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Input: ads7846 - fix integer overflow on Rt calculation

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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>
This commit is contained in:
Oleksij Rempel 2020-11-17 15:33:24 -08:00 committed by Dmitry Torokhov
parent 9c9509717b
commit 820830ec91
1 changed files with 3 additions and 2 deletions
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5
drivers/input/touchscreen/ads7846.c
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@ -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;
}