rtc: mc146818-lib: extract mc146818_avoid_UIP

Function mc146818_get_time() contains an elaborate mechanism of reading
the RTC time while no RTC update is in progress. It turns out that
reading the RTC alarm clock also requires avoiding the RTC update.
Therefore, the mechanism in mc146818_get_time() should be reused - so
extract it into a separate function.

The logic in mc146818_avoid_UIP() is same as in mc146818_get_time()
except that after every

        if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {

there is now "mdelay(1)".

To avoid producing a very unreadable patch, mc146818_get_time() will be
refactored to use mc146818_avoid_UIP() in the next patch.
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211210200131.153887-6-mat.jonczyk@o2.pl

drivers/rtc/rtc-mc146818-lib.c

@@ -8,6 +8,76 @@
 #include <linux/acpi.h>
 #endif
 
+/*
+ * Execute a function while the UIP (Update-in-progress) bit of the RTC is
+ * unset.
+ *
+ * Warning: callback may be executed more then once.
+ */
+bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
+			void *param)
+{
+	int i;
+	unsigned long flags;
+	unsigned char seconds;
+
+	for (i = 0; i < 10; i++) {
+		spin_lock_irqsave(&rtc_lock, flags);
+
+		/*
+		 * Check whether there is an update in progress during which the
+		 * readout is unspecified. The maximum update time is ~2ms. Poll
+		 * every msec for completion.
+		 *
+		 * Store the second value before checking UIP so a long lasting
+		 * NMI which happens to hit after the UIP check cannot make
+		 * an update cycle invisible.
+		 */
+		seconds = CMOS_READ(RTC_SECONDS);
+
+		if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
+			spin_unlock_irqrestore(&rtc_lock, flags);
+			mdelay(1);
+			continue;
+		}
+
+		/* Revalidate the above readout */
+		if (seconds != CMOS_READ(RTC_SECONDS)) {
+			spin_unlock_irqrestore(&rtc_lock, flags);
+			continue;
+		}
+
+		if (callback)
+			callback(seconds, param);
+
+		/*
+		 * Check for the UIP bit again. If it is set now then
+		 * the above values may contain garbage.
+		 */
+		if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
+			spin_unlock_irqrestore(&rtc_lock, flags);
+			mdelay(1);
+			continue;
+		}
+
+		/*
+		 * A NMI might have interrupted the above sequence so check
+		 * whether the seconds value has changed which indicates that
+		 * the NMI took longer than the UIP bit was set. Unlikely, but
+		 * possible and there is also virt...
+		 */
+		if (seconds != CMOS_READ(RTC_SECONDS)) {
+			spin_unlock_irqrestore(&rtc_lock, flags);
+			continue;
+		}
+		spin_unlock_irqrestore(&rtc_lock, flags);
+
+		return true;
+	}
+	return false;
+}
+EXPORT_SYMBOL_GPL(mc146818_avoid_UIP);
+
 /*
  * If the UIP (Update-in-progress) bit of the RTC is set for more then
  * 10ms, the RTC is apparently broken or not present.

include/linux/mc146818rtc.h

@@ -127,4 +127,7 @@ bool mc146818_does_rtc_work(void);
 unsigned int mc146818_get_time(struct rtc_time *time);
 int mc146818_set_time(struct rtc_time *time);
 
+bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
+			void *param);
+
 #endif /* _MC146818RTC_H */