x86/rtc: Rewrite & simplify mach_get_cmos_time() by deleting duplicated functionality

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 d2a632a8 ("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 3195ef59 ("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

arch/x86/kernel/rtc.c

@@ -4,11 +4,8 @@
  */
 #include <linux/platform_device.h>
 #include <linux/mc146818rtc.h>
-#include <linux/acpi.h>
-#include <linux/bcd.h>
 #include <linux/export.h>
 #include <linux/pnp.h>
-#include <linux/of.h>
 
 #include <asm/vsyscall.h>
 #include <asm/x86_init.h>
@@ -20,15 +17,12 @@
 /*
  * This is a special lock that is owned by the CPU and holds the index
  * register we are working with.  It is required for NMI access to the
- * CMOS/RTC registers.  See include/asm-i386/mc146818rtc.h for details.
+ * CMOS/RTC registers.  See arch/x86/include/asm/mc146818rtc.h for details.
  */
 volatile unsigned long cmos_lock;
 EXPORT_SYMBOL(cmos_lock);
 #endif /* CONFIG_X86_32 */
 
-/* For two digit years assume time is always after that */
-#define CMOS_YEARS_OFFS 2000
-
 DEFINE_SPINLOCK(rtc_lock);
 EXPORT_SYMBOL(rtc_lock);
 
@@ -62,8 +56,7 @@ int mach_set_rtc_mmss(const struct timespec64 *now)
 
 void mach_get_cmos_time(struct timespec64 *now)
 {
-	unsigned int status, year, mon, day, hour, min, sec, century = 0;
-	unsigned long flags;
+	struct rtc_time tm;
 
 	/*
 	 * If pm_trace abused the RTC as storage, set the timespec to 0,
@@ -74,51 +67,13 @@ void mach_get_cmos_time(struct timespec64 *now)
 		return;
 	}
 
-	spin_lock_irqsave(&rtc_lock, flags);
-
-	/*
-	 * If UIP is clear, then we have >= 244 microseconds before
-	 * RTC registers will be updated.  Spec sheet says that this
-	 * is the reliable way to read RTC - registers. If UIP is set
-	 * then the register access might be invalid.
-	 */
-	while ((CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP))
-		cpu_relax();
-
-	sec = CMOS_READ(RTC_SECONDS);
-	min = CMOS_READ(RTC_MINUTES);
-	hour = CMOS_READ(RTC_HOURS);
-	day = CMOS_READ(RTC_DAY_OF_MONTH);
-	mon = CMOS_READ(RTC_MONTH);
-	year = CMOS_READ(RTC_YEAR);
-
-#ifdef CONFIG_ACPI
-	if (acpi_gbl_FADT.header.revision >= FADT2_REVISION_ID &&
-	    acpi_gbl_FADT.century)
-		century = CMOS_READ(acpi_gbl_FADT.century);
-#endif
-
-	status = CMOS_READ(RTC_CONTROL);
-	WARN_ON_ONCE(RTC_ALWAYS_BCD && (status & RTC_DM_BINARY));
-
-	spin_unlock_irqrestore(&rtc_lock, flags);
-
-	if (RTC_ALWAYS_BCD || !(status & RTC_DM_BINARY)) {
-		sec = bcd2bin(sec);
-		min = bcd2bin(min);
-		hour = bcd2bin(hour);
-		day = bcd2bin(day);
-		mon = bcd2bin(mon);
-		year = bcd2bin(year);
+	if (mc146818_get_time(&tm)) {
+		pr_err("Unable to read current time from RTC\n");
+		now->tv_sec = now->tv_nsec = 0;
+		return;
 	}
 
-	if (century) {
-		century = bcd2bin(century);
-		year += century * 100;
-	} else
-		year += CMOS_YEARS_OFFS;
-
-	now->tv_sec = mktime64(year, mon, day, hour, min, sec);
+	now->tv_sec = rtc_tm_to_time64(&tm);
 	now->tv_nsec = 0;
 }