ALSA: xen-front: Implement Xen event channel handling

Handle Xen event channels:
  - create for all configured streams and publish
    corresponding ring references and event channels in Xen store,
    so backend can connect
  - implement event channels interrupt handlers
  - create and destroy event channels with respect to Xen bus state

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/xen/xen_snd_front.c b/sound/xen/xen_snd_front.c
index 70fa9168..277214d 100644
--- a/sound/xen/xen_snd_front.c
+++ b/sound/xen/xen_snd_front.c
@@ -18,9 +18,11 @@
 #include <xen/interface/io/sndif.h>
 
 #include "xen_snd_front.h"
+#include "xen_snd_front_evtchnl.h"
 
 static void xen_snd_drv_fini(struct xen_snd_front_info *front_info)
 {
+	xen_snd_front_evtchnl_free_all(front_info);
 }
 
 static int sndback_initwait(struct xen_snd_front_info *front_info)
@@ -32,7 +34,12 @@ static int sndback_initwait(struct xen_snd_front_info *front_info)
 	if (ret < 0)
 		return ret;
 
-	return 0;
+	/* create event channels for all streams and publish */
+	ret = xen_snd_front_evtchnl_create_all(front_info, num_streams);
+	if (ret < 0)
+		return ret;
+
+	return xen_snd_front_evtchnl_publish_all(front_info);
 }
 
 static int sndback_connect(struct xen_snd_front_info *front_info)