[alsa-devel] playback write error (DMA or IRQ trouble)
tiwai at suse.de
Mon Jun 2 11:24:56 CEST 2008
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).
> 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
> > to the hardware.
> > Correct me if am wrong;
> > .pointer implementation - passes the current buffer pointer. When the
> > 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?
> > 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.
> > 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
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