Re: [PATCH v4 07/16] PM / devfreq: tegra: Properly disable interrupts

[Dmitry Osipenko]

04.06.2019 17:06, Thierry Reding ÐÐÑÐÑ:
> On Tue, Jun 04, 2019 at 04:40:18PM +0300, Dmitry Osipenko wrote:
>> 04.06.2019 14:07, Thierry Reding ÐÐÑÐÑ:
>>> On Thu, May 02, 2019 at 02:38:06AM +0300, Dmitry Osipenko wrote:
>>>> There is no guarantee that interrupt handling isn't running in parallel
>>>> with tegra_actmon_disable_interrupts(), hence it is necessary to protect
>>>> DEV_CTRL register accesses and clear IRQ status with ACTMON's IRQ being
>>>> disabled in the Interrupt Controller in order to ensure that device
>>>> interrupt is indeed being disabled.
>>>>
>>>> Reviewed-by: Chanwoo Choi <cw00.choi@xxxxxxxxxxx>
>>>> Signed-off-by: Dmitry Osipenko <digetx@xxxxxxxxx>
>>>> ---
>>>>  drivers/devfreq/tegra-devfreq.c | 21 +++++++++++++++------
>>>>  1 file changed, 15 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/devfreq/tegra-devfreq.c b/drivers/devfreq/tegra-devfreq.c
>>>> index b65313fe3c2e..ce1eb97a2090 100644
>>>> --- a/drivers/devfreq/tegra-devfreq.c
>>>> +++ b/drivers/devfreq/tegra-devfreq.c
>>>> @@ -171,6 +171,8 @@ struct tegra_devfreq {
>>>>  	struct notifier_block	rate_change_nb;
>>>>  
>>>>  	struct tegra_devfreq_device devices[ARRAY_SIZE(actmon_device_configs)];
>>>> +
>>>> +	int irq;
>>>
>>> Interrupts are typically unsigned int.
>>>
>>>>  };
>>>>  
>>>>  struct tegra_actmon_emc_ratio {
>>>> @@ -417,6 +419,8 @@ static void tegra_actmon_disable_interrupts(struct tegra_devfreq *tegra)
>>>>  	u32 val;
>>>>  	unsigned int i;
>>>>  
>>>> +	disable_irq(tegra->irq);
>>>> +
>>>>  	for (i = 0; i < ARRAY_SIZE(tegra->devices); i++) {
>>>>  		dev = &tegra->devices[i];
>>>>  
>>>> @@ -427,9 +431,14 @@ static void tegra_actmon_disable_interrupts(struct tegra_devfreq *tegra)
>>>>  		val &= ~ACTMON_DEV_CTRL_CONSECUTIVE_ABOVE_WMARK_EN;
>>>>  
>>>>  		device_writel(dev, val, ACTMON_DEV_CTRL);
>>>> +
>>>> +		device_writel(dev, ACTMON_INTR_STATUS_CLEAR,
>>>> +			      ACTMON_DEV_INTR_STATUS);
>>>>  	}
>>>>  
>>>>  	actmon_write_barrier(tegra);
>>>> +
>>>> +	enable_irq(tegra->irq);
>>>
>>> Why do we enable interrupts after this? Is there any use in having the
>>> top-level interrupt enabled if nothing's going to generate an interrupt
>>> anyway?
>>
>> There is no real point in having the interrupt enabled other than to
>> keep the enable count balanced.
>>
>> IIUC, we will need to disable IRQ at the driver's probe time (after
>> requesting the IRQ) if we want to avoid that (not really necessary)
>> balancing. This is probably something that could be improved in a
>> follow-up patches, if desired.
>>
>>>>  }
>>>>  
>>>>  static void tegra_actmon_configure_device(struct tegra_devfreq *tegra,
>>>> @@ -604,7 +613,6 @@ static int tegra_devfreq_probe(struct platform_device *pdev)
>>>>  	struct resource *res;
>>>>  	unsigned int i;
>>>>  	unsigned long rate;
>>>> -	int irq;
>>>>  	int err;
>>>>  
>>>>  	tegra = devm_kzalloc(&pdev->dev, sizeof(*tegra), GFP_KERNEL);
>>>> @@ -673,15 +681,16 @@ static int tegra_devfreq_probe(struct platform_device *pdev)
>>>>  		dev_pm_opp_add(&pdev->dev, rate, 0);
>>>>  	}
>>>>  
>>>> -	irq = platform_get_irq(pdev, 0);
>>>> -	if (irq < 0) {
>>>> -		dev_err(&pdev->dev, "Failed to get IRQ: %d\n", irq);
>>>> -		return irq;
>>>> +	tegra->irq = platform_get_irq(pdev, 0);
>>>> +	if (tegra->irq < 0) {
>>>> +		err = tegra->irq;
>>>> +		dev_err(&pdev->dev, "Failed to get IRQ: %d\n", err);
>>>> +		return err;
>>>>  	}
>>>
>>> This is very oddly written. tegra->irq should really be an unsigned int
>>> since that's the standard type for interrupt numbers. But since you need
>>> to be able to detect errors from platform_get_irq() it now becomes
>>> natural to write this as:
>>>
>>> 	err = platform_get_irq(pdev, 0);
>>> 	if (err < 0) {
>>> 		dev_err(...);
>>> 		return err;
>>> 	}
>>>
>>> 	tegra->irq = err;
>>>
>>> Two birds with one stone. I suppose this could be done in a follow-up
>>> patch since it isn't practically wrong in your version, so either way:
>>>
>>> Acked-by: Thierry Reding <treding@xxxxxxxxxx>
>>>
>>
>> Thank you for the ACK! Although, I disagree with yours suggestion, to me
>> that makes code a bit less straightforward and it's not really
>> worthwhile to bloat the code just because technically IRQ's are unsigned
>> numbers (we don't care about that). It also makes me a bit uncomfortable
>> to see "err" assigned to a variable, I don't think it's a good practice.
> 
> Actually you should care that IRQs are unsigned. Implicit casting from
> a potentially negative value can hide bugs. That is, once you've passed
> that negative value into the IRQ API you loose the context that it could
> be an error code. Hence I think it makes sense to always store values in
> the native type, and only store them if they are actually valid.
> 
> In the above you have an error value in tegra->irq. In this particular
> case it's pretty harmless because you don't do anything with it, but if
> the circumstances were slightly different that could lead to problems
> down the road.
> 
> On the other hand what I was proposing makes it pretty clear from the
> context that err contains a valid interrupt number when it is assigned
> to tegra->irq. There's plenty of similar constructs in the kernel if you
> want to grep for it.
> 
> Also, it's not bloating the code at all. It's the exact same number of
> lines of code as your variant.

I agree that it is better to maintain proper typing everywhere in
general, I have been bitten so many times by typecasting bugs..
Opentegra's Bool (unsigned) -> BOOL (signed) casting horror was the most
recent one. Well, I guess indeed it won't hurt to apply your suggestion
in a follow-up patch to keep things a bit more consistent.