Concerning my attempts to use the aloop driver in a mildly complex ALSA setup, after spending hours reading ALSA code and playing with it with interactive gdb session, I have somehow progressed a little bit.

1. I have understood that because connecting one side of the snd_aloop was setting the hw params for the other side, this could explain the difference of behavior when changing the sequence of connecting the playback stream and the capture stream of the aloop.

2. I did some tweaks in my alsa conf

a. Replace the plugin used to downsample my playback stream from type 'plug' to the simpler type 'rate' because this is really just what I needed.

b. Changed the downsampled rate from 44100 to 48000 since 48000 being a multiple of 192000 has the potential to ease the hw params refinements related to the rate value.

c. Enforce multi client rate param to 192000 or else slave 'a' params were used for refining slave b params. This had the effect of using the full rate interval of slave 'a' to calculate slave 'b' buffer size [32000 192000] and for the upsampling case, the required buffer size was exceeding the client buffer size interval.

my conf now looks like this:

pcm.loop_44100_16 { type rate slave { pcm "hw:1,0,0" format S16_LE rate 48000 } }

pcm.loop_capture { type hw card 1 device 1 subdevice 0 }

pcm.split { type plug slave { pcm { type multi slaves { a { channels 8 pcm "hdmi:0,0" } # hdmi output b { channels 2 pcm "loop_44100_16" } # loopback record } bindings { 0 { slave a channel 0 } # Front Left 1 { slave a channel 1 } # Front Right 2 { slave a channel 2 } # Rear Left 3 { slave a channel 3 } # Rear right 4 { slave a channel 4 } # center 5 { slave a channel 5 } # LFE 6 { slave a channel 6 } # Side Left 7 { slave a channel 7 } # Side right 8 { slave b channel 0 } # left 9 { slave b channel 1 } # right } } rate 192000 } ttable [ [ 1 0 0 0 0 0 0 0 0.38 0 ] # Front left [ 0 1 0 0 0 0 0 0 0 0.38 ] # Front right [ 0 0 1 0 0 0 0 0 0.22 0 ] # Rear left [ 0 0 0 1 0 0 0 0 0 0.22 ] # Rear right [ 0 0 0 0 1 0 0 0 0.18 0.18 ] # center [ 0 0 0 0 0 1 0 0 0 0 ] # LFE [ 0 0 0 0 0 0 1 0 0.22 0 ] # Side left [ 0 0 0 0 0 0 0 1 0 0.22 ] # Side right ] }

I have already said it once in the past but again, by having being forced to spend hours studying the ALSA source code has made me appreciate the quality of its design and implementation but I find that requiring this type of devotion to correctly use a system may be a problem. I'm just throwing a suggestion that could reduce the problematic opacity. I did find the REFINE_DEBUG output very useful to understand what was happening to me. Maybe they could be included in the release build and outputed only when refine fails.

Concerning the last problem. Here it is 'loop_44100_16' refine sometimes works and sometimes it doesn't. Either something invisible to the param dumps is different or maybe some uninitialized variables adds some randomness to the result:

