Commit 820830ec authored by Oleksij Rempel's avatar Oleksij Rempel Committed by Dmitry Torokhov

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: ffa458c1 ("spi: ads7846 driver")
Co-developed-by: default avatarDavid Jander <david@protonic.nl>
Signed-off-by: default avatarDavid Jander <david@protonic.nl>
Signed-off-by: default avatarOleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20201113112240.1360-1-o.rempel@pengutronix.deSigned-off-by: default avatarDmitry Torokhov <dmitry.torokhov@gmail.com>
parent 9c950971
......@@ -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;
}
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment