[alsa-devel] problems in getting rid of set_xfer_align

Kai Vehmanen kvehmanen at eca.cx
Mon Oct 19 22:39:30 CEST 2009

Hello all,

here's one problem that I stumbled on with the deprecated 
snd_pcm_sw_params_set_xfer_align() function. I'm not sure if anything 
needs to be done to this, but at least FYI for other app developers that 
might hit the same issue.

In short, ecasound-2.7.0 was just released, and as one of the last 
commits, I dropped the call to snd_pcm_sw_params_set_xfer_align() as it 
has been deprecated since early 2008 in ALSA. Previously Ecasound had 
always set the xfer_align to 1.

Anyways, right after the release I got an error report (from Pierre, 
cc'ed) about bad glitches in audio with the new release, and the problem 
now appears to be deprecation of set_xfer_align. To summarize:

  - app relies on xfer_align=1, and has previously set it explicitly
  - in kernels before 2.6.25, the xfer_align is initialized to
    period_size in pcm_native.c and to 1 in pcm_oss.c; but in 2.6.25
    the field was removed altogether:
  - similar change was done to alsa-lib 1.0.16:

Now the problematic combination is:
  - new application (not anymore calling set_xfer_align())
  - old alsa-lib and kernel (still initializing xfer_size)
  - application used with an ALSA PCM that is strict
    about what period-sizes can be used (e.g. in the specific
    case we found out, ALSA PCM wouldn't accept a power-of-two
    period size)

This is common enough that some workaround is needed in apps, an in 
Ecasound's case case, I'll add some ifdef-magic to 2.7.1 so that 
xfer_align is still set if compiled against an older alsa-lib. That should 
cover majority of the cases...

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