diff mbox series

[v1] Input: touchscreen: ads7846.c: fix integer overflow on Rt calculation

Message ID 20201113112240.1360-1-o.rempel@pengutronix.de (mailing list archive)
State Accepted
Commit 23fd34a56ce11e1e90444c55a96fddc8398bbfa8
Headers show
Series [v1] Input: touchscreen: ads7846.c: fix integer overflow on Rt calculation | expand

Commit Message

Oleksij Rempel Nov. 13, 2020, 11:22 a.m. UTC
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>
---
 drivers/input/touchscreen/ads7846.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Comments

Dmitry Torokhov Nov. 18, 2020, 12:31 a.m. UTC | #1
On Fri, Nov 13, 2020 at 12:22:40PM +0100, Oleksij Rempel wrote:
> 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>

Applied, thank you.
diff mbox series

Patch

diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 652d9bcfdaee..78d7159e92fe 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -993,10 +993,11 @@  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;
 	}