hwmon: (ltc4282) Avoid overflow in maximum power calculation

hwmon: (ltc4282) Avoid overflow in maximum power calculation

During device initialization in ltc4282_set_max_limits(), the calculation
of the maximum power limit can suffer from a 32-bit integer overflow.

static int ltc4282_set_max_limits(struct ltc4282_state *st)
{
    ...
    st->power_max = DIV_ROUND_CLOSEST(st->vsense_max * DECA * MILLI,
                                      st->rsense) * st->vfs_out;
    ...
}

The result of DIV_ROUND_CLOSEST() evaluates to a 32-bit unsigned integer
on 32-bit architectures. This result is then multiplied by st->vfs_out,
which is a 16-bit unsigned integer. According to C promotion rules, since
both operands are 32-bit or smaller, the multiplication is performed in
32-bit precision.

If the device is configured with a low sense resistor value via the device
tree (for example, 100 nano-ohms, resulting in st->rsense = 1) and the
voltage is high, the division result can reach 343,750,000 and st->vfs_out
can be 33,280. The product of these values is approximately 11.44 trillion,
which exceeds the maximum capacity of a 32-bit integer and overflows
before being stored in st->power_max.

This overflow causes a truncated value to be assigned to st->power_max and
written to the hardware limit register. An incorrect maximum power limit
can trigger spurious power-bad faults or alarms, which may lead to the
shutdown of the monitored power rail.

Avoid the problem by calculating and storing the maximum power using 64-bit
variables.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: cbc29538dbf7d ("hwmon: Add driver for LTC4282")
Cc: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>