Our hardware saves 1024 frames of 16 bit stereo. I wrote an ALSA driver for it. PCM data is captured using arecord and saved to a wav file.
arecord -Dhw:0,0 -t wav -f S16_LE -r 44100 -c 2 -d 2 -v rec.wav
For debugging, the hardware generates a ramp of 2048 values in bits 11:0 with the ISR counter in bits 15:12. The wav file is viewed in audacity.
Expected pcm for 8 periods:
Ramp start end
0h, 7ffh
1800h, 1fffh
2000h, 27ffh
3800h, 3fffh
4000h, 47ffh
5800h, 5fffh
6000h, 67ffh
7800h, 7fffh
As seen in audacity
0h, 7ffh
0h, 7ffh
2000h, 27ffh
2000h, 27ffh
4000h, 47ffh
4000h, 47ffh
6000h, 67ffh
6000h, 67ffh
PCM ramp in audacity
The 1st period of pcm is duplicated twice. The big jump in the ramp indicates that the 2nd period of pcm is not present. The 3rd period of pcm is duplicated. The 4th period of data is is not present. The 5th period of pcm is duplicated. The 6th period of data is is not present.
In struct snd_pcm_hardware, I have:
.channels_min = 2,
.channels_max = 2,
.buffer_bytes_max = (8*4096),
.period_bytes_min = 4096,
.period_bytes_max = (2*4096),
.periods_min = 1,
.periods_max = 2,
Since there's no DMA, I use struct snd_pcm_ops .copy_user to copy data to an ALSA buffer passed in .copy_user.
I tried the settings below, but arecord had an error.
.periods_min = 1,
.periods_max = 1,
arecord: set_params:1411: Can't use period equal to buffer size 1024 == 1024)
I tried increasing period_max to 4, and it resulted in 4 duplicates
.period_bytes_max = (4*4096),
.periods_min = 1,
.periods_max = 4,
As seen in audacity
0h, 7ffh
0h, 7ffh
0h, 7ffh
0h, 7ffh
4000h, 47ffh
4000h, 47ffh
4000h, 47ffh
4000h, 47ffh
I verified that the ramp values are as expected in the ISR handler. In .copy_user, I verified that the pos argument isn't the same as the previous copy_user call and that the ramp values are as expected. Why is it duplicating? What can I do to fix it? Thanks in advance.
Added: Info printed by arecord
Recording WAVE 'rec.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
Bufsz 8192, srate 44100, nch 2
Opening PCM
PCM Prepare
Format 2
Rate 44100
Channels 2
Buffer size 2048
Period size 1024
stream : CAPTURE
access : RW_INTERLEAVED
format : S16_LE
PCMptr 0
subformat : STD
channels : 2
rate : 44100
exact rate : 44100 (44100/1)
msbits : 16
buffer_size : 2048
period_size : 1024
period_time : 23219
tstamp_mode : NONE
tstamp_type : MONOTONIC
period_step : 1
avail_min : 1024
period_event : 0
start_threshold : 1
stop_threshold : 2048
silence_threshold: 0
silence_size : 0
boundary : 4611686018427387904
appl_ptr : 0
hw_ptr : 0
question from:
https://stackoverflow.com/questions/65948007/a-period-of-pcm-data-from-alsa-is-duplicated 
