From: Oleksij Rempel o.rempel@pengutronix.de
[ 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: 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 Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/input/touchscreen/ads7846.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c index a61b2153ab8c2..b4ded36cc4162 100644 --- a/drivers/input/touchscreen/ads7846.c +++ b/drivers/input/touchscreen/ads7846.c @@ -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; }