hwclock: fix --systohc sets clock 0.5 seconds slow

quote from rh150493:

	The kernel code, when setting the BIOS clock notes that the clock time
	ticks to the next second 0.5 seconds after adjusting it  (see
	linux/arch/i386/kernel/time.c).

	hwclock --systohc sets the CMOS clock at the 1 second boundry and thus
	causes the clock to be wrong by 500ms each time it is reset.  If the
	clock is set every shutdown then the clock will have a reboot-count
	related drift as well as the natural drift problems of the clock. Note
	that this also mucks up the drift calculations, of course.

Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/hwclock/hwclock.c b/hwclock/hwclock.c
index 9731dadfbff552d8fead4ad3db442a2f336fa3b9..820c388d45ff8b6ea8a38e9fa0337864ee507126 100644 (file)
--- a/hwclock/hwclock.c
+++ b/hwclock/hwclock.c
@@ -517,14 +517,19 @@ set_hardware_clock_exact(const time_t sethwtime,
            "Delaying further to reach the next full second.\n"),
            time_diff(beginsystime, refsystime));
   
-  /* Now delay some more until Hardware Clock time newhwtime arrives */
+  /*
+   * Now delay some more until Hardware Clock time newhwtime arrives.  The -500
+   * ms is because the Hardware Clock always sets to your set time plus 500 ms
+   * (because it is designed to update to the next second precisely 500 ms
+   * after you finish the setting).
+   */
   do {
          float tdiff;
          gettimeofday(&nowsystime, NULL);
          tdiff = time_diff(nowsystime, beginsystime);
          if (tdiff < 0)
                  goto time_resync;     /* probably time was reset */
-  } while (time_diff(nowsystime, refsystime) < newhwtime - sethwtime);
+  } while (time_diff(nowsystime, refsystime) - 0.5 < newhwtime - sethwtime);
   
   set_hardware_clock(newhwtime, universal, testing);
 }

diff --git a/tests/ts-hwclock-systohc b/tests/ts-hwclock-systohc
index 522b597fa2e2151fdeac216e58df6f04dc4a01bf..6171ab30c2ce37d5026b807b5b7c21939f103353 100755 (executable)
--- a/tests/ts-hwclock-systohc
+++ b/tests/ts-hwclock-systohc
@@ -72,5 +72,5 @@ if [ "$DIFF" == "1" ]; then
        ts_failed "offset is $OFFSET"
 fi
 
-ts_ok
+ts_ok "offset is $OFFSET"