Re: virt_to_bus and >1G of memory (was MAX_DMA_ADDRESS ...)

H. Peter Anvin (hpa@transmeta.com)
12 Nov 1998 01:45:41 GMT

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

-- 
    PGP: 2047/2A960705 BA 03 D3 2C 14 A8 A8 BD  1E DF FE 69 EE 35 BD 74
    See http://www.zytor.com/~hpa/ for web page and full PGP public key
        I am Bahá'í -- ask me about it or see http://www.bahai.org/
   "To love another person is to see the face of God." -- Les Misérables


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/