From:   =?UTF-8?q?Mateusz=20Jo=C5=84czyk?= <mat.jonczyk@o2.pl>
To:     linux-rtc@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     =?UTF-8?q?Mateusz=20Jo=C5=84czyk?= <mat.jonczyk@o2.pl>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alessandro Zummo <a.zummo@towertech.it>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>
Subject: [PATCH v5 4/9] rtc-mc146818-lib: fix RTC presence check
Date:   Fri,  7 Jan 2022 13:49:29 +0100
Message-Id: <20220107124934.159878-5-mat.jonczyk@o2.pl>
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References: <20220107124934.159878-1-mat.jonczyk@o2.pl>
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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 20ms. 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 20 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>

---

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

v3: - add "EXPORT_SYMBOL_GPL(mc146818_does_rtc_work)",
    - change return type from mc146818_does_rtc_work to bool.

v4: - drop pr_err_ratelimited() in mc146818_get_time(), callers of this
      function are now required to notify the user,
    - tweak commit description.

v5: - increase maximum duration of the the UIP high pulse from 10 to 20ms,
      in case there are some very slow chips out there.

 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 d0f58cca5c20..b90a603d6b12 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -800,16 +800,14 @@ 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 ccd974b8a75a..9b04c304d909 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
+ * 20ms, 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 < 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 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 > 20) {
 		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
diff --git a/include/linux/mc146818rtc.h b/include/linux/mc146818rtc.h
index 0661af17a758..69c80c4325bf 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 */
 
+bool 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