use {readl|writel}_relaxed instead of readl/writel in i2c-designware-core ?

Jisheng Zhang jszhang at
Fri Feb 14 06:14:47 EST 2014

Dear Arnd,

On Fri, 14 Feb 2014 01:09:44 -0800
Arnd Bergmann <arnd at> wrote:

> On Friday 14 February 2014 15:54:38 Jisheng Zhang wrote:
> > Hi all,
> > 
> > The writel/readl is too expensive especially on Cortex A9 w/ outer L2
> > cache. This introduce i2c read/write error on Marvell Berlin SoCs when
> > there are L2 cache maintenance operations at the same time.
> > 
> > In our internal berlin bsp, we just replaced readl/writel with the relaxed
> > version. But AFAIK, the "relaxed" version doesn't exist on all
> > architectures. How to handle this issue? 
> In case of i2c-designware, this is safe because that driver does not perform
> DMA. In other drivers, you may have to be more careful, to ensure that all
> MMIO is serialized with DMA operations performed by the driver.
> > Any suggestions are appreciated.
> I would definitely welcome a patch that adds a default  _relaxed
> implementation to include/linux/io.h, like this:
> #ifndef readb_relaxed
> #define readb_relaxed(p) readb(p)
> #endif
> and then adds "#define readb_relaxed(p) readb_relaxed(p)" etc. to all
> architectures that have a non-macro definition for readb.

Thanks for the suggestions. I'll try to send out one RFC patch.


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