[alsa-devel] [PATCH 2/2] pcm:plugin: Fix sound capture via MMAP access

[sutar.mounesh at gmail.com]

From: Timo Wischer <twischer at de.adit-jv.com>

Distorted sound is heard if the capture device of an ioplug plugin is used,
which is accessed through any ALSA plugin (like copy, linear, ...) via MMAP access.
E.g. aplay -> loop -> copy -> arecord -M -> aplay -> hw:0
As mentioned in the ALSA API (see pcm/pcm.c:942):
The function #snd_pcm_avail_update() have to be called
before any mmap begin+commit operation.

Signed-off-by: Timo Wischer <twischer at de.adit-jv.com>
Signed-off-by: Mounesh Sutar <sutar.mounesh at gmail.com>

diff --git a/src/pcm/pcm_plugin.c b/src/pcm/pcm_plugin.c
index aaea2ab..ec57f6e 100644
--- a/src/pcm/pcm_plugin.c
+++ b/src/pcm/pcm_plugin.c
@@ -490,7 +490,14 @@ static snd_pcm_sframes_t snd_pcm_plugin_avail_update(snd_pcm_t *pcm)
 			snd_pcm_uframes_t slave_offset;
 			snd_pcm_uframes_t slave_frames = ULONG_MAX;
 			snd_pcm_sframes_t result;
-
+			/* As mentioned in the ALSA API (see pcm/pcm.c:942):
+			 * The function #snd_pcm_avail_update()
+			 * have to be called before any mmap begin+commit operation.
+			 * Otherwise the snd_pcm_areas_copy will not called a second time.
+			 * But this is needed, if the ring buffer wrap is reached and
+			 * there is more data available.
+			 */
+			slave_size = snd_pcm_avail_update(slave);
 			result = snd_pcm_mmap_begin(slave, &slave_areas, &slave_offset, &slave_frames);
 			if (result < 0) {
 				err = result;
-- 
2.7.4