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From:   Kenneth Sloat <ksloat@designlinxhs.com>
To:     "sean.anderson@seco.com" <sean.anderson@seco.com>,
        "michal.simek@xilinx.com" <michal.simek@xilinx.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kenneth Sloat <ksloat@designlinxhs.com>
Subject: [v2] pwm: xilinx: Fix u32 overflow issue in 32-bit width PWM mode.
Thread-Topic: [v2] pwm: xilinx: Fix u32 overflow issue in 32-bit width PWM
 mode.
Thread-Index: AQHZEJ7bv2gKFs5+xEGDSHiKdkn4og==
Date:   Thu, 15 Dec 2022 16:07:15 +0000
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From: Ken Sloat <ksloat@designlinxhs.com>=0A=
=0A=
This timer HW supports 8, 16 and 32-bit timer widths. This=0A=
driver currently uses a u32 to store the max possible value=0A=
of the timer. However, statements perform addition of 2 in=0A=
xilinx_pwm_apply() when calculating the period_cycles and=0A=
duty_cycles values. Since priv->max is a u32, this will=0A=
result in an overflow to 1 which will not only be incorrect=0A=
but fail on range comparison. This results in making it=0A=
impossible to set the PWM in this timer mode.=0A=
=0A=
There are two obvious solutions to the current problem:=0A=
1. Cast each instance where overflow occurs to u64.=0A=
2. Change priv->max from a u32 to a u64.=0A=
=0A=
Solution #1 requires more code modifications, and leaves=0A=
opportunity to introduce similar overflows if other math=0A=
statements are added in the future. These may also go=0A=
undetected if running in non 32-bit timer modes.=0A=
=0A=
Solution #2 is the much smaller and cleaner approach and=0A=
thus the chosen method in this patch.=0A=
=0A=
This was tested on a Zynq UltraScale+ with multiple=0A=
instances of the PWM IP.=0A=
=0A=
Signed-off-by: Ken Sloat <ksloat@designlinxhs.com>=0A=
---=0A=
Changes in v2:=0A=
	-Update commit comments to explain specifically where this=0A=
 	problem occurs as well as compare solutions.=0A=
=0A=
---=0A=
 include/clocksource/timer-xilinx.h | 2 +-=0A=
 1 file changed, 1 insertion(+), 1 deletion(-)=0A=
=0A=
diff --git a/include/clocksource/timer-xilinx.h b/include/clocksource/timer=
-xilinx.h=0A=
index c0f56fe6d22a..d116f18de899 100644=0A=
--- a/include/clocksource/timer-xilinx.h=0A=
+++ b/include/clocksource/timer-xilinx.h=0A=
@@ -41,7 +41,7 @@ struct regmap;=0A=
 struct xilinx_timer_priv {=0A=
 	struct regmap *map;=0A=
 	struct clk *clk;=0A=
-	u32 max;=0A=
+	u64 max;=0A=
 };=0A=
 =0A=
 /**=0A=
-- =0A=
2.17.1=0A=