[src/trunk]: src/share/man/man4 Document the hw.driverN.latency sysctl variable.

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

details:   https://anonhg.NetBSD.org/src/rev/aad0d2e25601
branches:  trunk
changeset: 827419:aad0d2e25601
user:      nat <nat%NetBSD.org@localhost>
date:      Fri Oct 27 19:00:07 2017 +0000

description:
Document the hw.driverN.latency sysctl variable.

diffstat:

 share/man/man4/audio.4 |  25 ++++++++++++++++++++++---
 1 files changed, 22 insertions(+), 3 deletions(-)

diffs (48 lines):

diff -r 3ed0d8b3783b -r aad0d2e25601 share/man/man4/audio.4
--- a/share/man/man4/audio.4    Fri Oct 27 18:16:09 2017 +0000
+++ b/share/man/man4/audio.4    Fri Oct 27 19:00:07 2017 +0000
@@ -1,4 +1,4 @@
-.\"    $NetBSD: audio.4,v 1.80 2017/07/03 21:30:58 wiz Exp $
+.\"    $NetBSD: audio.4,v 1.81 2017/10/27 19:00:07 nat Exp $
 .\"
 .\" Copyright (c) 1996 The NetBSD Foundation, Inc.
 .\" All rights reserved.
@@ -112,6 +112,7 @@
 .It hw.driverN.precision
 .It hw.driverN.frequency
 .It hw.driverN.channels
+.It hw.driverN.latency
 .It hw.driverN.multiuser
 .El
 .Pp
@@ -124,10 +125,28 @@
 For best results, values close to the underlying hardware should be chosen.
 These variables may only be changed when the sampling device is not in use.
 .Pp
-An additional
+The hw.driverN.latency
+.Xr sysctl 8
+variable controls the latency of the in-kernel mixer by varying the hardware
+blocksize.
+It accepts a value in milliseconds(ms), fractional values are not allowed.
+A value of zero will default to 150ms.
+.Pp
+If a static blocksize is enforced by the underlying hardware driver this value
+cannot be changed.
+.Pp
+For audio applications that do not specify a preferred blocksize when configuring
+the audio device, this will be the latency these applications have.
+.Pp
+For audio applications that
+.Xr mmap 2
+the audio device for play back the resultant latency is a third (1/3) of the value
+of the hw.driverN.latency variable.
+.Pp
+The hw.driverN.multiuser
 .Xr sysctl 8
 variable determines if multiple users are allowed to access the sampling
-device, hw.driverN.multiuser.
+device.
 .Pp
 By default it is set to false.
 This means that the sampling device may be only used by