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authorMateusz Jończyk <mat.jonczyk@o2.pl>2021-12-10 21:01:26 +0100
committerAlexandre Belloni <alexandre.belloni@bootlin.com>2021-12-16 21:50:06 +0100
commitea6fa4961aab8f90a8aa03575a98b4bda368d4b6 (patch)
tree853d26af3a37f115551895f5149164093f99795e /drivers/rtc/rtc-mc146818-lib.c
parent0dd8d6cb9eddfe637bcd821bbfd40ebd5a0737b9 (diff)
downloadlinux-ea6fa4961aab8f90a8aa03575a98b4bda368d4b6.tar.bz2
rtc: mc146818-lib: fix RTC presence check
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]. This problem was also reported in Fedora [4]. 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 to prevent an infinite loop there. The functions mc146818_get_time() and mc146818_does_rtc_work() will be refactored later in this patch series, in order to fix a separate problem with reading / setting the RTC alarm time. This is done so to avoid a confusion about what is being fixed when. 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. [4] https://bugzilla.redhat.com/show_bug.cgi?id=1936688 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> Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20211210200131.153887-5-mat.jonczyk@o2.pl
Diffstat (limited to 'drivers/rtc/rtc-mc146818-lib.c')
-rw-r--r--drivers/rtc/rtc-mc146818-lib.c34
1 files changed, 30 insertions, 4 deletions
diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index ccd974b8a75a..d8e67a01220e 100644
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/drivers/rtc/rtc-mc146818-lib.c
@@ -8,10 +8,36 @@
#include <linux/acpi.h>
#endif
+/*
+ * If the UIP (Update-in-progress) bit of the RTC is set for more then
+ * 10ms, the RTC is apparently broken or not present.
+ */
+bool mc146818_does_rtc_work(void)
+{
+ int i;
+ unsigned char val;
+ unsigned long flags;
+
+ for (i = 0; i < 10; 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 true;
+
+ mdelay(1);
+ }
+
+ return false;
+}
+EXPORT_SYMBOL_GPL(mc146818_does_rtc_work);
+
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 +46,13 @@ 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) {
memset(time, 0, sizeof(*time));
return -EIO;
}
+ iter_count++;
+
+ spin_lock_irqsave(&rtc_lock, flags);
/*
* Check whether there is an update in progress during which the