srefine 'loop_44100_16' (client) ACCESS: MMAP_INTERLEAVED MMAP_NONINTERLEAVED FORMAT: S16_LE S32_LE SUBFORMAT: STD SAMPLE_BITS: ALL FRAME_BITS: ALL CHANNELS: 2 RATE: 192000 PERIOD_TIME: (20 682667) PERIOD_SIZE: [4 131072] PERIOD_BYTES: ALL PERIODS: [2 32] BUFFER_TIME: (41 1365334) BUFFER_SIZE: [8 262144] BUFFER_BYTES: ALL TICK_TIME: ALL srefine 'loop_44100_16' (slave) ACCESS: MMAP_INTERLEAVED MMAP_NONINTERLEAVED MMAP_COMPLEX FORMAT: S16_LE SUBFORMAT: STD SAMPLE_BITS: ALL FRAME_BITS: ALL CHANNELS: 2 RATE: [48000 48001) PERIOD_TIME: (20 682667) PERIOD_SIZE: ALL PERIOD_BYTES: ALL PERIODS: ALL BUFFER_TIME: ALL BUFFER_SIZE: [2 65538) BUFFER_BYTES: ALL TICK_TIME: ALL REFINE called: ACCESS: MMAP_INTERLEAVED MMAP_NONINTERLEAVED MMAP_COMPLEX FORMAT: S16_LE SUBFORMAT: STD SAMPLE_BITS: ALL FRAME_BITS: ALL CHANNELS: 2 RATE: [48000 48001) PERIOD_TIME: (20 682667) PERIOD_SIZE: ALL PERIOD_BYTES: ALL PERIODS: ALL BUFFER_TIME: ALL BUFFER_SIZE: [2 65538) BUFFER_BYTES: ALL TICK_TIME: ALL refine done - result = 0

lano1106@Wailaba2 ~/dev/alsa_debug $ diff refine_success.txt refine_fail.txt 49c49,50 < refine done - result = 0 ---

refine done - result = -22

I think that maybe the fact that the slave buffer size min being outside the client buffer size interval could be the problem but then it should fail all the time and this would mean that there is an issue with pcm_rate.c since for rate intervals having single value both for the client and the slave I think that you cannot have both slave buffer size interval endpoints being inside the client interval.

I could be wrong as I have not yet totally assimilated the refine algorithm. I'll report back when I have nailed down this final problem.

Greetings, Olivier

On Fri, 2013-12-13 at 00:12 -0500, Olivier Langlois wrote:

I did a small experiment or maybe it was just luck but. I am getting the error messages on the playback side if I launch the playback side before the capture side.

By reversing the order, that is launch arecord before speaker-test, the errors go away but. ALSA forces me to match the sampling rate of the capture side:

lano1106@whippet2 ~ $ arecord -Dloop_capture -f S16_LE -c2 -r 44100 alsa.wav Recording WAVE 'alsa.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo

lano1106@whippet2 ~/dev/alsa-lib.git $ speaker-test -Dsplit -c8 -r192000 -F S32_LE

speaker-test 1.0.27.2

Playback device is split Stream parameters are 192000Hz, S32_LE, 8 channels Using 16 octaves of pink noise Rate set to 192000Hz (requested 192000Hz) Buffer size range from 296 to 131060 Period size range from 148 to 65476 Using max buffer size 131060 Periods = 4 Unable to set nperiods 4 for playback: Invalid argument Setting of hwparams failed: Invalid argument lano1106@whippet2 ~/dev/alsa-lib.git :( $ speaker-test -Dsplit -c8 -r44100 -F S32_LE

speaker-test 1.0.27.2

Playback device is split Stream parameters are 44100Hz, S32_LE, 8 channels Using 16 octaves of pink noise Rate set to 44100Hz (requested 44100Hz) Buffer size range from 32 to 131072 Period size range from 16 to 65536 Using max buffer size 131072 Periods = 4 was set period_size = 32768 was set buffer_size = 131072 0 - Front Left 4 - Center 1 - Front Right ^C 7 - Side Right 3 - Rear Right 2 - Rear Left 6 - Side Left 5 - LFE Time per period = 4.752270

Am I abusing the plug type plugin? I tough that it was suppose to adapt the source params with the sink expected params.

On Wed, 2013-12-11 at 01:33 -0500, Olivier Langlois wrote:

...

Hi,

I am trying to split a pcm stream and send it to my sound card and send a downmixed/downsampled copy to the playback device of the loopback card.

I am using speaker-test to test my config. the sound goes to the speakers and I am able to record the downmixed signal through the loopback capture device but I am seeing a bunch of error messages in the speaker-test output which makes me doubt that there is maybe something wrong in what I am doing.

