linux-kernel - [v2] pwm: xilinx: Fix u32 overflow issue in 32-bit width PWM mode.

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Message-ID: <SJ0P222MB0107490C5371B848EF04351CA1E19@SJ0P222MB0107.NAMP222.PROD.OUTLOOK.COM>
Date:   Thu, 15 Dec 2022 16:07:15 +0000
From:   Kenneth Sloat <ksloat@...ignlinxhs.com>
To:     "sean.anderson@...o.com" <sean.anderson@...o.com>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kenneth Sloat <ksloat@...ignlinxhs.com>
Subject: [v2] pwm: xilinx: Fix u32 overflow issue in 32-bit width PWM mode.

From: Ken Sloat <ksloat@...ignlinxhs.com>

This timer HW supports 8, 16 and 32-bit timer widths. This
driver currently uses a u32 to store the max possible value
of the timer. However, statements perform addition of 2 in
xilinx_pwm_apply() when calculating the period_cycles and
duty_cycles values. Since priv->max is a u32, this will
result in an overflow to 1 which will not only be incorrect
but fail on range comparison. This results in making it
impossible to set the PWM in this timer mode.

There are two obvious solutions to the current problem:
1. Cast each instance where overflow occurs to u64.
2. Change priv->max from a u32 to a u64.

Solution #1 requires more code modifications, and leaves
opportunity to introduce similar overflows if other math
statements are added in the future. These may also go
undetected if running in non 32-bit timer modes.

Solution #2 is the much smaller and cleaner approach and
thus the chosen method in this patch.

This was tested on a Zynq UltraScale+ with multiple
instances of the PWM IP.

Signed-off-by: Ken Sloat <ksloat@...ignlinxhs.com>
---
Changes in v2:
	-Update commit comments to explain specifically where this
 	problem occurs as well as compare solutions.

---
 include/clocksource/timer-xilinx.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/clocksource/timer-xilinx.h b/include/clocksource/timer-xilinx.h
index c0f56fe6d22a..d116f18de899 100644
--- a/include/clocksource/timer-xilinx.h
+++ b/include/clocksource/timer-xilinx.h
@@ -41,7 +41,7 @@ struct regmap;
 struct xilinx_timer_priv {
 	struct regmap *map;
 	struct clk *clk;
-	u32 max;
+	u64 max;
 };

 /**
-- 
2.17.1