[alsa-devel] plughw behaviour not consistent with hw

[Takashi Sakamoto]

Hi,

On 2018/11/01 13:44, Stefano Antonelli wrote:
> Does anyone know why plughw might be acting the way it is and how I can
> get it to deliver 10ms periods?  Equally appreciated would be any tips
> on how to debug this problem.

The 'plughw' PCM node of alsa-lib configuration space is assigned to
'plug' PCM plugin[1]. This PCM plugin can aggregate several PCM plugins
to perform resampling. For example, in a below case, two PCM plugins
(rate/route conversion PCM plugins) are aggregated by 'plug' PCM plugin
in front of 'hw' PCM plugin which direct accesses to ALSA PCM chracter
device.

$ LC_ALL=C arecord -v -Dplughw:1,0 /dev/null -fS16_LE
Recording WAVE '/dev/null' : Signed 16 bit Little Endian, Rate 8000 Hz, Mono
Plug PCM: Rate conversion PCM (48000, sformat=S16_LE)
Converter: libspeex (external)
Protocol version: 10002
Its setup is:
   stream       : CAPTURE
   access       : RW_INTERLEAVED
   format       : S16_LE
   ...
Slave: Route conversion PCM (sformat=S16_LE)
   Transformation table:
     0 <- 0*0.5 + 1*0.5
Its setup is:
   stream       : CAPTURE
   access       : MMAP_INTERLEAVED
   format       : S16_LE
   ...
Slave: Hardware PCM card 1 'HDA Intel PCH' device 0 subdevice 0
Its setup is:
   stream       : CAPTURE
   access       : MMAP_INTERLEAVED
   format       : S16_LE
   ...


Well, for capture direction, the most of PCM plugins in
alsa-lib/alsa-utils are programmed to do resampling in .avail_update()
callback of plugin framework in alsa-lib. The number of available PCM
frames is calculated in this timing. On the other hand, poll(2) system
call is executed against ALSA PCM character device. Even if poll(2)
returns to notify available PCM frames in the device, actual number of
available frames is a summary of resampling done by the
aggregated PCM plugins. As a result, you can see the behaviour because
plugins possibly have buffers for count gap between in/out frames.

[1] 
http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/conf/alsa.conf;h=bb00ff409cd9643b01e2d3f8e36dc9571ddb0c85;hb=HEAD#l193


Regards

Takashi Sakamoto