Re: [PATCH 4/7] pwm: jz4740: Improve algorithm of clock calculation

[Paul Cercueil <paul@crapouillou.net>]

Le ven. 9 août 2019 à 19:05, Uwe =?iso-8859-1?q?Kleine-K=F6nig?= 
<u.kleine-koenig@pengutronix.de> a écrit :
> On Fri, Aug 09, 2019 at 02:30:28PM +0200, Paul Cercueil wrote:
>>  The previous algorithm hardcoded details about how the TCU clocks 
>> work.
>>  The new algorithm will use clk_round_rate to find the perfect clock 
>> rate
>>  for the PWM channel.
>> 
>>  Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>>  Tested-by: Mathieu Malaterre <malat@debian.org>
>>  Tested-by: Artur Rojek <contact@artur-rojek.eu>
>>  ---
>>   drivers/pwm/pwm-jz4740.c | 60 
>> +++++++++++++++++++++++++++++-----------
>>   1 file changed, 44 insertions(+), 16 deletions(-)
>> 
>>  diff --git a/drivers/pwm/pwm-jz4740.c b/drivers/pwm/pwm-jz4740.c
>>  index 6ec8794d3b99..f20dc2e19240 100644
>>  --- a/drivers/pwm/pwm-jz4740.c
>>  +++ b/drivers/pwm/pwm-jz4740.c
>>  @@ -110,24 +110,56 @@ static int jz4740_pwm_apply(struct pwm_chip 
>> *chip, struct pwm_device *pwm,
>>   	struct jz4740_pwm_chip *jz4740 = to_jz4740(pwm->chip);
>>   	struct clk *clk = pwm_get_chip_data(pwm),
>>   		   *parent_clk = clk_get_parent(clk);
>>  -	unsigned long rate, period, duty;
>>  +	unsigned long rate, parent_rate, period, duty;
>>   	unsigned long long tmp;
>>  -	unsigned int prescaler = 0;
>>  +	int ret;
>> 
>>  -	rate = clk_get_rate(parent_clk);
>>  -	tmp = (unsigned long long)rate * state->period;
>>  -	do_div(tmp, 1000000000);
>>  -	period = tmp;
>>  +	parent_rate = clk_get_rate(parent_clk);
>>  +
>>  +	jz4740_pwm_disable(chip, pwm);
>> 
>>  -	while (period > 0xffff && prescaler < 6) {
>>  -		period >>= 2;
>>  -		rate >>= 2;
>>  -		++prescaler;
>>  +	/* Reset the clock to the maximum rate, and we'll reduce it if 
>> needed */
>>  +	ret = clk_set_max_rate(clk, parent_rate);
> 
> What is the purpose of this call? IIUC this limits the allowed range 
> of
> rates for clk. I assume the idea is to prevent other consumers to 
> change
> the rate in a way that makes it unsuitable for this pwm. But this only
> makes sense if you had a notifier for clk changes, doesn't it? I'm
> confused.

Nothing like that. The second call to clk_set_max_rate() might have set
a maximum clock rate that's lower than the parent's rate, and we want to
undo that.


> I think this doesn't match the commit log, you didn't even introduced 
> a
> call to clk_round_rate().

Right, I'll edit the commit message.


>>  +	if (ret) {
>>  +		dev_err(chip->dev, "Unable to set max rate: %d\n", ret);
>>  +		return ret;
>>   	}
>> 
>>  -	if (prescaler == 6)
>>  -		return -EINVAL;
>>  +	ret = clk_set_rate(clk, parent_rate);
>>  +	if (ret) {
>>  +		dev_err(chip->dev, "Unable to reset to parent rate (%lu Hz)",
>>  +			parent_rate);
>>  +		return ret;
>>  +	}
>>  +
>>  +	/*
>>  +	 * Limit the clock to a maximum rate that still gives us a period 
>> value
>>  +	 * which fits in 16 bits.
>>  +	 */
>>  +	tmp = 0xffffull * NSEC_PER_SEC;
>>  +	do_div(tmp, state->period);
>> 
>>  +	ret = clk_set_max_rate(clk, tmp);
> 
> And now you change the maximal rate again?

Basically, we start from the maximum clock rate we can get for that PWM
- which is the rate of the parent clk - and from that compute the 
maximum
clock rate that we can support that still gives us < 16-bits hardware
values for the period and duty.

We then pass that computed maximum clock rate to clk_set_max_rate(), 
which
may or may not update the current PWM clock's rate to match the new 
limits.
Finally we read back the PWM clock's rate and compute the period and 
duty
from that.


>>  +	if (ret) {
>>  +		dev_err(chip->dev, "Unable to set max rate: %d\n", ret);
>>  +		return ret;
>>  +	}
>>  +
>>  +	/*
>>  +	 * Read back the clock rate, as it may have been modified by
>>  +	 * clk_set_max_rate()
>>  +	 */
>>  +	rate = clk_get_rate(clk);
>>  +
>>  +	if (rate != parent_rate)
>>  +		dev_dbg(chip->dev, "PWM clock updated to %lu Hz\n", rate);
>>  +
>>  +	/* Calculate period value */
>>  +	tmp = (unsigned long long)rate * state->period;
>>  +	do_div(tmp, NSEC_PER_SEC);
>>  +	period = (unsigned long)tmp;
>>  +
>>  +	/* Calculate duty value */
>>   	tmp = (unsigned long long)period * state->duty_cycle;
>>   	do_div(tmp, state->period);
>>   	duty = period - tmp;
>>  @@ -135,14 +167,10 @@ static int jz4740_pwm_apply(struct pwm_chip 
>> *chip, struct pwm_device *pwm,
>>   	if (duty >= period)
>>   		duty = period - 1;
>> 
>>  -	jz4740_pwm_disable(chip, pwm);
>>  -
>>   	/* Set abrupt shutdown */
>>   	regmap_update_bits(jz4740->map, TCU_REG_TCSRc(pwm->hwpwm),
>>   			   TCU_TCSR_PWM_SD, TCU_TCSR_PWM_SD);
>> 
>>  -	clk_set_rate(clk, rate);
>>  -
> 
> It's not obvious to me why removing these two lines belong in the
> current patch.

They're not removed, they're both moved up in the function.


> Best regards
> Uwe
> 
> --
> Pengutronix e.K.                           | Uwe Kleine-König        
>     |
> Industrial Linux Solutions                 | 
> http://www.pengutronix.de/  |