Cannot use ALSA's `dmix` output

[jessicah]

Trying to run scream with the dmix output, it fails due to an unsupported sample format:

$ ./scream -o alsa -d dmix:CARD=sndrpihifiberry,DEV=0 -v
Using ASLA output
ALSA lib pcm.c:8847:(snd_pcm_set_params) Sample format not available for PLAYBACK: Invalid argument
snd_pcm_set_params: Invalid argument

It seems to be an issue with scream setting a "default" sample rate that doesn't match the available format(s), as indicated by using the raw output and piping into aplay:

$ ./scream -o raw | aplay -D dmix:CARD=sndrpihifiberry,DEV=0
Playing raw data 'stdin' : Unsigned 8 bit, Rate 8000 Hz, Mono
aplay: set_params:1387: Sample format non available
Available formats:
- S32_LE

And indeed, when running scream with plughw instead of dmix, you can see it starting with S16_LE/44kHz, then switching to S32_LE/48kHz:

$ ./scream -o alsa -d plughw:CARD=sndrpihifiberry,DEV=0 -v -u
Using ALSA output
Plug PCM: Hardware PCM card 3 'snd_rpi_hifiberry_dacplus' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : S16_LE
  subformat    : STD
  channels     : 2
  rate         : 44100
  exact rate   : 44100 (44100/1)
  msbits       : 16
  buffer_size  : 2205
  period_size  : 735
  period_time  : 16666
  tstamp_mode  : NONE
  tstamp_type  : MONOTONIC
  period_step  : 1
  avail_min    : 735
  period_event : 0
  start_threshold  : 2205
  stop_threshold   : 2205
  silence_threshold: 0
  silence_size : 0
  boundary     : 1156055040
  appl_ptr     : 0
  hw_ptr       : 0
Starting unicast receiver
Channel 0 mapped to Front Left
Channel 1 mapped to Front Right
Plug PCM: Hardware PCM card 3 'snd_rpi_hifiberry_dacplus' device 0 subdevice 0
Its setup is:
  stream       : PLAYBACK
  access       : RW_INTERLEAVED
  format       : S32_LE
  subformat    : STD
  channels     : 2
  rate         : 48000
  exact rate   : 48000 (48000/1)
  msbits       : 32
  buffer_size  : 2400
  period_size  : 600
  period_time  : 12500
  tstamp_mode  : NONE
  tstamp_type  : MONOTONIC
  period_step  : 1
  avail_min    : 600
  period_event : 0
  start_threshold  : 2400
  stop_threshold   : 2400
  silence_threshold: 0
  silence_size : 0
  boundary     : 1258291200
  appl_ptr     : 0
  hw_ptr       : 0
Switched format to sample rate 48000, sample size 32 and 2 channels.

My goal is to be able to run both scream and squeezelite side-by-side on the same system, which would require using the dmix endpoint, otherwise I cannot run both.

[jessicah]

So I modified

scream/Receivers/unix/alsa.c

Line 106 in acb8fc7

if (setup_alsa(&ao_data.snd, SND_PCM_FORMAT_S16_LE, 44100, latency, alsa_device, ao_data.receiver_format.channels, &ao_data.channel_map) == -1) {

Changing it to (..., SND_PCM_FORMAT_S32_LE, 48000, ...), and then it was able to bind to the dmix endpoint.

Perhaps a config option, or maybe querying ALSA for the current config and available configs, so it doesn't try to set a config that the hardware device doesn't support.

I'm assuming that closing the ALSA device on runtime format changes is to avoid ALSA's mixing as well?