[PATCH] hwmon: (pmbus/lm25066) Fix PMBus coefficient calculations

[Guenter Roeck <linux@roeck-us.net>]

In lm25066_probe(), the PMBus coefficients for current and power are
scaled based on the shunt resistor value. The calculation evaluates the
multiplication using 32-bit arithmetic because info->m is an int and
shunt is a u32:

static int lm25066_probe(struct i2c_client *client) {
    ...
    info->m[PSC_CURRENT_IN] = info->m[PSC_CURRENT_IN] * shunt / 1000;
    info->m[PSC_POWER] = info->m[PSC_POWER] * shunt / 1000;
    ...
}

For large coefficients like 26882 (LM25056) or 15076 (LM5066i), a device
tree shunt-resistor-micro-ohms value exceeding approximately 159,000
(159 mOhm, which is physically valid for low-current applications) causes
the intermediate product to exceed UINT_MAX (4,294,967,295). This results
in a silent wraparound before the division by 1000.

Furthermore, if the wrapped value has the most significant bit set,
converting it back to the signed int info->m results in negative
coefficients. This logic error leads to drastically corrupted current and
power readings, which can cause erratic thermal or power management
behavior in the system.

Fix the problem by using 64-bit operations for the multiply/divide
operations. This can still overflow, but only for unreasonably large
shunt resistor values.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: 94ee5fcc240fe ("hwmon: (pmbus/lm25066) Support configurable sense resistor values")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 drivers/hwmon/pmbus/lm25066.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/pmbus/lm25066.c b/drivers/hwmon/pmbus/lm25066.c
index f20991001727..497802c646e9 100644
--- a/drivers/hwmon/pmbus/lm25066.c
+++ b/drivers/hwmon/pmbus/lm25066.c
@@ -14,6 +14,7 @@
 #include <linux/slab.h>
 #include <linux/i2c.h>
 #include <linux/log2.h>
+#include <linux/math.h>
 #include <linux/of.h>
 #include "pmbus.h"
 
@@ -579,8 +580,8 @@ static int lm25066_probe(struct i2c_client *client)
 	if (of_property_read_u32(client->dev.of_node, "shunt-resistor-micro-ohms", &shunt))
 		shunt = 1000;
 
-	info->m[PSC_CURRENT_IN] = info->m[PSC_CURRENT_IN] * shunt / 1000;
-	info->m[PSC_POWER] = info->m[PSC_POWER] * shunt / 1000;
+	info->m[PSC_CURRENT_IN] = DIV_ROUND_CLOSEST_ULL((u64)info->m[PSC_CURRENT_IN] * shunt, 1000);
+	info->m[PSC_POWER] = DIV_ROUND_CLOSEST_ULL((u64)info->m[PSC_POWER] * shunt, 1000);
 
 #if IS_ENABLED(CONFIG_SENSORS_LM25066_REGULATOR)
 	/* LM25056 doesn't support OPERATION */
-- 
2.45.2