From mboxrd@z Thu Jan  1 00:00:00 1970
From: Clemens Ladisch <clemens@ladisch.de>
Subject: Re: _mmap_playback_avail() short
Date: Mon, 28 Nov 2011 22:12:53 +0100
Message-ID: <4ED3F955.4090904@ladisch.de>
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To: Alan Horstmann <gineera@aspect135.co.uk>
Cc: Takashi Iwai <tiwai@suse.de>, alsa-devel@alsa-project.org
List-Id: alsa-devel@alsa-project.org

Alan Horstmann wrote:
>> It seems that at a period boundary event snd_pcm_mmap_playback_avail() may
>> not return a full period size, but is a few bytes short.  If avail_min is
>> set to one whole period (which Portaudio at present does),
>> snd_pcm_direct_poll_revents() zeroes the revent due to the check of
>> snd_pcm_mmap_playback_avail() against avail_min.  POLLOUT is therefore not
>> indicated, and the app does not write any data, causing an Xrun.

This sounds like a bug in the rate plugin.  snd_pcm_rate_sync_hwptr()
tries to properly align pointer values at period boundaries, but its
algorithm doesn't work when a period does not start at the buffer start
(i.e., when the number of periods is not an integer), and this
restriction is not actually enforced by snd_pcm_rate_hw_refine_cchange().

As a workaround, try snd_pcm_hw_params_set_periods_integer().


Regards,
Clemens