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

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Message-ID: <672b1277-545c-27f0-a086-b47ce002d498@amd.com>
Date:   Thu, 15 Dec 2022 13:09:39 +0100
From:   Michal Simek <michal.simek@....com>
To:     Kenneth Sloat <ksloat@...ignlinxhs.com>,
        "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>
Subject: Re: [PATCH] pwm: xilinx: Fix overflow issue in 32-bit width PWM mode.



On 12/12/22 14:59, Kenneth Sloat wrote:
> This timer HW supports 8, 16 and 32-bit timer widths. This
> driver uses a u32 to store the max value of the timer.
> Because addition is done to this max value, when operating
> in 32-bit mode, this will result in overflow that makes it
> impossible to set the timer period and thus the PWM itself.
> 
> To fix this, simply make max a u64. This was tested on a
> Zynq UltraScale+.

Can you please be more accurate where that overflow is happening.
I see that value is set only at probe like

priv->max = BIT_ULL(width) - 1;


No doubt that there are calculation based on u64 types.


> 
> Signed-off-by: Ken Sloat <ksloat@...ignlinxhs.com>
> ---
>   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
> 

Thanks,
Michal