From 19dbe52930f6dd95b788e4e8bffcd6da12e310c7 Mon Sep 17 00:00:00 2001
From: Miroslav Lichvar <mlichvar@redhat.com>
Date: Fri, 10 Jan 2014 13:25:39 +0100
Subject: [PATCH] Update linux_freq_scale and linux_hz documentation

---
 chrony.texi.in | 28 +++++++++-------------------
 sys_linux.c    |  4 +---
 2 files changed, 10 insertions(+), 22 deletions(-)

diff --git a/chrony.texi.in b/chrony.texi.in
index a984f8a..9429f50 100644
--- a/chrony.texi.in
+++ b/chrony.texi.in
@@ -1126,7 +1126,7 @@ directives can occur in any order in the file.
 * keyfile directive::           Specify location of file containing keys
 * leapsectz directive::         Read leap second data from tz database
 * linux_freq_scale directive::  Define a non-standard value to compensate the kernel frequency bias
-* linux_hz directive::          Define a non-standard value of the kernel HZ constant
+* linux_hz directive::          Define a non-standard value of the kernel USER_HZ constant
 * local directive::             Allow unsynchronised machine to act as server
 * lock_all directive::          Require that chronyd be locked into RAM
 * log directive::               Make daemon log certain sets of information
@@ -1794,16 +1794,10 @@ Wed Dec 31 23:59:60 UTC 2008
 @subsection linux_freq_scale
 (This option only applies to Linux).
 
-By default, chronyd will find the value of @code{HZ} and @code{SHIFT_HZ} from
-kernel header files at compile time.  An internal value called
-@code{freq_scale} is calculated from this.  By default it is (1<<SHIFT_HZ)/HZ,
-except for the case HZ=100, when special case code is used which leads to the
-value 128/128.125.  If you're running chronyd on the system where it was built,
-the value it has should be right, and you don't need to worry about this
-option.
-
-This option is provided for people who move a pre-built chronyd onto a system
-where the method by which the kernel computes the reciprocal of this value has been changed or where the HZ and SHIFT_HZ constants differ from those on the system where chronyd was built.
+This option sets a scale factor needed to control the frequency of the clock by
+the @code{adjtimex()} system call exactly.  By default, the value is determined
+by the version of the running kernel.  In recent kernels it is always 1.0 (i.e.
+no scaling is needed).
 
 An example of the command is
 
@@ -1816,14 +1810,10 @@ linux_freq_scale 0.99902439
 @subsection linux_hz
 (This option only applies to Linux).
 
-By default, chronyd will find the value of @code{HZ} from a kernel header file
-at compile time.  @code{HZ} is the nominal number of timer interrupts per
-second.  If you're running chronyd on the system where it was built, the value
-it has should be right, and you don't need to worry about this option.
-
-This option is provided for people who move a pre-built chronyd onto a system
-where the value of HZ in the kernel headers has been changed from the default
-value.
+This option defines the value of the kernel @code{USER_HZ} constant, which is
+needed to use the @code{adjtimex()} system call correctly.  By default, its
+value is determined from the running kernel automatically and there should
+rarely be a need to use this option.
 
 An example of the command is
 
diff --git a/sys_linux.c b/sys_linux.c
index 27c18fc..078b122 100644
--- a/sys_linux.c
+++ b/sys_linux.c
@@ -83,9 +83,7 @@ static int nominal_tick;
    per day). */
 static double freq_scale;
 
-/* The HZ value from the kernel header file (may be over-ridden from config
-   file, e.g. if chronyd binary is moved to a box whose kernel was built with a
-   different HZ value). */
+/* The kernel HZ constant (USER_HZ in recent kernels). */
 static int hz;
 static double dhz; /* And dbl prec version of same for arithmetic */