rtc: mc146818: Detect and handle broken RTCs

The recent fix for handling the UIP bit unearthed another issue in the RTC
code. If the RTC is advertised but the readout is straight 0xFF because
it's not available, the old code just proceeded with crappy values, but the
new code hangs because it waits for the UIP bit to become low.

Add a sanity check in the RTC CMOS probe function which reads the RTC_VALID
register (Register D) which should have bit 0-6 cleared. If that's not the
case then fail to register the CMOS.

Add the same check to mc146818_get_time(), warn once when the condition
is true and invalidate the rtc_time data.

Reported-by: Mickaël Salaün <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Mickaël Salaün <[email protected]>
Acked-by: Alexandre Belloni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: KAGA-KOKO

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