why the PAGE_OFFSET is not 0xc0000000 on m68k platform

why the PAGE_OFFSET is not 0xc0000000 on m68k platform

[Lanttor]

Hi,

I see the definition of PAGE_OFFSET on m68k platform like that:

#define PAGE_OFFSET (PAGE_OFFSET_RAW)

#ifdef CONFIG_MMU
#ifndef CONFIG_SUN3
#define PAGE_OFFSET_RAW         0x00000000
#else
#define PAGE_OFFSET_RAW         0x0E000000
#endif
#else
#define PAGE_OFFSET_RAW         CONFIG_RAMBASE
#endif

In my understanding, the kernel page mapping depends on PAGE_OFFSET.
For example, if PAGE_OFFSET is 0x00000000 (assuming 256M memory), the
kernel virtual address will be at (0x00000000 - 0x10000000), right?

Could this incur some side effect? I see the definition of PAGE_OFFSET
is 0xc0000000 on much platforms.

Thanks,

-- 
Best Regards,
Lanttor

Re: why the PAGE_OFFSET is not 0xc0000000 on m68k platform

[Andreas Schwab]

Lanttor <lanttor.guo@freescale.com> writes:

> In my understanding, the kernel page mapping depends on PAGE_OFFSET.
> For example, if PAGE_OFFSET is 0x00000000 (assuming 256M memory), the
> kernel virtual address will be at (0x00000000 - 0x10000000), right?

On m68k, the kernel virtual addresses are in a separate address space
from user space, thus the value of PAGE_OFFSET is somewhat arbitrary.

> Could this incur some side effect? I see the definition of PAGE_OFFSET
> is 0xc0000000 on much platforms.

On those platforms kernel and user space share the same address space,
and PAGE_OFFSET makes sure that they don't overlap.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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