Input: ads7846 - fix integer overflow on Rt calculation

In some rare cases the 32 bit Rt value will overflow if z2 and x is max,
z1 is minimal value and x_plate_ohms is relatively high (for example 800
ohm). This would happen on some screen age with low pressure.

There are two possible fixes:
- make Rt 64bit
- reorder calculation to avoid overflow

The second variant seems to be preferable, since 64 bit calculation on
32 bit system is a bit more expensive.

Fixes: ffa458c1bd9b6f653008d450f337602f3d52a646 ("spi: ads7846 driver")
Co-developed-by: David Jander <david@protonic.nl>
Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20201113112240.1360-1-o.rempel@pengutronix.de
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 95e89f675ad5ff01eb159b1c159d9cb58b4a93e5..636b901c13748b8e1433b6aa42d80badbc5c458a 100644 (file)
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -802,10 +802,11 @@ static void ads7846_report_state(struct ads7846 *ts)
                /* compute touch pressure resistance using equation #2 */
                Rt = z2;
                Rt -= z1;
-               Rt *= x;
                Rt *= ts->x_plate_ohms;
+               Rt = DIV_ROUND_CLOSEST(Rt, 16);
+               Rt *= x;
                Rt /= z1;
-               Rt = (Rt + 2047) >> 12;
+               Rt = DIV_ROUND_CLOSEST(Rt, 256);
        } else {
                Rt = 0;
        }