There is considered by some to be a duplex bug in alsa-OSS, which affects for
example audacity, which uses portaudio. As this affects myself very much I
have investigated.
The cause is that the OSS ioctl SNDCTL_DSP_CHANNELS cannot set separately the
playback and capture channels. With the original OSS this was not a problem
AFAICT because separate playback and capture devices are created. However,
with the alsa-emulation, only a single device is created. Thus it is not
possible to record 4 chs while playing 2.
It seems to me that some method of separately setting playback and capture
channels should be included in the emulation. This could be through new
ioctls eg SNDCTL_DSP_PCHANNELS and SNDCTL_DSP_CCHANNELS; I have devised an
alternative using the existing SNDCTL_DSP_CHANNELS, by passing 'channels'
calculated as (playback_channels + 256 * capture_channels) so that in effect
the first 8 bits are playback, the 2nd 8 bits are capture. The result is
compatible with the existing, as numbers < 255 are treated in the existing
way, setting playback and capture the same if the sub-streams are active.
Thus it has simply added an enhanced mode.
Diff is attached; I wonder what your thoughts are on this?? I have been able
to very simply patch portaudio to use this facility and the system now works
great.
Regards Alan
--- alsa-kernel/core/oss/pcm_oss-orig.c 2006-04-13 11:51:24.000000000 +0100
+++ alsa-kernel/core/oss/pcm_oss.c 2006-06-19 11:08:07.000000000 +0100
@@ -1156,25 +1156,67 @@
return substream->runtime->oss.rate;
}
+static int snd_pcm_oss_get_substream_channels(struct snd_pcm_oss_file *pcm_oss_file, int substrindx)
+{
+ struct snd_pcm_substream *substream;
+ int err = -1;
+
+ if (substrindx < 0 || substrindx > 1)
+ return err;
+ substream = pcm_oss_file->streams[substrindx];
+ if (substream == NULL)
+ return 0; /* Don't complain, consider as zero active channels */
+ return substream->runtime->oss.channels;
+}
+
static int snd_pcm_oss_set_channels(struct snd_pcm_oss_file *pcm_oss_file, unsigned int channels)
{
- int idx;
+ struct snd_pcm_substream *substream;
+ int playback_channels, capture_channels, channels_set = -1;
+
if (channels < 1)
channels = 1;
- if (channels > 128)
+ if (channels > 65025)
return -EINVAL;
- for (idx = 1; idx >= 0; --idx) {
- struct snd_pcm_substream *substream = pcm_oss_file->streams[idx];
- struct snd_pcm_runtime *runtime;
- if (substream == NULL)
- continue;
- runtime = substream->runtime;
- if (runtime->oss.channels != channels) {
- runtime->oss.params = 1;
- runtime->oss.channels = channels;
+
+ playback_channels = channels & 255;
+ /* lower 8 bits always set playback channels */
+ if (playback_channels > 128)
+ return -EINVAL;
+ substream = pcm_oss_file->streams[SNDRV_PCM_STREAM_PLAYBACK];
+ printk("SND OSS: p/b chs %i\n", playback_channels);
+ if (substream != NULL)
+ if (substream->runtime->oss.channels != playback_channels) {
+ substream->runtime->oss.params = 1;
+ substream->runtime->oss.channels = playback_channels;
}
+
+ substream = pcm_oss_file->streams[SNDRV_PCM_STREAM_CAPTURE];
+ if (channels >255) { /* Enhanced channel set mode */
+ capture_channels = channels / 256 & 255;
+ /* 2nd 8 bits set capture channels */
+ if (capture_channels > 128)
+ return -EINVAL;
+ if (substream != NULL)
+ if (substream->runtime->oss.channels != capture_channels) {
+ substream->runtime->oss.params = 1;
+ substream->runtime->oss.channels = capture_channels;
+ }
+ channels_set = snd_pcm_oss_get_substream_channels(
+ pcm_oss_file, SNDRV_PCM_STREAM_PLAYBACK)
+ + 256 * snd_pcm_oss_get_substream_channels(
+ pcm_oss_file, SNDRV_PCM_STREAM_CAPTURE);
+ } else { /* Standard mode, same num channels play and capture */
+ if (substream != NULL)
+ if (substream->runtime->oss.channels != channels) {
+ /* Capture channels same as playback */
+ substream->runtime->oss.params = 1;
+ substream->runtime->oss.channels = channels;
+ }
+ channels_set = snd_pcm_oss_get_channels(pcm_oss_file);
}
- return snd_pcm_oss_get_channels(pcm_oss_file);
+ printk("SND OSS: chs set %i\n", channels_set);
+ return channels_set;
}
static int snd_pcm_oss_get_channels(struct snd_pcm_oss_file *pcm_oss_file)
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