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
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.