* [PATCH v1] Input: touchscreen: ads7846.c: fix integer overflow on Rt calculation
@ 2020-11-13 11:22 Oleksij Rempel
2020-11-18 0:31 ` Dmitry Torokhov
0 siblings, 1 reply; 2+ messages in thread
From: Oleksij Rempel @ 2020-11-13 11:22 UTC (permalink / raw)
To: Dmitry Torokhov, Alexandru Ardelean, Mark Brown
Cc: Oleksij Rempel, David Jander, kernel, linux-kernel, linux-input,
linux-spi
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(-)
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;
}
--
2.28.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v1] Input: touchscreen: ads7846.c: fix integer overflow on Rt calculation
2020-11-13 11:22 [PATCH v1] Input: touchscreen: ads7846.c: fix integer overflow on Rt calculation Oleksij Rempel
@ 2020-11-18 0:31 ` Dmitry Torokhov
0 siblings, 0 replies; 2+ messages in thread
From: Dmitry Torokhov @ 2020-11-18 0:31 UTC (permalink / raw)
To: Oleksij Rempel
Cc: Alexandru Ardelean, Mark Brown, David Jander, kernel,
linux-kernel, linux-input, linux-spi
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.
--
Dmitry
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