[alsa-devel] [PATCH] Fix JACK plugin buffer sizes

[John McKay]

I was having a conversation on the Archlinux forums[1] about using the
alsa-plugins JACK plugin. While one of the bugs that we found has
already been fixed in the git alsa-lib, there is another bug relating
to buffer sizes.

Currently snd_pcm_hw_params_set_buffer_size_near() doesn't really work
when you are using this plugin unless you happen to request a buffer
size that almost exactly matches a whole multiple of periods that the
JACK is asking for. After some debugging, I discovered the reason is
that some plugins, notably the 'plug' plugin that most everybody is
going to use to change frequency/format directly attempt to manipulate
the buffer size. This causes the code in pcm_ioplug.c to fail because it
cannot find a number of periods/period time that can fit the buffer.

The included patch stops this by specifying the list of acceptable
buffer sizes as well, so that when other plugins try to change the
buffer it always picks one that will work. It does have some problems
in the JACK plugin now offers a number of periods/sizes that don't
exist but in practice asking for huge buffers just seems to give you a
more sane value.

[1] https://bbs.archlinux.org/viewtopic.php?id=250116

--- jack/pcm_jack.c	2019-05-10 06:57:24.000000000 +0000
+++ jack/pcm_jack.c	2019-10-27 03:43:36.053734268 +0000
@@ -428,6 +428,7 @@
 	unsigned int format = SND_PCM_FORMAT_FLOAT;
 	unsigned int rate = jack_get_sample_rate(jack->client);
 	unsigned int psize_list[MAX_PERIODS_MULTIPLE];
+	unsigned int bsize_list[MAX_PERIODS_MULTIPLE];
 	unsigned int nframes = jack_get_buffer_size(jack->client);
 	unsigned int jack_buffer_bytes = (snd_pcm_format_size(format, nframes) *
 					  jack->num_ports);
@@ -441,6 +442,9 @@
 	for (i = 1; i <= ARRAY_SIZE(psize_list); i++)
 		psize_list[i-1] = jack_buffer_bytes * i;
 
+	for (i = 1; i <= ARRAY_SIZE(bsize_list); i++)
+		bsize_list[i-1] = jack_buffer_bytes * (i + 1);
+
 	jack->sample_bits = snd_pcm_format_physical_width(format);
 	if ((err = snd_pcm_ioplug_set_param_list(&jack->io, SND_PCM_IOPLUG_HW_ACCESS,
 						 ARRAY_SIZE(access_list), access_list)) < 0 ||
@@ -452,6 +456,8 @@
 						   rate, rate)) < 0 ||
 	    (err = snd_pcm_ioplug_set_param_list(&jack->io, SND_PCM_IOPLUG_HW_PERIOD_BYTES,
 						 ARRAY_SIZE(psize_list), psize_list)) < 0 ||
+	    (err = snd_pcm_ioplug_set_param_list(&jack->io, SND_PCM_IOPLUG_HW_BUFFER_BYTES,
+						 ARRAY_SIZE(bsize_list), bsize_list)) < 0 ||
 	    (err = snd_pcm_ioplug_set_param_minmax(&jack->io, SND_PCM_IOPLUG_HW_PERIODS,
 						   2, 64)) < 0)
 		return err;