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Subject: Re: [PATCH v4 1/2] soc/tegra: pmc: Query PCLK clock rate at probe
 time
To:     Jon Hunter <jonathanh@nvidia.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Peter De Schrijver <pdeschrijver@nvidia.com>
Cc:     linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20190804202927.15014-1-digetx@gmail.com>
 <23856887-06b0-66a8-1df2-ef4d7b48dc68@nvidia.com>
 <854c90c4-9e42-2020-5fa7-8711203f56a5@gmail.com>
 <c5a5a059-c949-1a25-c7ac-59d88ad9a026@nvidia.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <ff1a790f-baba-dd0b-8cbe-f9468fbd56e3@gmail.com>
Date:   Mon, 23 Sep 2019 16:31:43 +0300
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23.09.2019 16:01, Jon Hunter пишет:
> 
> On 23/09/2019 13:49, Dmitry Osipenko wrote:
>> 23.09.2019 13:56, Jon Hunter пишет:
>>>
>>>
>>> On 04/08/2019 21:29, Dmitry Osipenko wrote:
>>>> It is possible to get a lockup if kernel decides to enter LP2 cpuidle
>>>> from some clk-notifier, in that case CCF's "prepare" mutex is kept locked
>>>> and thus clk_get_rate(pclk) blocks on the same mutex with interrupts being
>>>> disabled.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>>>> ---
>>>>
>>>> Changelog:
>>>>
>>>> v4: Added clk-notifier to track PCLK rate-changes, which may become useful
>>>>     in the future. That's done in response to v3 review comment from Peter
>>>>     De Schrijver.
>>>>
>>>>     Now properly handling case where clk pointer is intentionally NULL on
>>>>     the driver's probe.
>>>>
>>>> v3: Changed commit's message because I actually recalled what was the
>>>>     initial reason for the patch, since the problem reoccurred once again.
>>>>
>>>> v2: Addressed review comments that were made by Jon Hunter to v1 by
>>>>     not moving the memory barrier, replacing one missed clk_get_rate()
>>>>     with pmc->rate, handling possible clk_get_rate() error on probe and
>>>>     slightly adjusting the commits message.
>>>>
>>>>  drivers/soc/tegra/pmc.c | 71 ++++++++++++++++++++++++++++++-----------
>>>>  1 file changed, 53 insertions(+), 18 deletions(-)
>>>>
>>>> diff --git a/drivers/soc/tegra/pmc.c b/drivers/soc/tegra/pmc.c
>>>> index 9f9c1c677cf4..4e44943d0b26 100644
>>>> --- a/drivers/soc/tegra/pmc.c
>>>> +++ b/drivers/soc/tegra/pmc.c
>>>> @@ -309,6 +309,7 @@ static const char * const tegra210_reset_sources[] = {
>>>>   * @pctl_dev: pin controller exposed by the PMC
>>>>   * @domain: IRQ domain provided by the PMC
>>>>   * @irq: chip implementation for the IRQ domain
>>>> + * @clk_nb: pclk clock changes handler
>>>>   */
>>>>  struct tegra_pmc {
>>>>  	struct device *dev;
>>>> @@ -344,6 +345,8 @@ struct tegra_pmc {
>>>>  
>>>>  	struct irq_domain *domain;
>>>>  	struct irq_chip irq;
>>>> +
>>>> +	struct notifier_block clk_nb;
>>>>  };
>>>>  
>>>>  static struct tegra_pmc *pmc = &(struct tegra_pmc) {
>>>> @@ -1192,7 +1195,7 @@ static int tegra_io_pad_prepare(struct tegra_pmc *pmc, enum tegra_io_pad id,
>>>>  		return err;
>>>>  
>>>>  	if (pmc->clk) {
>>>> -		rate = clk_get_rate(pmc->clk);
>>>> +		rate = pmc->rate;
>>>>  		if (!rate) {
>>>>  			dev_err(pmc->dev, "failed to get clock rate\n");
>>>>  			return -ENODEV;
>>>
>>> So this error should never happen now, right? Assuming that rate is
>>> never set to 0. But ...
>>
>> Good catch!
>>
>>>> @@ -1433,6 +1436,7 @@ void tegra_pmc_set_suspend_mode(enum tegra_suspend_mode mode)
>>>>  void tegra_pmc_enter_suspend_mode(enum tegra_suspend_mode mode)
>>>>  {
>>>>  	unsigned long long rate = 0;
>>>> +	u64 ticks;
>>>>  	u32 value;
>>>>  
>>>>  	switch (mode) {
>>>> @@ -1441,31 +1445,22 @@ void tegra_pmc_enter_suspend_mode(enum tegra_suspend_mode mode)
>>>>  		break;
>>>>  
>>>>  	case TEGRA_SUSPEND_LP2:
>>>> -		rate = clk_get_rate(pmc->clk);
>>>> +		rate = pmc->rate;
>>>>  		break;
>>>>  
>>>>  	default:
>>>>  		break;
>>>>  	}
>>>>  
>>>> -	if (WARN_ON_ONCE(rate == 0))
>>>> -		rate = 100000000;
>>>> -
