On Tue, 16 Jun 2020, Takashi Iwai wrote:
> On Tue, 16 Jun 2020 15:17:43 +0200,
> Mark Hills wrote:
> >
> > +/* Update software pointer to match the hardware
> > + *
> > + * Return: 1 if we crossed a period threshold, otherwise 0
> > + */
> > +static int snd_echo_poll_substream(struct snd_pcm_substream *substream)
>
> This is a bit confusing name. One would imagine from the function
> name as if it were about the handling of poll() syscall.
Poll felt intuitive to me; maybe from FreeBSD where network drivers can
poll on a timer instead of interrupts. I do know about poll(), though.
How about snd_echo_update_substream()?
> > +{
> > + struct snd_pcm_runtime *runtime = substream->runtime;
> > + struct audiopipe *pipe = runtime->private_data;
> > + unsigned counter, step, period, last_period;
> > + size_t buffer_bytes;
> > +
> > + if (pipe->state != PIPE_STATE_STARTED)
> > + return 0;
> > +
> > + counter = le32_to_cpu(*pipe->dma_counter);
> > +
> > + period = bytes_to_frames(runtime, counter)
> > + / runtime->period_size;
> > + last_period = bytes_to_frames(runtime, pipe->last_counter)
> > + / runtime->period_size;
> > +
> > + if (period == last_period)
> > + return 0; /* don't do any work yet */
> > +
> > + step = counter - pipe->last_counter;
> > + pipe->last_counter = counter;
>
> Dose this calculation consider the overlap of 32bit integer of the
> counter? (I'm not sure whether the old code did it right, though.)
I believe it does, since (small - big) as unsigned gives small value. And
period is checked only for equality (not greater than). I'll add a comment
as such. I have run it with long streams.
Would it be clearer, or should it be that buffer_bytes to "unsigned"?
Though size_t conveys that it is a byte length, in memory.
In general I haven't deviated from existing code without need to, so I
inherited these types.
The same goes for the pattern of calculating "step" with unsigned values,
and then using a loop to wrap it to the buffer:
while (pipe->position >= buffer_bytes)
pipe->position -= buffer_bytes;
I've assumed this was a recognised pattern in ALSA code, preferred over
modulus.
--
Mark
