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 not used by the kernel.
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.p...
[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-...
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
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Hello, Please check this commit carefully as the RTC is a finicky beast and I'm quite new to kernel development.
I have tested this on three computers, though, and found no problems. Also, using the UIP (Update-in-progress) bit should be safe.
Greetings, Mateusz
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(-)
diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c index 670fd8a2970e..0fa66d1039b9 100644 --- a/drivers/rtc/rtc-cmos.c +++ b/drivers/rtc/rtc-cmos.c @@ -796,16 +796,14 @@ 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);