On Mon, Sep 28, 2020 at 02:42:43PM +0200, Arnd Bergmann wrote:

quoted

I need some idea how this numberspace is managed in order to
understand the code so I can review it, I guess it all makes perfect
sense but I need some background here.

I also had never understood this part before, and I'm still not
sure where the 0x900000 actually comes from, though my best
guess is that this was intended as a an OS specific number space,
with '9' being assigned to Linux (similar to the way Itanium and
MIPS do with their respective offsets). By the time EABI got added,
this was apparently no longer considered helpful.

It is an OS specific number space, originally designed to allow
RISC OS programs to be run under Linux.  There was indeed such a
project, but that died and the code ripped out. EABI, by using
SWI 0 - or more accurately, not reading the SWI opcode, trampled
over the ability for RISC OS programs to be run under Linux.

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