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Input: ads7846 - fix integer overflow on Rt calculation

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[ 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: ffa458c1bd ("spi: ads7846 driver")
Co-developed-by: David Jander <[email protected]>
Signed-off-by: David Jander <[email protected]>
Signed-off-by: Oleksij Rempel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dmitry Torokhov <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
This commit is contained in:
Oleksij Rempel 2020-11-17 15:33:24 -08:00 • committed by Greg Kroah-Hartman
commit 920c379029
1 changed files with 3 additions and 2 deletions
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5
drivers/input/touchscreen/ads7846.c
View file

@ -801,10 +801,11 @@ 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;
}