[PATCH 3/3] echoaudio: Address bugs in the interrupt handling

[Mark Hills]

On Tue, 16 Jun 2020, Giuliano Pochini wrote:

> On Tue, 16 Jun 2020 14:17:43 +0100
> Mark Hills <mark at xwax.org> wrote:
> 
> > Distorted audio appears occasionally, affecting either playback
> > or capture and requiring the affected substream to be closed by
> > all applications and re-opened.
> 
> Yes, it also happens here very rarely (one time every some months) and I 
> failed to reproduce the problem.

It frustrated me for years, but I had other work I needed to do. 
Eventually I was working around it several times a day :-/
 
[...]
> > * Check for progress using the counter from the hardware, not after
> >   it has been truncated to the buffer.
> > 
> >   There appears to be a possible bug if a whole ringbuffer advances
> >   between interrupts, it goes unnoticed.
> 
> In that case the stream must be restarted anyway due to xrun.

Yes, but I think it will go unnoticed, so you don't know to re-start.

[...]
> > * Remove the accessing of pipe->dma_counter twice in the interrupt
> >   handler:
> 
> Why twice?

Yes, the interrupt handler calls pcm_pointer() directly to make a 
decision. Then it calls snd_pcm_period_elapsed(), which itself goes on to 
make another call to pcm_pointer(), by this time the bus mastering of the 
interface has adjusted the counter.

The new code processes the counter only once, and this is easier to 
rationalise whether there are bugs.

-- 
Mark