[alsa-devel] arecord: buffer-size dependency on period-size and other params

[Rajwa, Marcin]

Hello,

I have spotted strange behavior of alsa-lib when I try to set 
buffer-size in arecord. It looks like the size of the buffer is very 
dependent on other parameters like period-size and rate for example. At 
first it sounds like normal behaviour, after all buffer size smaller 
than period size doesn’t make much sense. However what if we go in the 
reverse direction and set buffer-size much larger than period size? Then 
I see no reason to forbid such request (at least on the user space 
level). So to be specific I issue such command: arecod -Dplughw:0,8 -r 
16000 -c 2 -f S24_LE --buffer_size 1280000 tmp.wav -vvv. The arecord 
response for this is buffer size of 31992 frames, 255936 bytes. Now let 
me also tell the buffer size of 1280000 bytes is the maximum buffer size 
our driver allows, likewise max period is 268800 bytes. Now if inside 
our driver I double max period size limit I get buffer size accordingly 
bigger. Similarly, if I change the format in the command line request 
from S24_LE to i.e S16_LE then again I will get buffer size closer to my 
request (bigger). So below are my questions:

 1. Why alsa-lib behaves in a way I described above? Why I can not
    control buffer size independently from other parameters (providing
    it has physical sense).
 2. It looks like alsa-lib first calculates initial buffer-size base on
    other input params like mentioned period size or sampling rate and
    then ask driver to refine it according to its capabilities. Is that
    the case?
 3. I also tried to dump hw capabilities by adding –dump-hw-param flag
    to my command line request and here yet another surprise – in a row
    dedicated for buffer size it says “BUFFER_SIZE [12 4294967294]”.
    Where is this “4294967294” comes from, and on what basics is it
    calculated? I would expect at the bare minimum it should ask driver
    but in the snd_pcm_hw_param_dump() function I don’t see any
    communication with the driver.

Kind Regards,

Marcin Rajwa