[PATCH 4.14 058/191] pwm: lpss: Fix off by one error in base_unit math in pwm_lpss_prepare()

Greg Kroah-Hartman
2020-10-27T18:11:33Z

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 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;

--
2.25.1