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.