[alsa-devel] [PATCH alsa-lib 2/2] pcm_plugin: Fix return value of snd_pcm_rewind

[David Henningsson]

In case the rewind did not rewind as much as expected, e g due to
time delay between the latest avail update and the rewind, we must
properly account for that in the plugin layer.

Otherwise, the plugin's appl ptr and the hw's appl ptr become
unsynchronised, which is very bad, especially in mmap_shadow plugins,
e g, this could cause the overlapping memcpy in the softvol plugin
as seen here:
https://bugs.freedesktop.org/show_bug.cgi?id=64299

Signed-off-by: David Henningsson <david.henningsson at canonical.com>
---
 src/pcm/pcm_plugin.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Btw, I was also considering always syncing the appl.ptr (for playback)
with the slave in snd_pcm_plugin_avail_update, like we do for the
hw.ptr. Do you think this would be a good idea?

diff --git a/src/pcm/pcm_plugin.c b/src/pcm/pcm_plugin.c
index 96218a8..17157e8 100644
--- a/src/pcm/pcm_plugin.c
+++ b/src/pcm/pcm_plugin.c
@@ -219,9 +219,9 @@ static snd_pcm_sframes_t snd_pcm_plugin_rewind(snd_pcm_t *pcm, snd_pcm_uframes_t
 		snd_atomic_write_end(&plugin->watom);
 		return sframes;
 	}
-	snd_pcm_mmap_appl_backward(pcm, (snd_pcm_uframes_t) frames);
+	snd_pcm_mmap_appl_backward(pcm, (snd_pcm_uframes_t) sframes);
 	snd_atomic_write_end(&plugin->watom);
-	return (snd_pcm_sframes_t) frames;
+	return (snd_pcm_sframes_t) sframes;
 }
 
 static snd_pcm_sframes_t snd_pcm_plugin_forwardable(snd_pcm_t *pcm)
-- 
1.7.9.5