Re: [PATCH v2] pwm: imx27: Fix variable truncation in .apply()

[Frank Li <Frank.li@nxp.com>]

On Fri, May 22, 2026 at 04:13:48PM -0300, Ronaldo Nunez wrote:
> Fix a variable truncation when calculating period in microseconds as
> part of the solution for the ERR051198 in .apply() callback.
>
> Example scenario:
>  - Period of 3us (PWMPR = 196 and prescaler = 1)
>  - Expected value in tmp: 198000000000 (NSEC_PER_SEC * (196 + 2) * 1)
>  - Actual value is 431504384 (truncation to u32)
>
> Signed-off-by: Ronaldo Nunez <rnunez@baylibre.com>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

> Changes in v2:
> - Added example with actual PWMPR/prescaler values per Frank Li's feedback
> - Dropped testing section
> ---
>  drivers/pwm/pwm-imx27.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
> index 3d34cdc4a3a5..c8b801fcb525 100644
> --- a/drivers/pwm/pwm-imx27.c
> +++ b/drivers/pwm/pwm-imx27.c
> @@ -200,7 +200,7 @@ static void pwm_imx27_wait_fifo_slot(struct pwm_chip *chip,
>  static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  			   const struct pwm_state *state)
>  {
> -	unsigned long period_cycles, duty_cycles, prescale, period_us, tmp;
> +	unsigned long period_cycles, duty_cycles, prescale, period_us;
>  	struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
>  	unsigned long long c;
>  	unsigned long long clkrate;
> @@ -208,6 +208,7 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	int val;
>  	int ret;
>  	u32 cr;
> +	u64 tmp;
>
>  	clkrate = clk_get_rate(imx->clks[PWM_IMX27_PER].clk);
>  	c = clkrate * state->period;
> @@ -249,6 +250,11 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	val = readl(imx->mmio_base + MX3_PWMPR);
>  	val = val >= MX3_PWMPR_MAX ? MX3_PWMPR_MAX : val;
>  	cr = readl(imx->mmio_base + MX3_PWMCR);
> +
> +	/*
> +	 * tmp stores period in nanoseconds. Result fits in u64 since
> +	 * val <= 0xfffe and prescaler in [1, 0x1000].
> +	 */
>  	tmp = NSEC_PER_SEC * (u64)(val + 2) * MX3_PWMCR_PRESCALER_GET(cr);
>  	tmp = DIV_ROUND_UP_ULL(tmp, clkrate);
>  	period_us = DIV_ROUND_UP_ULL(tmp, 1000);
> --
> 2.53.0
>