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Re: IP32 and 32bit kernel

Giuseppe Sacco wrote:
> Hi Florian,
> Il giorno lun, 29/10/2007 alle 15.39 +0100, Florian Lohoff ha scritto:
> [...]
> > I guess xfsdump/restore use some interesting ioctl's which might
> > need compat wrappers for the 32/64 bit case. But this is how its
> > supposed to work.
> Thanks for this clarification, should this wrapper be in the userland
> packages?

Compat ioctls would be implemented on the kernel side. IIRC they are
just not implemented in the MIPS kernels, this is a feature missing

> Should I bug the Debian xfsdump? Anyway, I still would like to
> understand which are the reasons for providing a 64 bit kernel: is there
> any ip32 system with more than 4Gb of memory? Is 64 bit required for
> accessing some hardware stuff? Are there any other reason?

On MIPS, the physical RAM limit of 32bit Kernels is the size of KSEG0
(512 MB), which typically hosts also some I/O devices. More than that
need highmem. IP32 systems can have up to 2 GB RAM.

The real killer, however, is the need to access 64bit wide registers
atomically, implied by the hardware design. A 32bit kernel needs to
disable/enable interrupts around each register access.

The IP32 kernel used to be optionally 32bit or 64 bit, when it became
too painful to maintain the 32bit support was dropped.


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