[alsa-devel] what is exactly pcm.boundary?

[Takashi Sakamoto]

Hi,

On Jan 6 2017 03:10, Ricardo Biehl wrote:
> Well, in src/pcm/pcm_local.h  line ~469:
>
> 1	static inline snd_pcm_uframes_t snd_pcm_mmap_playback_avail(snd_pcm_t *pcm)
> 2	{
> 3		snd_pcm_sframes_t avail;
> 4		avail = *pcm->hw.ptr + pcm->buffer_size - *pcm->appl.ptr;
> 5		if (avail < 0)
> 6			avail += pcm->boundary;
> 7		else if ((snd_pcm_uframes_t) avail >= pcm->boundary)
> 8			avail -= pcm->boundary;
> 9		return avail;
> 10	}
>
> First, I assumed that both hw_ptr and appl_ptr always points to addresses inside
> a REGION of memory that is exactly the size of buffer = (period_size * periods).

No. Both of the 'appl_ptr' and 'hw_ptr' is not within the size of 
buffer. They round up to the size of boundary. The size of boundary is 
calculated in these lines.

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/sound/core/pcm_native.c?h=v4.10-rc2#n590

590	while (runtime->boundary * 2 <= LONG_MAX - runtime->buffer_size)
591		runtime->boundary *= 2;

You can also see current value of the boundary in PCM substream via 
procfs, like:
$ cat /proc/asound/card0/pcm0p/sub0/sw_params
tstamp_mode: ENABLE
period_step: 1
avail_min: 1088
start_threshold: 18446744073709551615
stop_threshold: 4971973988617027584
silence_threshold: 0
silence_size: 0
boundary: 4971973988617027584

You can see it's not the size of allocated memory because it's quite large.

The size of boundary is always multiple of the size of period in 
intermediate buffer for user space, up to UINT_MAX. Both of 'hw_ptr' and 
'appl_ptr' means total number of processed PCM frames in current PCM 
substream. They're not directly related to dedicated buffer for data 
transmission.


Regards

Takashi Sakamoto