[alsa-devel] [PATCH v2 4/6] pcm: direct: Round up of slave_app_ptr pointer if buffer size is less than 2 period size.

[vanitha.channaiah at in.bosch.com]

From: Vanitha Channaiah <vanitha.channaiah at in.bosch.com>

For buffer size less than two period size, the start position
of slave_app_ptr is rounded up in order to avoid xruns
For e.g.:
Considering below parameters and its values
Period size = 96
Buffer size = 191
slave_appl_ptr = slave_hw_ptr = unaligned value

Issue:
- During the start of the stream, app_ptr = hw_ptr = 0
- Application writes one period of data in the buffer i.e
  app_ptr = 96, hw_ptr = 0
- Now, the avail is just period-1 frames available.
  avail = hw_ptr + buffer_size - app_ptr = 95
  i.e. shortage of 1 frame space
- so application is waiting for the 1frame space to be available.
- slave_app_ptr and slave_hw_ptr would get updated to lower values
- This could lead to under run to occur.

Fix:
If we round Up the slave_app_ptr pointer,
- During the start of the stream, app_ptr = hw_ptr = 0
- Application writes one period of data in the buffer i.e
  app_ptr = 96, hw_ptr = 0
- Round Up of slave_app_ptr pointer leads to below calculation:
- slave_app_ptr rounded to 96
- slave_app_ptr and slave_hw_ptr would get updated to larger value nearing to 2 period size
- avail = greater than period size.
- Here, there is a lower chance of under run.

Signed-off-by: Vanitha Channaiah <vanitha.channaiah at in.bosch.com>
---
 src/pcm/pcm_direct.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
index 54d9900..b56da85 100644
--- a/src/pcm/pcm_direct.c
+++ b/src/pcm/pcm_direct.c
@@ -2043,10 +2043,12 @@ int snd_pcm_direct_parse_open_conf(snd_config_t *root, snd_config_t *conf,
 
 void snd_pcm_direct_reset_slave_ptr(snd_pcm_t *pcm, snd_pcm_direct_t *dmix)
 {
-
+	/* For buffer size less than two period size, the start position
+	 * of slave app ptr is rounded up in order to avoid xruns
+	 */
 	if (dmix->hw_ptr_alignment == SND_PCM_HW_PTR_ALIGNMENT_ROUNDUP ||
 		(dmix->hw_ptr_alignment == SND_PCM_HW_PTR_ALIGNMENT_AUTO &&
-		pcm->buffer_size <= pcm->period_size * 2))
+		pcm->buffer_size < pcm->period_size * 2))
 		dmix->slave_appl_ptr =
 			((dmix->slave_appl_ptr + dmix->slave_period_size - 1) /
 			dmix->slave_period_size) * dmix->slave_period_size;
-- 
2.7.4