linux-kernel - [PATCH V2] rtc: mc146818: Dont test for bit 0-5 in Register D

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Date:   Mon, 01 Feb 2021 20:24:17 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Serge Belyshev <belyshev@...ni.sinp.msu.ru>
Cc:     Dirk Gouders <dirk@...ders.net>,
        Mickaël Salaün <mic@...ikod.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH V2] rtc: mc146818: Dont test for bit 0-5 in Register D

The recent change to validate the RTC turned out to be overly tight.

While it cures the problem on the reporters machine it breaks machines
with Intel chipsets which use bit 0-5 of the D register. So check only
for bit 6 being 0 which is the case on these Intel machines as well.

Fixes: 211e5db19d15 ("rtc: mc146818: Detect and handle broken RTCs")
Reported-by: Serge Belyshev <belyshev@...ni.sinp.msu.ru>
Reported-by: Dirk Gouders <dirk@...ders.net>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
V2: Provide the actual delta patch. Should have stayed away from
    computers today....
---
 drivers/rtc/rtc-cmos.c         |    4 ++--
 drivers/rtc/rtc-mc146818-lib.c |    4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -805,8 +805,8 @@ cmos_do_probe(struct device *dev, struct
 
 	spin_lock_irq(&rtc_lock);
 
-	/* Ensure that the RTC is accessible. Bit 0-6 must be 0! */
-	if ((CMOS_READ(RTC_VALID) & 0x7f) != 0) {
+	/* 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");
 		retval = -ENXIO;
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/drivers/rtc/rtc-mc146818-lib.c
@@ -21,8 +21,8 @@ unsigned int mc146818_get_time(struct rt
 
 again:
 	spin_lock_irqsave(&rtc_lock, flags);
-	/* Ensure that the RTC is accessible. Bit 0-6 must be 0! */
-	if (WARN_ON_ONCE((CMOS_READ(RTC_VALID) & 0x7f) != 0)) {
+	/* 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);
 		memset(time, 0xff, sizeof(*time));
 		return 0;