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    Subject[PATCH 5.9 234/757] pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare()
    Date
    From: Hans de Goede <hdegoede@redhat.com>

    [ Upstream commit 181f4d2f44463fe09fe4df02e03095cb87151c29 ]

    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>
    Acked-by: Thierry Reding <thierry.reding@gmail.com>
    Signed-off-by: Hans de Goede <hdegoede@redhat.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20200903112337.4113-4-hdegoede@redhat.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 9d965ffe66d1e..43b1fc634af1a 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;

    --
    2.25.1


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