Re: [PATCH v2 2/3] pwm: bcm2835: allow pwm driver to be used in atomic context

[Stefan Wahren]

Hi Alexander,

Am 13.10.23 um 13:13 schrieb Alexander Stein:
> Hi,
>
> Am Freitag, 13. Oktober 2023, 13:04:48 CEST schrieb Stefan Wahren:
> > Hi Sean,
> >
> > Am 13.10.23 um 12:46 schrieb Sean Young:
> > > clk_get_rate() may do a mutex lock. Since the clock rate cannot change on
> > > an rpi, simply fetch it once.
> > does it mean you checked all possible SoCs (BCM2835, BCM2836, BCM2837,
> > BCM2711, BCM2712) for this change?
> >
> > Is it impossible that the real clock can never be influenced by turbo
> > mode like SPI?
> Assuming the clock can change, which I would, then a clock notifier seems
> appropriate. See [1] for an example.
i remember a similiar approach for the CPU frequency for the RPi. At end
the we decided to let the firmware handle it and don't use clock
notifier, see [2] and the related links for more background. The fact
that the VideoCore has the real control makes it hard.

I don't want to say that's impossible.

[2] -
https://patchwork.kernel.org/project/linux-arm-kernel/cover/20190520104708.11980-1-nsaenzjulienne@xxxxxxx/
> Best regards,
> Alexander
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
> commit/?id=90ad2cbe88c22d0215225ab9594eeead0eb24fde
>
> > Best regards
> >
> > > Signed-off-by: Sean Young <sean@xxxxxxxx>
> > > ---
> > >
> > >    drivers/pwm/pwm-bcm2835.c | 21 ++++++++++++---------
> > >    1 file changed, 12 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/pwm/pwm-bcm2835.c b/drivers/pwm/pwm-bcm2835.c
> > > index bdfc2a5ec0d6..59ea154dd657 100644
> > > --- a/drivers/pwm/pwm-bcm2835.c
> > > +++ b/drivers/pwm/pwm-bcm2835.c
> > > @@ -28,6 +28,7 @@ struct bcm2835_pwm {
> > >
> > >    	struct device *dev;
> > >    	void __iomem *base;
> > >    	struct clk *clk;
> > >
> > > +	unsigned long rate;
> > >
> > >    };
> > >
> > >    static inline struct bcm2835_pwm *to_bcm2835_pwm(struct pwm_chip *chip)
> > >
> > > @@ -63,17 +64,11 @@ static int bcm2835_pwm_apply(struct pwm_chip *chip,
> > > struct pwm_device *pwm,>
> > >    {
> > >
> > >    	struct bcm2835_pwm *pc = to_bcm2835_pwm(chip);
> > >
> > > -	unsigned long rate = clk_get_rate(pc->clk);
> > >
> > >    	unsigned long long period_cycles;
> > >    	u64 max_period;
> > >
> > >    	u32 val;
> > >
> > > -	if (!rate) {
> > > -		dev_err(pc->dev, "failed to get clock rate\n");
> > > -		return -EINVAL;
> > > -	}
> > > -
> > >
> > >    	/*
> > >
> > >    	 * period_cycles must be a 32 bit value, so period * rate /
> > >    	 NSEC_PER_SEC
> > >    	 * must be <= U32_MAX. As U32_MAX * NSEC_PER_SEC < U64_MAX the
> > >
> > > @@ -88,13 +83,13 @@ static int bcm2835_pwm_apply(struct pwm_chip *chip,
> > > struct pwm_device *pwm,>
> > >    	 * <=> period < ((U32_MAX * NSEC_PER_SEC + NSEC_PER_SEC/2) / rate
> > >    	 * <=> period <= ceil((U32_MAX * NSEC_PER_SEC + NSEC_PER_SEC/2) /
> rate)
> > >    	 - 1
> > >    	 */
> > >
> > > -	max_period = DIV_ROUND_UP_ULL((u64)U32_MAX * NSEC_PER_SEC +
> NSEC_PER_SEC
> > > / 2, rate) - 1; +	max_period = DIV_ROUND_UP_ULL((u64)U32_MAX *
> > > NSEC_PER_SEC + NSEC_PER_SEC / 2, pc->rate) - 1;>
> > >    	if (state->period > max_period)
> > >
> > >    		return -EINVAL;
> > >
> > >    	/* set period */
> > >
> > > -	period_cycles = DIV_ROUND_CLOSEST_ULL(state->period * rate,
> > > NSEC_PER_SEC); +	period_cycles = DIV_ROUND_CLOSEST_ULL(state->period *
> > > pc->rate, NSEC_PER_SEC);>
> > >    	/* don't accept a period that is too small */
> > >    	if (period_cycles < PERIOD_MIN)
> > >
> > > @@ -103,7 +98,7 @@ static int bcm2835_pwm_apply(struct pwm_chip *chip,
> > > struct pwm_device *pwm,>
> > >    	writel(period_cycles, pc->base + PERIOD(pwm->hwpwm));
> > >
> > >    	/* set duty cycle */
> > >
> > > -	val = DIV_ROUND_CLOSEST_ULL(state->duty_cycle * rate, NSEC_PER_SEC);
> > > +	val = DIV_ROUND_CLOSEST_ULL(state->duty_cycle * pc->rate,
> NSEC_PER_SEC);
> > >    	writel(val, pc->base + DUTY(pwm->hwpwm));
> > >
> > >    	/* set polarity */
> > >
> > > @@ -129,6 +124,7 @@ static const struct pwm_ops bcm2835_pwm_ops = {
> > >
> > >    	.request = bcm2835_pwm_request,
> > >    	.free = bcm2835_pwm_free,
> > >    	.apply = bcm2835_pwm_apply,
> > >
> > > +	.atomic = true,
> > >
> > >    	.owner = THIS_MODULE,
> > >
> > >    };
> > >
> > > @@ -156,6 +152,13 @@ static int bcm2835_pwm_probe(struct platform_device
> > > *pdev)>
> > >    	if (ret)
> > >
> > >    		return ret;
> > >
> > > +	pc->rate = clk_get_rate(pc->clk);
> > > +	if (!pc->rate) {
> > > +		dev_err(pc->dev, "failed to get clock rate\n");
> > > +		ret = -EINVAL;
> > > +		goto add_fail;
> > > +	}
> > > +
> > >
> > >    	pc->chip.dev = &pdev->dev;
> > >    	pc->chip.ops = &bcm2835_pwm_ops;
> > >    	pc->chip.npwm = 2;