[alsa-devel] On non-rewindability of resamplers

[Raymond Yau]

>>  >>
>>  >> The appl_ptr  can be placed in any position in the ring buffer for
the
>>  >> application to write data but the sound card fetch data from this
ring
>>  >> buffer sequentially, however snd_pcm_write() assume the maximum
distance
>>  >> between appl_ptr and hwptr is only one buffer
>>  >>
>>  >>
>>
https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/commit/sound/pci/hda/hda_intel.c?id=2ae66c26550cd94b0e2606a9275eb0ab7070ad0e
>>  >>
>>  >> Do you mean hwptr does not decrease by one period when you use
arbitrary
>>  >> period sizes for hda-Intel  ?
>>  >
>>  >
>>  > I cannot comment on this commit. But
>> "snd_hda_intel.align_buffer_size=1" indeed existed on my kernel command
>> line (for no good reason now - so removed), and I don't use a strange
>> period size.
Loaded sound module options
!!--------------------------

!!Module: snd_hda_intel
align_buffer_size : -1
bdl_pos_adj :
32,32,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1

Why only one entry for align_buffer_size if it is specific to hda
controller when the computer have two hda controllers ?

https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/1324426

https://launchpadlibrarian.net/176540072/Symptom_PulseAudioLog.txt

May 29 06:57:49 Lihkin pulseaudio[1874]: [alsa-sink-92HD81B1X5 Analog]
alsa-sink.c: ALSA woke us up to write new data to the device, but there was
actually nothing to write!

Most likely this is a bug in the ALSA driver 'snd_hda_intel'. Please report
this issue to the ALSA developers.
We were woken up with POLLOUT set -- however a subsequent snd_pcm_avail()
returned 0 or another value < min_avail.

snd_pcm_avail() returned a value that is exceptionally large: 181132 bytes
(1026 ms).
Most likely this is a bug in the ALSA driver 'snd_hda_intel'. Please report
this issue to the ALSA developers.

Seem pulseaudio can use strange period size

May 29 11:36:52 Lihkin pulseaudio[1306]: [alsa-sink-HDMI 0] alsa-util.c:
Its setup is:
  stream       : PLAYBACK
   access       : MMAP_INTERLEAVED
   format       : S16_LE
   subformat    : STD
   channels     : 2
   rate         : 44100
   exact rate   : 44100 (44100/1)
   msbits       : 16
   buffer_size  : 3520
   period_size  : 352
   period_time  : 7981
   tstamp_mode  : ENABLE
   period_step  : 1
   avail_min    : 352
   period_event : 1
   start_threshold  : -1
   stop_threshold   : 7926335344172072960
   silence_threshold: 0
   silence_size : 0
   boundary     : 7926335344172072960
   appl_ptr     : 2916662
   hw_ptr       : 2958425

start_threshold  : -1

This mean the playback will automatically start if pulseaudio write any
audio data instead of start after write  pre buffer

Try aplay with  any arbitrary period size/period time and xrun_debug ,
check whether hw_ptr is updated on every period update in the system log

aplay  -v --period-size=352 --buffer-size=3520   -Dhw:0,0 stereo.wav

http://www.alsa-project.org/main/index.php/XRUN_Debug

/proc/asound/card#/pcm0p/xrun_debug

Replace '#' with your card number (usually 0). This proc file can enable
various debugging tools. The CONFIG_SND_PCM_XRUN_DEBUG,
CONFIG_SND_VERBOSE_PROCFS, CONFIG_SND_DEBUG options must be enabled in your
kernel (if xrun_debug proc file is present - this feature is enabled).

# Enable basic debugging, do jiffies check and dump position on each period
and hardware pointer update calls
# Usefull when the lowlevel (specific) hardware driver is somehow broken
echo 29 > /proc/asound/card0/pcm0p/xrun_debug