mpc755, sandpoint port and 1Gb of ram

mpc755, sandpoint port and 1Gb of ram

[Michael J. Accetta]

Hello all,

We have a custom board built around the mpc755 and patterned after the
Sandpoint reference design using the MontaVista Pro 3.0 distribution.
This is a base 2.4.18 kernel which, according to the MV web, page includes the
2.4.17 linuxppc code.  When 1Gb ram is used, CONFIG_HIGHMEM is required
to access the top 256Mb.  However, the sandpoint port does

   io_block_mapping(0xfe000000, 0xfe000000, 0x02000000, _PAGE_IO);

whichs maps all of virtual 0xfexxxxxx to physical 0xfexxxxxx.  This call
ends up using BAT#0 to map the entire block.  However, the CONFIG_HIGHMEM
support in the base ppc port sets PKMAP_BASE to 0xfe000000 and expects
these virtual addresses to be available for dynamic mapping via kmap()
of high memory.  The conflicting mapping in the BAT takes precedence,
wreaking havoc with any accesses to high memory.  In our case NFS mounts
of the root file system was the first high memory user and init could
not be found.

I've made a stab at fixing this by configuring PKMAP_BASE down to 0xfc000000
and things now seem to work fine.  Is this a correct solution?  Are there
likely to be any negative side effects of this approach?  Are there
better solutions?

Mike Accetta
Laurel Networks, Inc.

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Re: mpc755, sandpoint port and 1Gb of ram

[Mark A. Greer]

Michael J. Accetta wrote:

>Hello all,
>
>We have a custom board built around the mpc755 and patterned after the
>Sandpoint reference design using the MontaVista Pro 3.0 distribution.
>This is a base 2.4.18 kernel which, according to the MV web, page includes the
>2.4.17 linuxppc code.  When 1Gb ram is used, CONFIG_HIGHMEM is required
>to access the top 256Mb.  However, the sandpoint port does
>
>   io_block_mapping(0xfe000000, 0xfe000000, 0x02000000, _PAGE_IO);
>
>whichs maps all of virtual 0xfexxxxxx to physical 0xfexxxxxx.  This call
>ends up using BAT#0 to map the entire block.  However, the CONFIG_HIGHMEM
>support in the base ppc port sets PKMAP_BASE to 0xfe000000 and expects
>these virtual addresses to be available for dynamic mapping via kmap()
>of high memory.  The conflicting mapping in the BAT takes precedence,
>wreaking havoc with any accesses to high memory.  In our case NFS mounts
>of the root file system was the first high memory user and init could
>not be found.
>
>I've made a stab at fixing this by configuring PKMAP_BASE down to 0xfc000000
>and things now seem to work fine.  Is this a correct solution?  Are there
>likely to be any negative side effects of this approach?  Are there
>better solutions?
>
>
>
You have basically 2 choices:

a) change the mapping such that the io_block_mapping doesn't cover the
default PKMAP_BASE area
b) change PKMAP_BASE--which is what you did--however, if you didn't, you
should use the config facility that allows you to do this (look at 'Code
maturity level options'/'Prompt for advanced kernel configuration options').

Its probably just a personal preference but I think changing the mapping
and leaving PKMAP_BASE alone is a better way.  Someday I will do that
for the sandpoint port.

In short, what you did, assuming you did it properly, should work fine.

Mark


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Re: mpc755, sandpoint port and 1Gb of ram

[Michael J. Accetta]

Mark

    Its probably just a personal preference but I think changing the mapping
    and leaving PKMAP_BASE alone is a better way.  Someday I will do that
    for the sandpoint port.

    In short, what you did, assuming you did it properly, should work fine.

Thanks much for the confirmation.  That did seem less risky than
changing the sandpoint I/O mapping without understanding where all the
I/O addresses were expected to be (which I don't).  I would concur,
though, that that approach seems a better someday solution.

I am using the configure process to change PKMAP_BASE.

Mike

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