[PATCH] pcm: dmix: fix incorrect boundary handling in snd_pcm_dmix_sync_area

[Maarten Baert]

The available size calculation did not handle wraparound correctly. Also, the
underrun handling code confused slave_buffer_size with slave_boundary, causing
another integer overflow that results in too many samples being skipped.

Signed-off-by: Maarten Baert <maarten-baert at hotmail.com>
---
  src/pcm/pcm_dmix.c | 16 +++++++++-------
  1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/src/pcm/pcm_dmix.c b/src/pcm/pcm_dmix.c
index 843fa316..b5fdb240 100644
--- a/src/pcm/pcm_dmix.c
+++ b/src/pcm/pcm_dmix.c
@@ -315,11 +315,12 @@ static void snd_pcm_dmix_sync_area(snd_pcm_t *pcm)
  	/* check the available size in the local buffer
  	 * last_appl_ptr keeps the last updated position
  	 */
-	size = dmix->appl_ptr - dmix->last_appl_ptr;
+	if (dmix->appl_ptr >= dmix->last_appl_ptr)
+		size = dmix->appl_ptr - dmix->last_appl_ptr;
+	else
+		size = dmix->appl_ptr + (pcm->boundary - dmix->last_appl_ptr);
  	if (! size)
  		return;
-	if (size >= pcm->boundary / 2)
-		size = pcm->boundary - size;
  
  	/* the slave_app_ptr can be far behind the slave_hw_ptr */
  	/* reduce mixing and errors here - just skip not catched writes */
@@ -328,18 +329,19 @@ static void snd_pcm_dmix_sync_area(snd_pcm_t *pcm)
  	else
  		slave_size = dmix->slave_appl_ptr + (dmix->slave_boundary - dmix->slave_hw_ptr);
  	if (slave_size > dmix->slave_buffer_size) {
-		transfer = dmix->slave_buffer_size - slave_size;
+		transfer = dmix->slave_boundary - slave_size;
  		if (transfer > size)
  			transfer = size;
  		dmix->last_appl_ptr += transfer;
  		dmix->last_appl_ptr %= pcm->boundary;
  		dmix->slave_appl_ptr += transfer;
  		dmix->slave_appl_ptr %= dmix->slave_boundary;
-		size = dmix->appl_ptr - dmix->last_appl_ptr;
+		if (dmix->appl_ptr >= dmix->last_appl_ptr)
+			size = dmix->appl_ptr - dmix->last_appl_ptr;
+		else
+			size = dmix->appl_ptr + (pcm->boundary - dmix->last_appl_ptr);
  		if (! size)
  			return;
-		if (size >= pcm->boundary / 2)
-			size = pcm->boundary - size;
  	}
  
  	/* check the available size in the slave PCM buffer */
-- 
2.26.2