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Subject: Re: [PATCH v4 1/2] soc/tegra: pmc: Query PCLK clock rate at probe
 time
From:   Dmitry Osipenko <digetx@gmail.com>
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>
 <ff1a790f-baba-dd0b-8cbe-f9468fbd56e3@gmail.com>
Message-ID: <546ec91f-2486-21e1-9fdf-b3d087e176ed@gmail.com>
Date:   Mon, 23 Sep 2019 16:36:47 +0300
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23.09.2019 16:31, Dmitry Osipenko пишет:
> 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 will be fine without having the clk notifier or without the locking
within the notifier that I suggested below]

> 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?
>