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

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

[Ronaldo Nunez]

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>
---
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

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

[Frank Li]

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
>

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

[Uwe Kleine-König]

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Hello Ronaldo,

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>

Thanks for your patch. I added

Fixes: a25351e4c774 ("pwm: imx27: Workaround of the pwm output bug when decrease the duty cycle")

to the trailers and applied it to

https://git.kernel.org/pub/scm/linux/kernel/git/ukleinek/linux.git pwm/fixes

. I haven't made up my mind if I create another pull request to get the
patches from the pwm/fixes branch into 7.1. If not, it will go into
7.2-rc1.

Sashiko found a few more issues in the imx27 driver, would you like to
address these, too?

Best regards
Uwe

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