| Message ID | 20201113112240.1360-1-o.rempel@pengutronix.de (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 820830ec918f6c3dcd77a54a1c6198ab57407916 |
| Headers | show |
| Series | [v1] Input: touchscreen: ads7846.c: fix integer overflow on Rt calculation | expand |
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 --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; }
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(-)