linux-kernel - Re: [PATCH V2 6/6] coresight-stm: adding driver for CoreSight STM component

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Date:	Fri, 05 Feb 2016 15:30:19 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Chunyan Zhang <zhang.chunyan@...aro.org>,
	mathieu.poirier@...aro.org, mark.rutland@....com, al.grant@....com,
	corbet@....net, zhang.lyra@...il.com, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, tor@...com, broonie@...nel.org,
	mike.leach@....com, linux-api@...r.kernel.org,
	Russell King <linux@....linux.org.uk>, pratikp@...eaurora.org,
	nicolas.guion@...com
Subject: Re: [PATCH V2 6/6] coresight-stm: adding driver for CoreSight STM component

On Friday 05 February 2016 15:06:20 Alexander Shishkin wrote:
> Chunyan Zhang <zhang.chunyan@...aro.org> writes:
> 
> > +#ifndef CONFIG_64BIT
> > +static inline void __raw_writeq(u64 val, volatile void __iomem *addr)
> > +{
> > +     asm volatile("strd %1, %0"
> > +                  : "+Qo" (*(volatile u64 __force *)addr)
> > +                  : "r" (val));
> > +}
> 
> Is it really ok to do this for all !64bit arms, inside a driver, just
> like that? I'm not an expert, but I'm pretty sure there's more to it.

It's normally device dependent whether this works or not, on 32-bit
architectures, a 64-bit access to an I/O bus tends to get split into
two 32 bit accesses and the order might not be the as what was
intended.

We have functions in include/linux/io-64-nonatomic-hi-lo.h
and include/linux/io-64-nonatomic-lo-hi.h that are meant to
do this right. Maybe the driver can be changed to use whichever
one is correct for it.

	Arnd