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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, Denis Kirjanov <kda@linux-powerpc.org>,
        "Guenter Roeck" <linux@roeck-us.net>,
        "Luuk Paulussen" <luuk.paulussen@alliedtelesis.co.nz>
Date:   Fri, 24 Apr 2020 00:07:05 +0100
Message-ID: <lsq.1587683028.381735639@decadent.org.uk>
Subject: [PATCH 3.16 198/245] hwmon: (adt7475) Make volt2reg return same
 reg as reg2volt input
In-Reply-To: <lsq.1587683027.831233700@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
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3.16.83-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Luuk Paulussen <luuk.paulussen@alliedtelesis.co.nz>

commit cf3ca1877574a306c0207cbf7fdf25419d9229df upstream.

reg2volt returns the voltage that matches a given register value.
Converting this back the other way with volt2reg didn't return the same
register value because it used truncation instead of rounding.

This meant that values read from sysfs could not be written back to sysfs
to set back the same register value.

With this change, volt2reg will return the same value for every voltage
previously returned by reg2volt (for the set of possible input values)

Signed-off-by: Luuk Paulussen <luuk.paulussen@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20191205231659.1301-1-luuk.paulussen@alliedtelesis.co.nz
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 drivers/hwmon/adt7475.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/drivers/hwmon/adt7475.c
+++ b/drivers/hwmon/adt7475.c
@@ -268,9 +268,10 @@ static inline u16 volt2reg(int channel,
 	long reg;
 
 	if (bypass_attn & (1 << channel))
-		reg = (volt * 1024) / 2250;
+		reg = DIV_ROUND_CLOSEST(volt * 1024, 2250);
 	else
-		reg = (volt * r[1] * 1024) / ((r[0] + r[1]) * 2250);
+		reg = DIV_ROUND_CLOSEST(volt * r[1] * 1024,
+					(r[0] + r[1]) * 2250);
 	return clamp_val(reg, 0, 1023) & (0xff << 2);
 }