Re: [PATCH v6 1/4] gpio: mvebu: fix pwm get_state period calculation
From: Uwe Kleine-König <hidden>
Date: 2021-01-07 14:31:17
Also in:
linux-arm-kernel, linux-devicetree, linux-pwm
On Wed, Jan 06, 2021 at 09:37:37AM +0200, Baruch Siach wrote:
quoted hunk ↗ jump to hunk
The period is the sum of on and off values. Reported-by: Russell King <linux@armlinux.org.uk> Fixes: 757642f9a584e ("gpio: mvebu: Add limited PWM support") Signed-off-by: Baruch Siach <baruch@tkos.co.il> --- v6: divide (on + off) sum to reduce rounding error (RMK) --- drivers/gpio/gpio-mvebu.c | 19 ++++++++----------- 1 file changed, 8 insertions(+), 11 deletions(-)diff --git a/drivers/gpio/gpio-mvebu.c b/drivers/gpio/gpio-mvebu.c index 672681a976f5..a912a8fed197 100644 --- a/drivers/gpio/gpio-mvebu.c +++ b/drivers/gpio/gpio-mvebu.c@@ -676,20 +676,17 @@ static void mvebu_pwm_get_state(struct pwm_chip *chip, else state->duty_cycle = 1; + val = (unsigned long long) u; /* on duration */ regmap_read(mvpwm->regs, mvebu_pwmreg_blink_off_duration(mvpwm), &u); - val = (unsigned long long) u * NSEC_PER_SEC; + val += (unsigned long long) u; /* period = on + off duration */ + val *= NSEC_PER_SEC; do_div(val, mvpwm->clk_rate); - if (val < state->duty_cycle) { + if (val > UINT_MAX) + state->period = UINT_MAX;
state->period is an u64, so there is no reason to not use values greater than UINT_MAX.
+ else if (val) + state->period = val; + else state->period = 1;
This case assigning 1 looks strange. An explanation in a comment would
be great. I wonder if this is a hardware property or if it is only used
to not report 0 in case that mvpwm->clk_rate is high.
I found a few further shortcommings in the mvebu_pwm implementation while
looking through it:
a) The rounding problem that RMK found is also present in .apply
There we have:
val = clk_rate * (period - duty_cycle) / NSEC_PER_SEC
while
val = clk_rate * period / NSEC_PER_SEC - on
would be more exact.
b) To make
pwm_get_state(pwm, &state);
pwm_apply_state(pwm, &state);
idempotent .get_state should round up the division results.
c) .apply also has a check for val being zero and configures at least 1
cycle for the on and off intervals. Is this a hardware imposed
limitation?
Best regards
Uwe
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