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

[Mark Hills <mark@xwax.org>]

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