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

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Message-ID: <cc563313-32d7-4098-b93a-1fa06b8afb02@gmx.net>
Date:   Fri, 13 Oct 2023 13:44:03 +0200
From:   Stefan Wahren <wahrenst@....net>
To:     Alexander Stein <alexander.stein@...tq-group.com>,
        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>
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 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@suse.de/
>
> 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;
>