linux-kernel - Re: [PATCH v17 1/2] pwm: add microchip soft ip corePWM driver

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Date:   Wed, 17 May 2023 12:20:30 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Daire McNamara <daire.mcnamara@...rochip.com>,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v17 1/2] pwm: add microchip soft ip corePWM driver

Hello Conor,

I found one remaining issue:

On Fri, Apr 21, 2023 at 10:27:09AM +0100, Conor Dooley wrote:
> +static u64 mchp_core_pwm_calc_duty(const struct pwm_state *state, u64 clk_rate,
> +				   u8 prescale, u8 period_steps)
> +{
> +	u64 duty_steps, tmp;
> +
> +	/*
> +	 * Calculate the duty cycle in multiples of the prescaled period:
> +	 * duty_steps = duty_in_ns / step_in_ns
> +	 * step_in_ns = (prescale * NSEC_PER_SEC) / clk_rate
> +	 * The code below is rearranged slightly to only divide once.
> +	 */
> +	tmp = (prescale + 1) * NSEC_PER_SEC;

If prescale > 4 this overflows on 32bit archs, doesn't it?
(I think prescale + 1 is promoted to unsigned int, then the
multiplication is done and only then the range is extended to u64.

> +	duty_steps = mul_u64_u64_div_u64(state->duty_cycle, clk_rate, tmp);
> +
> +	return duty_steps;
> +}

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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