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

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Date:	Tue, 17 Dec 2013 11:51:36 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Thierry Reding <thierry.reding@...il.com>
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 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?

> > +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?
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