linux-kernel - Re: [PATCH] PM / OPP: Initialize regulator pointer to an error value

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Date:	Tue, 16 Feb 2016 06:30:59 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linaro-kernel@...ts.linaro.org, Rafael Wysocki <rjw@...ysocki.net>,
	Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Jon Hunter <jonathanh@...dia.com>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH] PM / OPP: Initialize regulator pointer to an error value

Cc'ing Mark as well.

On 15-02-16, 21:38, Arnd Bergmann wrote:
> There is usually something else wrong if you have to check for both.
> Why exactly do you need to check for both IS_ERR and NULL?

And here is the reasoning behind it:
- It is normally said that 'NULL' is a valid clk. The same is
  applicable to regulators as well, right? At least, that is what
  below says:

  commit 4a511de96d69 ("cpufreq: cpufreq-cpu0: NULL is a valid
  regulator")

- And so I left the regulator pointer to NULL in OPP core.
- But then I realized that its not safe to call many regulator core
  APIs with NULL regulator, as those caused the crashes reported by
  multiple people now.
- clk APIs guarantee that they return early when NULL clk is passed to
  them.
- Do we need to do the same for regulator core as well ?

- And so I initialized the pointer to an error value now, as
  initializing it to NULL (possibly a valid regulator, in theory)
  isn't the right thing to do.

> > diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
> > index d7cd4e265766..146b6197d598 100644
> > --- a/drivers/base/power/opp/core.c
> > +++ b/drivers/base/power/opp/core.c
> > @@ -257,7 +257,7 @@ unsigned long dev_pm_opp_get_max_volt_latency(struct device *dev)
> >  	}
> >  
> >  	reg = dev_opp->regulator;
> > -	if (IS_ERR_OR_NULL(reg)) {
> > +	if (IS_ERR(reg)) {
> >  		/* Regulator may not be required for device */
> >  		if (reg)
> >  			dev_err(dev, "%s: Invalid regulator (%ld)\n", __func__,
> > @@ -798,6 +798,9 @@ static struct device_opp *_add_device_opp(struct device *dev)
> >  		of_node_put(np);
> >  	}
> >  
> > +	/* Set regulator to a non-NULL error value */
> > +	dev_opp->regulator = ERR_PTR(-EFAULT);
> > +
> >  	/* Find clk for the device */
> >  	dev_opp->clk = clk_get(dev, NULL);
> >  	if (IS_ERR(dev_opp->clk)) {
> 
> -EFAULT has a very specific meaning (accessing an invalid pointer from
> user space), I don't think you want that one.

Sorry, wasn't aware of those requirements. What Rafael suggested is
the right thing to do then.

-- 
viresh