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

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Date:   Fri, 13 Oct 2023 13:13:50 +0200
From:   Alexander Stein <alexander.stein@...tq-group.com>
To:     Sean Young <sean@...s.org>, linux-media@...r.kernel.org,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Florian Fainelli <florian.fainelli@...adcom.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Stefan Wahren <wahrenst@....net>
Cc:     linux-pwm@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] pwm: bcm2835: allow pwm driver to be used in atomic context

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.

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@...s.org>
> > ---
> > 
> >   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;


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