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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>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <854c90c4-9e42-2020-5fa7-8711203f56a5@gmail.com>
Date:   Mon, 23 Sep 2019 15:49:52 +0300
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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?