Re: [PATCH v8 03/17] pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare()

Thierry Reding
2020-08-31T11:31:47Z

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On Sun, Aug 30, 2020 at 02:57:39PM +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>
> Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> 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(-)

Acked-by: Thierry Reding <thierry.reding@gmail.com>

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