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

From: Arnd Bergmann
Date: Fri Sep 26 2014 - 05:29:32 EST

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

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.

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