[PATCH alsa-lib 08/12] pcm: hw: add a helper function to request hwsync without side-effects

[Takashi Sakamoto <o-takashi@sakamocchi.jp>]

SNDRV_PCM_IOCTL_SYNC_PTR command for ioctl(2) with
SNDRV_PCM_SYNC_PTR_HWSYNC flag has an effect to update hw_ptr.
This is an alternative of SNDRV_PCM_IOCTL_HWSYNC but caller can get
current status simultaneously.

This commit adds a helper function just to issue hwsync. To avoid
side-effect to change appl_ptr and avail_min, this commit uses
SNDRV_PCM_SYNC_PTR_APPL and SNDRV_PCM_SYNC_PTR_AVAIL_MIN flags.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
---
 src/pcm/pcm_hw.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/pcm/pcm_hw.c b/src/pcm/pcm_hw.c
index c6abf170..43c37b83 100644
--- a/src/pcm/pcm_hw.c
+++ b/src/pcm/pcm_hw.c
@@ -151,6 +151,21 @@ static int sync_ptr(snd_pcm_hw_t *hw, unsigned int flags)
 	return 0;
 }
 
+static int request_hwsync(snd_pcm_hw_t *hw)
+{
+	if (!hw->mmap_status_fallbacked)
+		return 0;
+
+	/*
+	 * Query both of control/status data to avoid unexpected change of
+	 * control data in kernel space.
+	 */
+	return sync_ptr1(hw,
+			 SNDRV_PCM_SYNC_PTR_HWSYNC |
+			 SNDRV_PCM_SYNC_PTR_APPL |
+			 SNDRV_PCM_SYNC_PTR_AVAIL_MIN);
+}
+
 static int query_state(snd_pcm_hw_t *hw)
 {
 	if (!hw->mmap_status_fallbacked)
@@ -579,8 +594,8 @@ static int snd_pcm_hw_hwsync(snd_pcm_t *pcm)
 	snd_pcm_hw_t *hw = pcm->private_data;
 	int fd = hw->fd, err;
 	if (SNDRV_PROTOCOL_VERSION(2, 0, 3) <= hw->version) {
-		if (hw->sync_ptr) {
-			err = sync_ptr1(hw, SNDRV_PCM_SYNC_PTR_HWSYNC);
+		if (hw->mmap_status_fallbacked) {
+			err = request_hwsync(hw);
 			if (err < 0)
 				return err;
 		} else {
-- 
2.11.0