Re: [PATCH 5/6 v2] sound: Add n64 driver

Takashi Iwai
2021-01-13T12:05:32Z

On Wed, 13 Jan 2021 12:57:16 +0100,
Lauri Kasanen wrote:
>
> On Mon, 11 Jan 2021 16:25:08 +0100
> Takashi Iwai <tiwai@suse.de> wrote:
>
> > On Mon, 11 Jan 2021 13:02:22 +0100,
> > Lauri Kasanen wrote:
> > > This order gives correct pointer advancing etc, but now it's hitting a
> > > new problem: the pcm core is reusing the buffer from under the audio
> > > card. It's writing new data to the area that is currently being read by
> > > DMA.
> >
> > Could you elaborate? I still don't get what's going on there.
>
> I figured it out. Turns out the hw registers were double-buffered in a
> way that requires two periods' worth of buffers. The IRQ fires when one
> buffer is finished and another is queued, not when everything is
> finished as I first thought.
>
> There doesn't seem to be a way to request the PCM core to keep two
> periods' distance instead of one? I will deploy memcpy then.

We may return to the first approach, i.e. just use nextpos. But then
snd_pcm_period_elapsed() has to be called right after the trigger
callback without the IRQ, because the trigger START already queued the
full period and the position advances. So the first period-elapsed
has to be called from a work or such offload instead of IRQ.
In anyway, it's a bit tricky, yeah.

Takashi