Re: [PATCH v3 03/15] pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare()

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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On Sat, Jun 20, 2020 at 02:17:46PM +0200, Hans de Goede wrote:
> According to the data-sheet the way the PWM controller works is that
> each input clock-cycle the base_unit gets added to a N bit counter and
> that counter overflowing determines the PWM output frequency.
> 
> So assuming e.g. a 16 bit counter this means that if base_unit is set to 1,
> after 65535 input clock-cycles the counter has been increased from 0 to
> 65535 and it will overflow on the next cycle, so it will overflow after
> every 65536 clock cycles and thus the calculations done in
> pwm_lpss_prepare() should use 65536 and not 65535.
> 
> This commit fixes this. Note this also aligns the calculations in
> pwm_lpss_prepare() with those in pwm_lpss_get_state().
> 
> Note this effectively reverts commit 684309e5043e ("pwm: lpss: Avoid
> potential overflow of base_unit"). The next patch in this series really
> fixes the potential overflow of the base_unit value.
> 
> Fixes: 684309e5043e ("pwm: lpss: Avoid potential overflow of base_unit")
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v3:
> - Add Fixes tag
> - Add Reviewed-by: Andy Shevchenko tag
> ---
>  drivers/pwm/pwm-lpss.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
> index 9d965ffe66d1..43b1fc634af1 100644
> --- a/drivers/pwm/pwm-lpss.c
> +++ b/drivers/pwm/pwm-lpss.c
> @@ -93,7 +93,7 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
>  	 * The equation is:
>  	 * base_unit = round(base_unit_range * freq / c)
>  	 */
> -	base_unit_range = BIT(lpwm->info->base_unit_bits) - 1;
> +	base_unit_range = BIT(lpwm->info->base_unit_bits);
>  	freq *= base_unit_range;
>  
>  	base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
> @@ -104,8 +104,8 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
>  
>  	orig_ctrl = ctrl = pwm_lpss_read(pwm);
>  	ctrl &= ~PWM_ON_TIME_DIV_MASK;
> -	ctrl &= ~(base_unit_range << PWM_BASE_UNIT_SHIFT);
> -	base_unit &= base_unit_range;
> +	ctrl &= ~((base_unit_range - 1) << PWM_BASE_UNIT_SHIFT);
> +	base_unit &= (base_unit_range - 1);
>  	ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
>  	ctrl |= on_time_div;

I willing to believe your change is right, what I don't like is that the
calculation is really hard to follow. But that's nothing I want to
burden on you to improve. (If however you are motivated, adding some
comments about the hardware would probably help.)

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Thanks
Uwe

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

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