Re: [PATCH v9 4/6] pwm: dwc: use clock rate in hz to avoid rounding issues
From: Ben Dooks
Date: Mon Sep 11 2023 - 18:16:56 EST
On 07/09/2023 22:34, Uwe Kleine-König wrote:
Hello,
[Dropped William Salmon and Jude Onyenegecha from the list of recipents,
their email addresses don't seem to work any more.]
On Thu, Sep 07, 2023 at 05:12:40PM +0100, Ben Dooks wrote:
As noted, the clock-rate when not a nice multiple of ns is probably
going to end up with inacurate calculations, as well as on a non pci
system the rate may change (although we've not put a clock rate
change notifier in this code yet) so we also add some quick checks
of the rate when we do any calculations with it.
Signed-off-by; Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
Reported-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
v9:
- fixed commit spelling
- changed to use codethink email instead of sifive
v8:
- fixup post rename
- move to earlier in series
---
drivers/pwm/pwm-dwc-core.c | 24 +++++++++++++++---------
drivers/pwm/pwm-dwc.h | 2 +-
2 files changed, 16 insertions(+), 10 deletions(-)
diff --git a/drivers/pwm/pwm-dwc-core.c b/drivers/pwm/pwm-dwc-core.c
index 3fc281a78c9a..3b856685029d 100644
--- a/drivers/pwm/pwm-dwc-core.c
+++ b/drivers/pwm/pwm-dwc-core.c
@@ -49,13 +49,14 @@ static int __dwc_pwm_configure_timer(struct dwc_pwm *dwc,
* periods and check are the result within HW limits between 1 and
* 2^32 periods.
*/
- tmp = DIV_ROUND_CLOSEST_ULL(state->duty_cycle, dwc->clk_ns);
+ tmp = state->duty_cycle * dwc->clk_rate;
This might overflow. You can prevent this by asserting that clk_rate is
<= NSEC_PER_SEC and using mul_u64_u64_div_u64.
+ tmp = DIV_ROUND_CLOSEST_ULL(tmp, NSEC_PER_SEC);
if (tmp < 1 || tmp > (1ULL << 32))
return -ERANGE;
low = tmp - 1;
- tmp = DIV_ROUND_CLOSEST_ULL(state->period - state->duty_cycle,
- dwc->clk_ns);
+ tmp = (state->period - state->duty_cycle) * dwc->clk_rate;
+ tmp = DIV_ROUND_CLOSEST_ULL(tmp, NSEC_PER_SEC);
if (tmp < 1 || tmp > (1ULL << 32))
return -ERANGE;
high = tmp - 1;
@@ -121,11 +122,14 @@ static int dwc_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
struct pwm_state *state)
{
struct dwc_pwm *dwc = to_dwc_pwm(chip);
+ unsigned long clk_rate;
u64 duty, period;
u32 ctrl, ld, ld2;
pm_runtime_get_sync(chip->dev);
+ clk_rate = dwc->clk_rate;
+
ctrl = dwc_pwm_readl(dwc, DWC_TIM_CTRL(pwm->hwpwm));
ld = dwc_pwm_readl(dwc, DWC_TIM_LD_CNT(pwm->hwpwm));
ld2 = dwc_pwm_readl(dwc, DWC_TIM_LD_CNT2(pwm->hwpwm));
@@ -137,17 +141,19 @@ static int dwc_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
* based on the timer load-count only.
*/
if (ctrl & DWC_TIM_CTRL_PWM) {
- duty = (ld + 1) * dwc->clk_ns;
- period = (ld2 + 1) * dwc->clk_ns;
+ duty = ld + 1;
+ period = ld2 + 1;
period += duty;
} else {
- duty = (ld + 1) * dwc->clk_ns;
+ duty = ld + 1;
period = duty * 2;
}
+ duty *= NSEC_PER_SEC;
+ period *= NSEC_PER_SEC;
A comment that/why this cannot overflow would be nice. (I didn't check,
maybe it can?)
I /think/ that as long as NSEC_PER_SEC 2^32 then this shouldn't
overflow.
+ state->period = DIV_ROUND_CLOSEST_ULL(period, clk_rate);
+ state->duty_cycle = DIV_ROUND_CLOSEST_ULL(duty, clk_rate);
Without having thought deeply about this, I think you need to round up
here. Hmm, but given that .apply() uses round-closest, too, this needs
to be addressed separately.
(The ugly thing about round-closest is that .apply(mypwm,
.get_state(mypwm)) isn't idempotent in general. Consider a PWM that can
implement period = 41.7ns and period = 42.4 ns. If it's configured with
42.4, .get_state will return period = 42. Reapplying this will configure
for 41.7ns. This won't happen with the PCI clkrate, but it might in the
of case. Another reason to use rounding-down in .apply is that
mul_u64_u64_div_u64 doesn't have a round-nearest variant.)
state->polarity = PWM_POLARITY_INVERSED;
- state->period = period;
- state->duty_cycle = duty;
pm_runtime_put_sync(chip->dev);
@@ -168,7 +174,7 @@ struct dwc_pwm *dwc_pwm_alloc(struct device *dev)
if (!dwc)
return NULL;
- dwc->clk_ns = 10;
+ dwc->clk_rate = NSEC_PER_SEC / 10;
dwc->chip.dev = dev;
dwc->chip.ops = &dwc_pwm_ops;
dwc->chip.npwm = DWC_TIMERS_TOTAL;
diff --git a/drivers/pwm/pwm-dwc.h b/drivers/pwm/pwm-dwc.h
index 64795247c54c..e0a940fd6e87 100644
--- a/drivers/pwm/pwm-dwc.h
+++ b/drivers/pwm/pwm-dwc.h
@@ -42,7 +42,7 @@ struct dwc_pwm_ctx {
struct dwc_pwm {
struct pwm_chip chip;
void __iomem *base;
- unsigned int clk_ns;
+ unsigned long clk_rate;
Given that clk_ns was only introduced in patch #2 I think it would be
cleaner to squash these two patches together.
I'll have a look at how much work re-ordering the patches would
be.
--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius
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