>>>> -	if (rate != pmc->rate) {
>>>> -		u64 ticks;
>>>> -
>>>> -		ticks = pmc->cpu_good_time * rate + USEC_PER_SEC - 1;
>>>> -		do_div(ticks, USEC_PER_SEC);
>>>> -		tegra_pmc_writel(pmc, ticks, PMC_CPUPWRGOOD_TIMER);
>>>> +	ticks = pmc->cpu_good_time * rate + USEC_PER_SEC - 1;
>>>> +	do_div(ticks, USEC_PER_SEC);
>>>> +	tegra_pmc_writel(pmc, ticks, PMC_CPUPWRGOOD_TIMER);
>>>>  
>>>> -		ticks = pmc->cpu_off_time * rate + USEC_PER_SEC - 1;
>>>> -		do_div(ticks, USEC_PER_SEC);
>>>> -		tegra_pmc_writel(pmc, ticks, PMC_CPUPWROFF_TIMER);
>>>> +	ticks = pmc->cpu_off_time * rate + USEC_PER_SEC - 1;
>>>> +	do_div(ticks, USEC_PER_SEC);
>>>> +	tegra_pmc_writel(pmc, ticks, PMC_CPUPWROFF_TIMER);
>>>>  
>>>> -		wmb();
>>>> -
>>>> -		pmc->rate = rate;
>>>> -	}
>>>> +	wmb();
>>>>  
>>>>  	value = tegra_pmc_readl(pmc, PMC_CNTRL);
>>>>  	value &= ~PMC_CNTRL_SIDE_EFFECT_LP0;
>>>> @@ -2019,6 +2014,20 @@ static int tegra_pmc_irq_init(struct tegra_pmc *pmc)
>>>>  	return 0;
>>>>  }
>>>>  
>>>> +static int tegra_pmc_clk_notify_cb(struct notifier_block *nb,
>>>> +				   unsigned long action, void *ptr)
>>>> +{
>>>> +	struct clk_notifier_data *data = ptr;
>>>> +	struct tegra_pmc *pmc;
>>>> +
>>>> +	if (action == POST_RATE_CHANGE) {
>>>> +		pmc = container_of(nb, struct tegra_pmc, clk_nb);
>>>> +		pmc->rate = data->new_rate;
>>>> +	}
>>>> +
>>>> +	return NOTIFY_OK;
>>>> +}
>>>> +
>>>>  static int tegra_pmc_probe(struct platform_device *pdev)
>>>>  {
>>>>  	void __iomem *base;
>>>> @@ -2082,6 +2091,30 @@ static int tegra_pmc_probe(struct platform_device *pdev)
>>>>  		pmc->clk = NULL;
>>>>  	}
>>>>  
>>>> +	/*
>>>> +	 * PCLK clock rate can't be retrieved using CLK API because it
>>>> +	 * causes lockup if CPU enters LP2 idle state from some other
>>>> +	 * CLK notifier, hence we're caching the rate's value locally.
>>>> +	 */
>>>> +	if (pmc->clk) {
>>>> +		pmc->clk_nb.notifier_call = tegra_pmc_clk_notify_cb;
>>>> +		err = clk_notifier_register(pmc->clk, &pmc->clk_nb);
>>>> +		if (err) {
>>>> +			dev_err(&pdev->dev,
>>>> +				"failed to register clk notifier\n");
>>>> +			return err;
>>>> +		}
>>>> +
>>>> +		pmc->rate = clk_get_rate(pmc->clk);
>>>> +	}
>>>> +
>>>> +	if (!pmc->rate) {
>>>> +		if (pmc->clk)
>>>> +			dev_err(&pdev->dev, "failed to get pclk rate\n");
>>>> +
>>>> +		pmc->rate = 100000000;
>>>
>>> I wonder if we should just let this fail. Or set to 0 so that if the
>>> rate is not set we will never suspend or configure the IO pads? I could
>>> run some quick tests to see if there are any problems by failing here.
>>
>> Do you mean to fail the PMC driver to probe? I guess that will be fatal
>> and system won't be in a useful state, from a user perspective that
>> should be equal to a hang on boot with a black screen. On the other
>> hand, it seems that failing tegra_io_pad_prepare() should have similar
>> fatal result.
>>
>> I'm wondering whether that IO PAD misconfiguration could be harmful. If
>> not, then looks like falling back to 100Mhz should be good enough. In
>> practice clk_get_rate() shouldn't ever fail unless there is some serious
>> bug in clk/. What do you think?
> 
> Exactly. I think that if clk_get_rate() is failing then something bad is
> happening. I can see if this causes any obvious problems across the
> different boards we test, but it would be great to get rid of this
> 100MHz value (unless Peter knows of a good reason to keep it).

Okay!

Peter, do you have any thoughts about whether it worth to keep the
100MHz workaround?

BTW.. looking at tegra_io_pad_prepare() again, I think that it should be
fine to simply keep the clk_get_rate() there.

It also looks like clk notifier should actually take powergates_lock to
be really robust and not potentially race with tegra_io_pad_prepare(). I
can fix up it in v5, but.. maybe it will be better to postpone the clk
notifier addition until there will be a real use-case for PMC clk
freq-scaling and for now assume that clk rate is static?