, Jun 2, 2008 at 2:54 PM, Takashi Iwai tiwai@suse.de wrote:
...
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@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@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
-- 
-Harsha