[src/trunk]: src/sys/arch/arm/broadcom Reduce latency a bit. From nat@

[skrll <skrll%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/37b87b3dab0c
branches:  trunk
changeset: 809712:37b87b3dab0c
user:      skrll <skrll%NetBSD.org@localhost>
date:      Tue Jul 28 21:24:43 2015 +0000

description:
Reduce latency a bit. From nat@

diffstat:

 sys/arch/arm/broadcom/bcm2835_vcaudio.c |  18 ++++++++++--------
 1 files changed, 10 insertions(+), 8 deletions(-)

diffs (47 lines):

diff -r 195141aa93a2 -r 37b87b3dab0c sys/arch/arm/broadcom/bcm2835_vcaudio.c
--- a/sys/arch/arm/broadcom/bcm2835_vcaudio.c   Tue Jul 28 19:46:42 2015 +0000
+++ b/sys/arch/arm/broadcom/bcm2835_vcaudio.c   Tue Jul 28 21:24:43 2015 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: bcm2835_vcaudio.c,v 1.9 2015/03/15 18:31:29 jmcneill Exp $ */
+/* $NetBSD: bcm2835_vcaudio.c,v 1.10 2015/07/28 21:24:43 skrll Exp $ */
 
 /*-
  * Copyright (c) 2013 Jared D. McNeill <jmcneill%invisible.ca@localhost>
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: bcm2835_vcaudio.c,v 1.9 2015/03/15 18:31:29 jmcneill Exp $");
+__KERNEL_RCSID(0, "$NetBSD: bcm2835_vcaudio.c,v 1.10 2015/07/28 21:24:43 skrll Exp $");
 
 #include <sys/param.h>
 #include <sys/types.h>
@@ -81,20 +81,22 @@
        VCAUDIO_DEST_HDMI = 2,
 };
 
-
 /*
- * Standard message size is 4000 bytes and VCHIQ can accept 16 messages.
+ * Maximum message size is 4000 bytes and VCHIQ can accept 16 messages.
  *
  * 4000 bytes of 16bit 48kHz stereo is approximately 21ms.
  *
  * We get complete messages at ~10ms intervals.
  *
- * Setting blocksize to 2 x 4000 means that we send approx 42ms of audio. We
- * prefill by two blocks before starting audio meaning we have 83ms of latency.
+ * Setting blocksize to 4 x 1600 means that we send approx 33ms of audio. We
+ * prefill by two blocks before starting audio meaning we have 50ms of latency.
+ * 
+ * Six messages of 1600 bytes was chosen working back from a desired latency of
+ * 50ms.
  */
 
-#define VCAUDIO_MSGSIZE                4000
-#define VCAUDIO_NUMMSGS                2
+#define VCAUDIO_MSGSIZE                1600
+#define VCAUDIO_NUMMSGS                4
 #define VCAUDIO_BLOCKSIZE      (VCAUDIO_MSGSIZE * VCAUDIO_NUMMSGS)
 #define VCAUDIO_BUFFERSIZE     128000
 #define VCAUDIO_PREFILLCOUNT   2