Hello, Please do not apply this patch, it causes trouble on my Intel Kaby Lake laptop. (see below).
W dniu 07.07.2021 o 23:38, Mateusz Jończyk pisze:
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.
According to Intel's documentation [2]:
The update cycle [of CMOS RTC registers, which happens every second] will start at least 488 μs after the UIP bit of register A is asserted, and the entire cycle does not take more than 1984 μs to complete. The time and date RAM locations (0–9) are disconnected from the external bus during this time.
Therefore, trying to access the RTC_HOURS register during the update time is undefined behavior. In my tests, it frequently returned values from the wrong register, such as the seconds or minutes register instead of the hours register and the values failed the "< 24" comparison.
Indeed, the wrong readouts happen when the UIP bit is set.
I was not aware of this and blamed my firmware for accessing the RTC clock independently.
BTW, cmos_set_alarm() in drivers/rtc/rtc-cmos.c does not check for UIP bit. As it writes to registers in the 0-9 range, it may fail to set the alarm time.
Greetings,
Mateusz
[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-...
[1] AMD SB700/710/750 Register Reference Guide, page 308, https://developer.amd.com/wordpress/media/2012/10/43009_sb7xx_rrg_pub_1.00.p...
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 Cc: stable@vger.kernel.org
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");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");