[PATCH RESEND v2 3/6] rtc-mc146818-lib: fix RTC presence check

["Mateusz Jończyk" <mat.jonczyk@o2.pl>]

To prevent an infinite loop in mc146818_get_time(),
commit 211e5db19d15 ("rtc: mc146818: Detect and handle broken RTCs")
added a check for RTC availability. Together with a later fix, it
checked if bit 6 in register 0x0d is cleared. This, however, caused a
false negative on a motherboard with an AMD SB710 southbridge; according
to the specification [1], bit 6 of register 0x0d of this chipset is a
scratchbit.

This caused a regression in Linux 5.11 - the RTC was determined broken
by the kernel and not used by rtc-cmos.c [3].

As a better alternative, check whether the UIP ("Update-in-progress")
bit is set for longer then 10ms. If that is the case, then apparently
the RTC is either absent (and all register reads return 0xff) or broken.
Also limit the number of loop iterations in mc146818_get_time() to 10.

In a previous approach to this problem, I implemented a check whether
the RTC_HOURS register contains a value <= 24. This, however, sometimes
did not work correctly on my Intel Kaby Lake laptop. According to
Intel's documentation [2], "the time and date RAM locations (0-9) are
disconnected from the external bus" during the update cycle so reading
this register without checking the UIP bit is incorrect.

[1] AMD SB700/710/750 Register Reference Guide, page 308,
https://developer.amd.com/wordpress/media/2012/10/43009_sb7xx_rrg_pub_1.00.pdf

[2] 7th Generation Intel ® Processor Family I/O for U/Y Platforms [...] Datasheet
Volume 1 of 2, page 209
Intel's Document Number: 334658-006,
https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/7th-and-8th-gen-core-family-mobile-u-y-processor-lines-i-o-datasheet-vol-1.pdf

[3] Functions in arch/x86/kernel/rtc.c apparently were using it.

Fixes: 211e5db19d15 ("rtc: mc146818: Detect and handle broken RTCs")
Fixes: ebb22a059436 ("rtc: mc146818: Dont test for bit 0-5 in Register D")
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>

---
 drivers/rtc/rtc-cmos.c         | 10 ++++------
 drivers/rtc/rtc-mc146818-lib.c | 34 ++++++++++++++++++++++++++++++----
 include/linux/mc146818rtc.h    |  1 +
 3 files changed, 35 insertions(+), 10 deletions(-)

v2: Tweak commit description, remove "Cc: stable" (I'll send it manually
after more regression testing).

diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index 643433d984ab..f353a41dfe8c 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -833,16 +833,14 @@ cmos_do_probe(struct device *dev, struct resource *ports, int rtc_irq)
 
 	rename_region(ports, dev_name(&cmos_rtc.rtc->dev));
 
-	spin_lock_irq(&rtc_lock);
-
-	/* Ensure that the RTC is accessible. Bit 6 must be 0! */
-	if ((CMOS_READ(RTC_VALID) & 0x40) != 0) {
-		spin_unlock_irq(&rtc_lock);
-		dev_warn(dev, "not accessible\n");
+	if (!mc146818_does_rtc_work()) {
+		dev_warn(dev, "broken or not accessible\n");
 		retval = -ENXIO;
 		goto cleanup1;
 	}
 
+	spin_lock_irq(&rtc_lock);
+
 	if (!(flags & CMOS_RTC_FLAGS_NOFREQ)) {
 		/* force periodic irq to CMOS reset default of 1024Hz;
 		 *
diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index dcfaf09946ee..9175e11baf26 100644
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/drivers/rtc/rtc-mc146818-lib.c
@@ -8,10 +8,35 @@
 #include <linux/acpi.h>
 #endif
 
+/*
+ * If the UIP (Update-in-progress) bit of the RTC is set for more then
+ * 10ms, the RTC apparently is broken or not present.
+ */
+unsigned int mc146818_does_rtc_work(void)
+{
+	int i;
+	unsigned char val;
+	unsigned long flags;
+
+	for (i = 0; i < 20; i++) {
+		spin_lock_irqsave(&rtc_lock, flags);
+		val = CMOS_READ(RTC_FREQ_SELECT);
+		spin_unlock_irqrestore(&rtc_lock, flags);
+
+		if ((val & RTC_UIP) == 0)
+			return 1;
+
+		udelay(500);
+	}
+
+	return 0;
+}
+
 unsigned int mc146818_get_time(struct rtc_time *time)
 {
 	unsigned char ctrl;
 	unsigned long flags;
+	unsigned int iter_count = 0;
 	unsigned char century = 0;
 	bool retry;
 
@@ -20,13 +45,14 @@ unsigned int mc146818_get_time(struct rtc_time *time)
 #endif
 
 again:
-	spin_lock_irqsave(&rtc_lock, flags);
-	/* 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);
+	if (iter_count > 10) {
+		pr_err_ratelimited("Unable to read current time from RTC\n");
 		memset(time, 0xff, sizeof(*time));
 		return 0;
 	}
+	iter_count++;
+
+	spin_lock_irqsave(&rtc_lock, flags);
 
 	/*
 	 * Check whether there is an update in progress during which the
diff --git a/include/linux/mc146818rtc.h b/include/linux/mc146818rtc.h
index 0661af17a758..046a03df1e56 100644
--- a/include/linux/mc146818rtc.h
+++ b/include/linux/mc146818rtc.h
@@ -123,6 +123,7 @@ struct cmos_rtc_board_info {
 #define RTC_IO_EXTENT_USED      RTC_IO_EXTENT
 #endif /* ARCH_RTC_LOCATION */
 
+unsigned int mc146818_does_rtc_work(void);
 unsigned int mc146818_get_time(struct rtc_time *time);
 int mc146818_set_time(struct rtc_time *time);
 
-- 
2.25.1