linux-kernel - [PATCH 2/4] rtc: ds1307: Fix off-by-one issue with wday for rx8130

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Message-ID: <20251219-ds1307-rx8901-add-v1-2-b13f346ebe93@axis.com>
Date: Fri, 19 Dec 2025 13:10:36 +0100
From: Fredrik M Olsson <fredrik.m.olsson@...s.com>
To: Alexandre Belloni <alexandre.belloni@...tlin.com>, Rob Herring
	<robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>, Nobuhiro Iwamatsu <nobuhiro.iwamatsu.x90@...l.toshiba>
CC: <linux-rtc@...r.kernel.org>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Fredrik M Olsson <fredrik.m.olsson@...s.com>,
	<kernel@...s.com>
Subject: [PATCH 2/4] rtc: ds1307: Fix off-by-one issue with wday for rx8130

The RTC represent each weekday with a individual bit set in the WDAY
register, where the 0th bit represent the first day of the week and the
6th bit represents the last day of the week. For each passed day the
chip performs a rotary-left-shift by one to advance the weekday by one.

The tm_wday field represent weekdays by a value in the range of 0-6.

The fls() function return the bit index of the last bit set. To handle
when there are no bits set it will return 0, and if the 0th bit is set
it will return 1, and if the 1st bit is set it will return 2, and so on.

In order to make the result of the fls() function fall into the expected
range of 0-6 (instead of 1-7) this patch subtracts one from the result
(which matches how the value is written in ds1307_set_time()).

Fixes: 204756f016726 ("rtc: ds1307: Fix wday settings for rx8130")
Signed-off-by: Fredrik M Olsson <fredrik.m.olsson@...s.com>
---
 drivers/rtc/rtc-ds1307.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-ds1307.c b/drivers/rtc/rtc-ds1307.c
index 7205c59ff729..bf42c250ea7d 100644
--- a/drivers/rtc/rtc-ds1307.c
+++ b/drivers/rtc/rtc-ds1307.c
@@ -308,7 +308,7 @@ static int ds1307_get_time(struct device *dev, struct rtc_time *t)
 	t->tm_hour = bcd2bin(tmp);
 	/* rx8130 is bit position, not BCD */
 	if (ds1307->type == rx_8130)
-		t->tm_wday = fls(regs[DS1307_REG_WDAY] & 0x7f);
+		t->tm_wday = fls(regs[DS1307_REG_WDAY] & 0x7f) - 1;
 	else
 		t->tm_wday = bcd2bin(regs[DS1307_REG_WDAY] & 0x07) - 1;
 	t->tm_mday = bcd2bin(regs[DS1307_REG_MDAY] & 0x3f);

-- 
2.43.0