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SubjectRe: virt_to_bus and >1G of memory (was MAX_DMA_ADDRESS ...)
Date
Followup to:  <199811111858.TAA03864@gloin.fi.muni.cz>
By author: Jan Kasprzak <kas@informatics.muni.cz>
In newsgroup: linux.dev.kernel
>
> In my previous mail I wrote:
> : And another question related to the virt_to_bus on x86:
> : This function is defined as (x) & ~PAGE_OFFSET. It works for the current
> : value of the PAGE_OFFSET, but IMHO it does not work when I change the
> : PAGE_OFFSET to 0x70000000, as suggested in <asm-i386/page.h> for >1GB RAM
> : machines. Shouldn't the virt_to_bus be defined as (x)-PAGE_OFFSET ?
> : The physical memory is mapped 1:1 from the PAGE_OFFSET virtual address, IIRC.
>
> Can anybody explain this to me? Is (x)-PAGE_OFFSET at any chance
> slower or less efficient than (x)&~PAGE_OFFSET?
>

They are different if PAGE_OFFSET isn't a power of 2.

-hpa
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