hwmon: (corsair-psu) Fix linear11 calculation

In corsairpsu_linear11_to_int(), the mantissa is extracted using bitwise
operations and cast to s16 before being shifted left:

static int corsairpsu_linear11_to_int(const u16 val, const int scale)
{
    ...
    const int mant = (((s16)(val & 0x7ff)) << 5) >> 5;
    ...
}

Due to C integer promotion rules, the masked value (which is always
positive) is promoted to a 32-bit integer before the left shift. As a
result, the sign bit is never extended to bit 31 of the promoted integer.

When the device hardware reports a negative temperature in Linear11 format
(such as an ambient temperature probe reporting sub-zero), the negative
mantissa is parsed incorrectly as a massive positive value. For example,
-1 becomes 2047, which scales to 2047 degrees Celsius.

Fix the problem by type casting the result of the left shift operation
to s16.

Another problem is left-shifting of negative values. In C, the result of
left-shifting negative values is undefined. Use a multiplication instead
to avoid the problem.

Also use a local s64 variable to store temporary results, change
the return value type from int to long, and clamp the final value
to LONG_MIN and LONG_MAX to avoid under- and overflow issues while
retaining as much information as possible.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Cc: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/20260804034811.2385506-1-linux@roeck-us.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

0 files changed, 0 insertions, 0 deletions