To prevent an infinite loop, it is necessary to ascertain the RTC is
present. Previous code was checking if bit 6 in register 0x0D is
cleared. This caused a false negative on a motherboard with an AMD SB710
southbridge; according to the specification [1], bit 6 of register 0x0D
on this chipset is a scratchbit.
Use the RTC_HOURS register instead, which is expected to contain a value
not larger then 24, in BCD format.
Caveat: when I was playing with
while true; do cat /sys/class/rtc/rtc0/time; done
I sometimes triggered this mechanism on my HP laptop. It appears that
CMOS_READ(RTC_VALID) was sometimes reading the number of seconds from
previous loop iteration. This happens very rarely, though, and this patch
does not make it any more likely.
[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
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@xxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Alessandro Zummo <a.zummo@xxxxxxxxxxxx>
Cc: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
drivers/rtc/rtc-cmos.c | 6 +++---
drivers/rtc/rtc-mc146818-lib.c | 10 ++++++++--
2 files changed, 11 insertions(+), 5 deletions(-)
diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index 670fd8a2970e..b0102fb31b3f 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -798,10 +798,10 @@ cmos_do_probe(struct device *dev, struct resource *ports, int rtc_irq)
spin_lock_irq(&rtc_lock);
- /* Ensure that the RTC is accessible. Bit 6 must be 0! */
- if ((CMOS_READ(RTC_VALID) & 0x40) != 0) {
+ /* Ensure that the RTC is accessible (RTC_HOURS is in BCD format) */
+ if (CMOS_READ(RTC_HOURS) > 0x24) {
spin_unlock_irq(&rtc_lock);
- dev_warn(dev, "not accessible\n");
+ dev_warn(dev, "not accessible or not working correctly\n");
retval = -ENXIO;
goto cleanup1;
}
diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index dcfaf09946ee..1d69c3c13257 100644
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/drivers/rtc/rtc-mc146818-lib.c
@@ -21,9 +21,15 @@ unsigned int mc146818_get_time(struct rtc_time *time)
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)) {
+
+ /*
+ * Ensure that the RTC is accessible, to avoid an infinite loop.
+ * RTC_HOURS is in BCD format.
+ */
+ if (CMOS_READ(RTC_HOURS) > 0x24) {
spin_unlock_irqrestore(&rtc_lock, flags);
+ pr_warn_once("Real-time clock is not accessible or not "
+ "working correctly\n");
memset(time, 0xff, sizeof(*time));
return 0;
}
--
2.25.1
