Linux 2.4.25 / MPC8280 / ram config vs Linux instability?

["Absolut Hunter" <absoluthunter@comcast.net>]

A useful piece to this might be that I am attempting to map these two memory
regions contiguously, so that Linux doesn't see any memory holes.

0-2MB SRAM
2MB-34MB SDRAM

I would be open to any other ways that SRAM / SDRAM might be mapped to work,
and still provide a performance increase for the kernel.

-Russell

-----Original Message-----
From: Absolut Hunter [mailto:absoluthunter@comcast.net] 
Sent: Monday, July 11, 2005 6:38 AM
To: 'linuxppc-embedded@ozlabs.org'
Subject: Linux 2.4.25 / MPC8280 / ram config vs Linux instability?

Anybody,

I ran into a peculiar issue, though it could be user error. 

I am using U-boot to configure up the SDRAM and SRAM in our system before we
launch Linux. We have 32 Mb of SDRAM and 2 MB of SRAM in the system. 

If I leave the SRAM mapped at higher addresses 0x10000000 and SDRAM at
0x00000000, like U-boot defaults to Linux works great. However, my goal is
to map SRAM <8ns speeds> down to the address 0x00000000, to map the vector
table, so I can achieve a very fast interrupt response time.

I hacked U-boot a little bit to remap these addresses, after all the init
code was finished. U-boot relocates itself to higher memory, i.e. the last
2MB's of SDRAM and all is well within U-boot. I ran a 14 hour test on the
memory area to proof it. Now we have 0x02200000 <34M> memory size being
reported to Linux, in the bd->memsize. However, now Linux crashes randomly
all over the place, mostly sig11's task: swapper. I know this is not an
issue with SDRAM stability, and I can't imagine SRAM having problems seeing
as its 8ns capable and we're running at 15ns at the moment (66mhz).

Linux reports BAT mappings as BAT2=32Mb, BAT3=2MB when all 34 Megs are
presented via U-boot. 

Does anyone have any idea what would cause this instability? Physically all
memory seems to be great if used by itself. I know very little about how
Linux manages its pages, BATS, TBLs, etc... Are the limits on odd memory
sizes? Or certain configurations that it expects to see?

Thanks in advance.

-Russell McGuire