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Commit 4ea501f3 authored by Oleksij Rempel's avatar Oleksij Rempel Committed by Greg Kroah-Hartman
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Input: ads7846 - fix integer overflow on Rt calculation 



[ Upstream commit 820830ec918f6c3dcd77a54a1c6198ab57407916 ]

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.de


Signed-off-by: default avatarDmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 6dc7552c

drivers/input/touchscreen/ads7846.c

+3 −2
Original line number Diff line number Diff line
@@ -785,10 +785,11 @@ 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;

	}