Merge tag 'timers-urgent-2021-01-31' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer fix from Thomas Gleixner:
 "A fix for handling advertised, but non-existent 146818 RTCs correctly.

  With the recent UIP handling changes the time readout of non-existent
  RTCs hangs forever as the read returns always 0xFF which means the UIP
  bit is set.

  Sanity check the RTC before registering by checking the RTC_VALID
  register for correctness"

* tag 'timers-urgent-2021-01-31' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  rtc: mc146818: Detect and handle broken RTCs

Author: torvalds

drivers/rtc/rtc-cmos.c

@@ -805,6 +805,14 @@ cmos_do_probe(struct device *dev, struct resource *ports, int rtc_irq)
 
 	spin_lock_irq(&rtc_lock);
 
+	/* Ensure that the RTC is accessible. Bit 0-6 must be 0! */
+	if ((CMOS_READ(RTC_VALID) & 0x7f) != 0) {
+		spin_unlock_irq(&rtc_lock);
+		dev_warn(dev, "not accessible\n");
+		retval = -ENXIO;
+		goto cleanup1;
+	}
+
 	if (!(flags & CMOS_RTC_FLAGS_NOFREQ)) {
 		/* force periodic irq to CMOS reset default of 1024Hz;
 		 *

drivers/rtc/rtc-mc146818-lib.c

@@ -21,6 +21,13 @@ unsigned int mc146818_get_time(struct rtc_time *time)
 
 again:
 	spin_lock_irqsave(&rtc_lock, flags);
+	/* Ensure that the RTC is accessible. Bit 0-6 must be 0! */
+	if (WARN_ON_ONCE((CMOS_READ(RTC_VALID) & 0x7f) != 0)) {
+		spin_unlock_irqrestore(&rtc_lock, flags);
+		memset(time, 0xff, sizeof(*time));
+		return 0;
+	}
+
 	/*
 	 * Check whether there is an update in progress during which the
 	 * readout is unspecified. The maximum update time is ~2ms. Poll