rtc: Check return value from mc146818_get_time()

There are 4 users of mc146818_get_time() and none of them was checking
the return value from this function. Change this.

Print the appropriate warnings in callers of mc146818_get_time() instead
of in the function mc146818_get_time() itself, in order not to add
strings to rtc-mc146818-lib.c, which is kind of a library.

The callers of alpha_rtc_read_time() and cmos_read_time() may use the
contents of (struct rtc_time *) even when the functions return a failure
code. Therefore, set the contents of (struct rtc_time *) to 0x00,
which looks more sensible then 0xff and aligns with the (possibly
stale?) comment in cmos_read_time:

/*
* If pm_trace abused the RTC for storage, set the timespec to 0,
* which tells the caller that this RTC value is unusable.
*/

For consistency, do this in mc146818_get_time().

Note: hpet_rtc_interrupt() may call mc146818_get_time() many times a
second. It is very unlikely, though, that the RTC suddenly stops
working and mc146818_get_time() would consistently fail.

Only compile-tested on alpha.

Signed-off-by: Mateusz Jończyk <[e-mail]>
Cc: Richard Henderson <[e-mail]>
Cc: Ivan Kokshaysky <[e-mail]>
Cc: Matt Turner <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Cc: Alexandre Belloni <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/alpha/kernel/rtc.c

···
 static int
 alpha_rtc_read_time(struct device *dev, struct rtc_time *tm)
 {
-	mc146818_get_time(tm);
+	int ret = mc146818_get_time(tm);
+
+	if (ret < 0) {
+		dev_err_ratelimited(dev, "unable to read current time\n");
+		return ret;
+	}
 
 	/* Adjust for non-default epochs.  It's easier to depend on the
 	   generic __get_rtc_time and adjust the epoch here than create

arch/x86/kernel/hpet.c

···
 	hpet_rtc_timer_reinit();
 	memset(&curr_time, 0, sizeof(struct rtc_time));
 
-	if (hpet_rtc_flags & (RTC_UIE | RTC_AIE))
-		mc146818_get_time(&curr_time);
+	if (hpet_rtc_flags & (RTC_UIE | RTC_AIE)) {
+		if (unlikely(mc146818_get_time(&curr_time) < 0)) {
+			pr_err_ratelimited("unable to read current time from RTC\n");
+			return IRQ_HANDLED;
+		}
+	}
 
 	if (hpet_rtc_flags & RTC_UIE &&
 	    curr_time.tm_sec != hpet_prev_update_sec) {

drivers/base/power/trace.c

···
 	struct rtc_time time;
 	unsigned int val;
 
-	mc146818_get_time(&time);
+	if (mc146818_get_time(&time) < 0) {
+		pr_err("Unable to read current time from RTC\n");
+		return 0;
+	}
+
 	pr_info("RTC time: %ptRt, date: %ptRd\n", &time, &time);
 	val = time.tm_year;				/* 100 years */
 	if (val > 100)

drivers/rtc/rtc-cmos.c

···
 
 static int cmos_read_time(struct device *dev, struct rtc_time *t)
 {
+	int ret;
+
 	/*
 	 * If pm_trace abused the RTC for storage, set the timespec to 0,
 	 * which tells the caller that this RTC value is unusable.
···
 	if (!pm_trace_rtc_valid())
 		return -EIO;
 
-	mc146818_get_time(t);
+	ret = mc146818_get_time(t);
+	if (ret < 0) {
+		dev_err_ratelimited(dev, "unable to read current time\n");
+		return ret;
+	}
+
 	return 0;
 }
 

drivers/rtc/rtc-mc146818-lib.c

···
 	/* Ensure that the RTC is accessible. Bit 6 must be 0! */
 	if (WARN_ON_ONCE((CMOS_READ(RTC_VALID) & 0x40) != 0)) {
 		spin_unlock_irqrestore(&rtc_lock, flags);
-		memset(time, 0xff, sizeof(*time));
+		memset(time, 0, sizeof(*time));
 		return -EIO;
 	}