memcpy alignment for DEVICE_nGnRnE

[mbohan@codeaurora.org (Michael Bohan)]

Hi Catalin,

On Fri, Feb 21, 2014 at 10:53:08AM +0000, Catalin Marinas wrote:
> On Fri, Feb 21, 2014 at 09:58:27AM +0000, Zhou Zhu wrote:
> > We faced one issue using memcpy for memory type DEVICE_nGnRnE 
> > (pgprot_noncached). If the address is not aligned, exception would 
> > happen due to the alignment of this type could not be handled by hardware.
> 
> Indeed, that's expected. This memory type is the equivalent of Strongly
> Ordered.
> 
> > Is there any plan and ongoing patch to support this or we need to keep 
> > address aligned for memcpy using pgprot_noncached memory in drivers?
> 
> You either change the memory type to pgprot_writecombine() or use the
> right API (memcpy_toio/memcpy_fromio, these could be further optimised,
> similar to powerpc for example, but I didn't get there yet).

How should we handle Device Memory with copy_from_user / copy_to_user?
Should we follow the same scheme and create
copy_from_user_io / copy_to_user_io, or rather enforce that the stock
routines handle alignment?

Thanks,
Mike

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