This is a note to let you know that I've just added the patch titled pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare() to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: pwm-lpss-fix-off-by-one-error-in-base_unit-math-in-p.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit af477ccfb2fe7b1c65ca345fa87677c3ffeb3664 Author: Hans de Goede <hdegoede@xxxxxxxxxx> Date: Thu Sep 3 13:23:23 2020 +0200 pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare() [ 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@xxxxxxxxxxxxxxx> Acked-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> Acked-by: Thierry Reding <thierry.reding@xxxxxxxxx> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx> Link: https://patchwork.freedesktop.org/patch/msgid/20200903112337.4113-4-hdegoede@xxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c index 7a4a6406cf69a..da63c029aa286 100644 --- a/drivers/pwm/pwm-lpss.c +++ b/drivers/pwm/pwm-lpss.c @@ -105,7 +105,7 @@ 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); @@ -116,8 +116,8 @@ 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;