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

Uwe Kleine-König
2020-06-22T07:26:18Z

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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

--
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