linux-kernel - Re: [PATCH v3 00/17] Cross-architecture definitions of relaxed MMIO accessors

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Date:	Fri, 26 Sep 2014 11:28:38 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	Will Deacon <will.deacon@....com>,
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Subject: Re: [PATCH v3 00/17] Cross-architecture definitions of relaxed MMIO accessors

On Friday 26 September 2014 09:40:19 Russell King - ARM Linux wrote:
> 
> How would a 32-bit architecture know whether it should read the least
> significant 32-bit or the most significant 32-bit part of the 64-bit
> register first.  What would be right for one driver may not ben correct
> for another.  Hence, this decision should only be made by the driver
> wanting the accessor, and not having the accessor symbol defined should
> be the trigger for the driver to handle the problem themselves.

Some 32-bit architectures can trigger 64-bit bus cycles using well
defined accesses using register pairs. Meta seems to fit into this
category:

static inline u64 __raw_readq(const volatile void __iomem *addr)
{
       u64 ret;
       asm volatile("GETL %0,%t0,[%1]"
                    : "=da" (ret)
                    : "da" (addr)
                    : "memory");
       return ret;
}

Most other architectures I think cannot do this however, and would
turn the access into two separate bus cycles, which in addition to
the problem you mentioned could also result in side-effects from
doing an access at the wrong offset, so we definitely can't rely
on having these functions.

	Arnd
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