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From:   Eliav Farber <farbere@amazon.com>
To:     <jdelvare@suse.com>, <linux@roeck-us.net>, <robh+dt@kernel.org>,
        <p.zabel@pengutronix.de>, <rtanwar@maxlinear.com>,
        <linux-hwmon@vger.kernel.org>, <devicetree@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     <farbere@amazon.com>, <hhhawa@amazon.com>, <jonnyc@amazon.com>,
        <andriy.shevchenko@intel.com>
Subject: [PATCH v4 05/21] hwmon: (mr75203) fix voltage equation for negative source input
Date:   Tue, 6 Sep 2022 08:33:40 +0000
Message-ID: <20220906083356.21067-6-farbere@amazon.com>
In-Reply-To: <20220906083356.21067-1-farbere@amazon.com>
References: <20220906083356.21067-1-farbere@amazon.com>
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According to Moortec Embedded Voltage Monitor (MEVM) series 3 data
sheet, the minimum input signal is -100mv and maximum input signal
is +1000mv.

The equation used to convert the digital word to voltage uses mixed
types (*val signed and n unsigned), and on 64 bit machines also has
different size, since sizeof(u32) = 4 and sizeof(long) = 8.

So when measuring a negative input, n will be small enough, such that
PVT_N_CONST * n < PVT_R_CONST, and the result of
(PVT_N_CONST * n - PVT_R_CONST) will overflow to a very big positive
32 bit number. Then when storing the result in *val it will be the same
value just in 64 bit (instead of it representing a negative number which
will what happen when sizeof(long) = 4).

When -1023 <= (PVT_N_CONST * n - PVT_R_CONST) <= -1
dividing the number by 1024 should result of in 0, but because ">> 10"
is used it results in -1 (0xf...fffff).

This change fixes the sign problem and supports negative values by
casting n to long and replacing the shift right with div operation.

Fixes: 9d823351a337 ("hwmon: Add hardware monitoring driver for Moortec MR75203 PVT controller")
Signed-off-by: Eliav Farber <farbere@amazon.com>
---
V4 -> V3:
- Remove unrelated change (add of empty line).

V3 -> V2:
- Fix equation to support negative values instead of limiting value to
  zero.

 drivers/hwmon/mr75203.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwmon/mr75203.c b/drivers/hwmon/mr75203.c
index a209f5d95f4b..78dc471e843c 100644
--- a/drivers/hwmon/mr75203.c
+++ b/drivers/hwmon/mr75203.c
@@ -202,7 +202,7 @@ static int pvt_read_in(struct device *dev, u32 attr, int channel, long *val)
 
 		n &= SAMPLE_DATA_MSK;
 		/* Convert the N bitstream count into voltage */
-		*val = (PVT_N_CONST * n - PVT_R_CONST) >> PVT_CONV_BITS;
+		*val = (PVT_N_CONST * (long)n - PVT_R_CONST) / (1 << PVT_CONV_BITS);
 
 		return 0;
 	default:
-- 
2.37.1