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

[Takashi Iwai <tiwai@suse.de>]

On Wed, 17 Jun 2020 12:51:05 +0200,
Mark Hills wrote:
> 
> On Tue, 16 Jun 2020, Takashi Iwai wrote:
> 
> > On Tue, 16 Jun 2020 16:01:11 +0200,
> > Mark Hills wrote:
> > > 
> > > 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()?
> > 
> > Yes, it's more self-explanatory.  Or even better
> > snd_echo_update_substream_position() :)
> 
> Out of interest, these are static but the names are globally qualified. I 
> see this elsewhere, so I followed, but is this agreed convention?
> 
> Because it could be update_substream_position() :)

It's fine to use any name for a local function.


> > > > > +{
> > > > > +	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.
> > 
> > The problem is that the last_period calculation can be wrong if the
> > period size isn't aligned.  e.g. when period size is 44100, around the
> > boundary position, it gets a different last_period value although it
> > still in the same period.
> 
> Agreed, yes.
> 
> In which case I think it's clearer to not infer anything about periods 
> from the current counter or position, and (effectively) accumulate it.
> 
> Would you please make suggestions on the code below?
> 
> Because it allowed for further simplification whilst fixing the bugs.
> 
> In the end, modulo became clearer than loops and I think it has the bonus 
> of being less risky should the counter make a large step.
> 
> I'll run for a longer test today.
> 
> ---
> 
> /* Update software pointer to match the hardware
>  *
>  * \pre chip->lock is held
>  */
> static void snd_echo_update_substream_position(struct echoaudio *chip,
> 					struct snd_pcm_substream *substream)
> {
> 	struct snd_pcm_runtime *runtime = substream->runtime;
> 	struct audiopipe *pipe = runtime->private_data;
> 	u32 counter, step;
> 	size_t period_bytes;
> 
> 	if (pipe->state != PIPE_STATE_STARTED)
> 		return;
> 
> 	period_bytes = frames_to_bytes(runtime, runtime->period_size);
> 
> 	counter = le32_to_cpu(*pipe->dma_counter);
> 
> 	step = counter - pipe->last_counter;  /* handles wrapping of counter */
> 	step -= step % period_bytes;  /* acknowledge whole periods only */
> 
> 	if (step == 0)
> 		return;  /* haven't advanced a whole period yet */
> 	pipe->last_counter += step;  /* does not always wrap on a period */
> 	pipe->position += step;
> 
> 	/* wraparound the buffer */
> 	pipe->position %= frames_to_bytes(runtime, runtime->buffer_size);
> 
> 	/* notify only once, even if multiple periods elapsed */
> 	spin_unlock(&chip->lock);
> 	snd_pcm_period_elapsed(substream);
> 	spin_lock(&chip->lock);
> }
> 
> static irqreturn_t snd_echo_interrupt(int irq, void *dev_id)
> {
> 	struct echoaudio *chip = dev_id;
> 	int ss, st;
> 
> 	spin_lock(&chip->lock);
> 	st = service_irq(chip);
> 	if (st < 0) {
> 		spin_unlock(&chip->lock);
> 		return IRQ_NONE;
> 	}
> 	/* The hardware doesn't tell us which substream caused the irq,
> 	thus we have to check all running substreams. */
> 	for (ss = 0; ss < DSP_MAXPIPES; ss++) {
> 		struct snd_pcm_substream *substream;
> 
> 		substream = chip->substream[ss];
> 		if (substream)
> 			snd_echo_update_substream_position(chip, substream);
> 	}
> 	spin_unlock(&chip->lock);
> 
> #ifdef ECHOCARD_HAS_MIDI
> 	if (st > 0 && chip->midi_in) {
> 		snd_rawmidi_receive(chip->midi_in, chip->midi_buffer, st);
> 		dev_dbg(chip->card->dev, "rawmidi_iread=%d\n", st);
> 	}
> #endif
> 	return IRQ_HANDLED;
> }
> 
> static snd_pcm_uframes_t pcm_pointer(struct snd_pcm_substream *substream)
> {
> 	struct snd_pcm_runtime *runtime = substream->runtime;
> 	struct audiopipe *pipe = runtime->private_data;
> 
> 	return bytes_to_frames(runtime, pipe->position);

I guess this misses the update of the precise position; in your code,
pipe->position gets updated only with the period size at irq handler.


IMO, we should have the code like:

static bool update_stream_position(struct snd_pcm_substream *substream)
{
	// update pipe->position and others, returns true if period elapsed
}

static irqreturn_t snd_echo_interrupt()
{
	spin_lock(&chip->lock);
	....
	if (update_stream_position(substream)) {
		spin_unlock(&chip->lock);
		snd_pcm_period_elapsed(substream);
		spin_lock(&chip->lock);
	}
	....
	spin_unlock(&chip->lock);
	return IRQ_HANDLED;
}

static snd_pcm_uframes_t pcm_pointer(struct snd_pcm_substream *substream)
{
	....
	update_stream_position(substream);
	return bytes_to_frames(runtime, pipe->position);
}


Note that snd_pcm_period_elapsed() isn't needed (must not be done)
from the pointer callback.  The PCM core would detect and handle
properly if the period gets elapsed there.


thanks,

Takashi