linux-kernel - Re: [PATCHv7 1/4] pwm: Add Freescale FTM PWM driver support

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Date:	Tue, 17 Dec 2013 13:24:33 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Xiubo Li <Li.Xiubo@...escale.com>, mark.rutland@....com,
	s.hauer@...gutronix.de, galak@...eaurora.org,
	swarren@...dotorg.org, t.figa@...sung.com, grant.likely@...aro.org,
	matt.porter@...aro.org, rob@...dley.net, tomasz.figa@...il.com,
	ian.campbell@...rix.com, pawel.moll@....com,
	rob.herring@...xeda.com, linux-arm-kernel@...ts.infradead.org,
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	Alison Wang <b18965@...escale.com>,
	Jingchang Lu <b35083@...escale.com>
Subject: Re: [PATCHv7 1/4] pwm: Add Freescale FTM PWM driver support

On Tue, Dec 17, 2013 at 11:51:36AM +0000, Russell King - ARM Linux wrote:
> On Tue, Dec 17, 2013 at 12:10:22PM +0100, Thierry Reding wrote:
> > On Fri, Dec 13, 2013 at 04:57:04PM +0800, Xiubo Li wrote:
> > > +static inline u32 fsl_pwm_readl(struct fsl_pwm_chip *fpc,
> > > +		const void __iomem *addr)
> > > +{
> > > +	u32 val;
> > > +
> > > +	val = __raw_readl(addr);
> > > +
> > > +	if (likely(fpc->big_endian))
> > 
> > The likely() probably isn't very useful in this case. But if you want to
> > keep it, it should at least be reversed, since little-endian is actually
> > the default (you have to specify the big-endian property to activate the
> > big endian mode).
> > 
> > > +		val = be32_to_cpu(val);
> > > +	else
> > > +		val = le32_to_cpu(val);
> 
> This will also cause sparse errors, because when sparse is enabled, these
> expect __le32 or __be32 arguments, not u32.
> 
> > > +	rmb();
> > 
> > I'd prefer the rmb() to follow the __raw_readl() immediately to make the
> > relationship more explicit.
> 
> A better question to ask is: why is this barrier here?  What memory
> ordering operations is it trying to serialise?

I suppose that this was done so that the accesses would essentially
remain the same as those performed by readl() and writel().

> > > +static inline void fsl_pwm_writel(struct fsl_pwm_chip *fpc,
> > > +		u32 val, void __iomem *addr)
> > > +{
> > > +	wmb();
> > > +	if (likely(fpc->big_endian))
> > > +		val = cpu_to_be32(val);
> > > +	else
> > > +		val = cpu_to_le32(val);
> > > +
> > > +	__raw_writel(val, addr);
> > 
> > Same here. wmb() should precede __raw_writel() immediately.
> 
> Same comments here - what memory operations is the wmb() trying to
> serialise?  Does this PWM driver somehow end up doing DMA?

Not that I can see. But if my understanding is correct, not using the
barriers would allow the compiler and CPU to reorder accesses, and by
that cause the register accesses to potentially happen in the wrong
order.

Thierry

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