So I am asking help from the experts:

My conf:

pcm.loop_44100_16 { type plug slave { pcm "hw:1,0,0" format S16_LE rate 44100 } }

pcm.loop_capture { type hw card 1 device 1 subdevice 0 }

pcm.split { type plug slave.pcm { type multi slaves { a { channels 8 pcm "hdmi:0,0" } # hdmi output b { channels 2 pcm "loop_44100_16" } # loopback record } bindings { 0 { slave a channel 0 } # Front Left 1 { slave a channel 1 } # Front Right 2 { slave a channel 2 } # Rear Left 3 { slave a channel 3 } # Rear right 4 { slave a channel 4 } # center 5 { slave a channel 5 } # LFE 6 { slave a channel 6 } # Side Left 7 { slave a channel 7 } # Side right 8 { slave b channel 0 } # left 9 { slave b channel 1 } # right } } ttable [ [ 1 0 0 0 0 0 0 0 0.38 0 ] # Front left [ 0 1 0 0 0 0 0 0 0 0.38 ] # Front right [ 0 0 1 0 0 0 0 0 0.22 0 ] # Rear left [ 0 0 0 1 0 0 0 0 0 0.22 ] # Rear right [ 0 0 0 0 1 0 0 0 0.18 0.18 ] # center [ 0 0 0 0 0 1 0 0 0 0 ] # LFE [ 0 0 0 0 0 0 1 0 0.22 0 ] # Side left [ 0 0 0 0 0 0 0 1 0 0.22 ] # Side right ] }

$ speaker-test -Dsplit -c8 -r192000 -F S32_LE

speaker-test 1.0.27.2

Playback device is split Stream parameters are 192000Hz, S32_LE, 8 channels Using 16 octaves of pink noise Rate set to 192000Hz (requested 192000Hz) Buffer size range from 296 to 131072 Period size range from 148 to 65536 Using max buffer size 131072 Periods = 4 was set period_size = 65536 was set buffer_size = 131072 0 - Front Left Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy 4 - Center Write error: -32,Broken pipe Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy 1 - Front Right Write error: -32,Broken pipe Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy 7 - Side Right Write error: -32,Broken pipe Write error: -32,Broken pipe Can't recovery from underrun, prepare failed: Device or resource busy

lano1106@whippet2 ~ $ aplay -l **** List of PLAYBACK Hardware Devices **** card 0: HDMI [HDA ATI HDMI], device 3: HDMI 0 [HDMI 0] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: HDMI [HDA ATI HDMI], device 7: HDMI 1 [HDMI 1] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: HDMI [HDA ATI HDMI], device 8: HDMI 2 [HDMI 2] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: HDMI [HDA ATI HDMI], device 9: HDMI 3 [HDMI 3] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: HDMI [HDA ATI HDMI], device 10: HDMI 4 [HDMI 4] Subdevices: 1/1 Subdevice #0: subdevice #0 card 0: HDMI [HDA ATI HDMI], device 11: HDMI 5 [HDMI 5] Subdevices: 1/1 Subdevice #0: subdevice #0 card 1: Loopback [Loopback], device 0: Loopback PCM [Loopback PCM] Subdevices: 8/8 Subdevice #0: subdevice #0 Subdevice #1: subdevice #1 Subdevice #2: subdevice #2 Subdevice #3: subdevice #3 Subdevice #4: subdevice #4 Subdevice #5: subdevice #5 Subdevice #6: subdevice #6 Subdevice #7: subdevice #7 card 1: Loopback [Loopback], device 1: Loopback PCM [Loopback PCM] Subdevices: 8/8 Subdevice #0: subdevice #0 Subdevice #1: subdevice #1 Subdevice #2: subdevice #2 Subdevice #3: subdevice #3 Subdevice #4: subdevice #4 Subdevice #5: subdevice #5 Subdevice #6: subdevice #6 Subdevice #7: subdevice #7

Thank you,

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