Where does physical RAM start in kseg0?

Where does physical RAM start in kseg0?

[Erik J. Green]

Hi again, everyone;

A question about kseg0:  Do system designers usually set things up such that
kseg0 begins at the start of physical memory, regardless of the xkphys offset at
which RAM starts?

Or is it necessary to add the offset at which RAM starts to the kseg0 base,
making it possible that the system designers could start RAM addresses high
enough (above 512M) to make kseg0 unusable?  Does anyone have an implementation
in which this is the case?  

If kseg0 provides a window beginning at physical address 0, that means I'm going
to try using Ralf's mapped kernel option, or I'll have to get the kernel up and
running in 64 bit only mode (I believe 32 bit compat binaries would still work,
since kuseg can be mapped).

Erik



-- 
Erik J. Green
erik@greendragon.org

Re: Where does physical RAM start in kseg0?

[Ralf Baechle]

On Sun, Apr 13, 2003 at 02:13:51AM +0000, Erik J. Green wrote:

> 
> A question about kseg0:  Do system designers usually set things up such that
> kseg0 begins at the start of physical memory, regardless of the xkphys
> offset at which RAM starts?

XKPHYS and KSEG0 map the same physical address space so the offsets is
the same.  Keep also in mind that XKPHYS maps the physical address space
8 times due to the 3 bits of the address that encode a caching mode.

> Or is it necessary to add the offset at which RAM starts to the kseg0 base,
> making it possible that the system designers could start RAM addresses high
> enough (above 512M) to make kseg0 unusable?  Does anyone have an
> implementation in which this is the case?  

There is no requirement at all for a system to have physical memory in
KSEG0 - the Octane to my knowledge one example.  What every sane system
needs to have in KSEG0 are exception handlers.  Of course they could also
reside in a ROM but that's insane so you should expect at least a few kb
of RAM at physical address zero.

> If kseg0 provides a window beginning at physical address 0, that means
> I'm going to try using Ralf's mapped kernel option, or I'll have to get
> the kernel up and running in 64 bit only mode (I believe 32 bit compat
> binaries would still work, since kuseg can be mapped).

Due to the Octane's funky address space layout and the current tools
limitations the kernel will have to run in CKSEG2 instead of KSEG0 ...

  Ralf

Re: Where does physical RAM start in kseg0?

[Erik J. Green]

Quoting Ralf Baechle <ralf@linux-mips.org>:

Ralf wrote:
> There is no requirement at all for a system to have physical memory in
> KSEG0 - the Octane to my knowledge one example.  What every sane system
> needs to have in KSEG0 are exception handlers.  Of course they could also
> reside in a ROM but that's insane so you should expect at least a few kb
> of RAM at physical address zero.

At this point I wouldn't put anything past SGI, but it's good to know my 
suspicions about kseg0 are confirmed.
> 
> Due to the Octane's funky address space layout and the current tools
> limitations the kernel will have to run in CKSEG2 instead of KSEG0 ...
> 
>   Ralf

/nod.  Onward and upward.

Thanks,
Erik



-- 
Erik J. Green
erik@greendragon.org

Re: Where does physical RAM start in kseg0?

[Maciej W. Rozycki]

On Sun, 13 Apr 2003, Ralf Baechle wrote:

> Due to the Octane's funky address space layout and the current tools
> limitations the kernel will have to run in CKSEG2 instead of KSEG0 ...

 Recent tools (like these at my site) should be fine for making an XPHYS
kernel -- it's the assumption of being in the 32-bit address space made
here and there in the kernel that bites...

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +