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