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There are functions in drivers/rtc/rtc-mc146818-lib.c that handle
reading from / writing to the CMOS RTC clock. mach_get_cmos_time() in
arch/x86/kernel/rtc.c did not use them and was mostly a duplicate of
mc146818_get_time(). Modify mach_get_cmos_time() to use
mc146818_get_time() and remove the duplicated functionality.
mach_get_cmos_time() used a different algorithm than
mc146818_get_time(), but these functions are equivalent. The major
differences are:
- mc146818_get_time() is better refined and handles various edge
conditions,
- when the UIP ("Update in progress") bit of the RTC is set,
mach_get_cmos_time() was busy waiting with cpu_relax() while
mc146818_get_time() is using mdelay(1) in every loop iteration.
(However, there is my commit merged for Linux 5.20 / 6.0 to decrease
this period to 100us:
commit d2a632a8a117 ("rtc: mc146818-lib: reduce RTC_UIP polling period")
),
- mach_get_cmos_time() assumed that the RTC year is >= 2000, which
may not be true on some old boxes with a dead battery,
- mach_get_cmos_time() was holding the rtc_lock for a long time
and could hang if the RTC is broken or not present.
The RTC writing counterpart, mach_set_rtc_mmss() is already using
mc146818_get_time() from drivers/rtc. This was done in
commit 3195ef59cb42 ("x86: Do full rtc synchronization with ntp")
It appears that mach_get_cmos_time() was simply forgotten.
mach_get_cmos_time() is really used only in read_persistent_clock64(),
which is called only in a few places in kernel/time/timekeeping.c .
[ mingo: These changes are not supposed to change behavior, but they are
not identity transformations either, as mc146818_get_time() is a
better but different implementation of the same logic - so
regressions are possible in principle. ]
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220813131034.768527-1-mat.jonczyk@o2.pl
fc04b2ccf0