Problem booting Linux 2.6 on Virtex-4

David Baird dhbaird at
Mon Jan 14 16:12:38 EST 2008


I'm having trouble with getting Linux to boot farther than early_init.
 Things start to go obviously wrong after early_init calls memset to
clear the .bss section.  The first symptom I noticed was that the
stack (which currently contains the link register) was getting zeroed
by memset.  This doesn't make any sense because the stack does not
intersect the .bss section;  I confirmed that the do not intersect by
using JTAG+xmd to read the registers and also by inserting print

So, I experimented further and discovered that different memory
regions seem to be aliased on to each other every 2*32*256 bytes.


    char * ptr = 0;
    unsigned long addr = 0xc0001234; // pick some address
    int n = 1; // pick some integer

    // Now write values to two different addresses:
    ptr[addr] = 42;
    ptr[addr + n*2*32*256] = 24;
    volatile asm ("nop");

    // Now mysteriously, ptr[addr] == 24

The reason I have written 32*256 is because that corresponds to the
number of bytes per cache line times the number of cache lines ...
because I was suspicious this might be related to caching.  However,
with experimentation, I found that the aliasing does not occur every
32*256 bytes, but rather every 32*512 bytes (hence 2*32*256).
Anyways, I looked in embed_config (and confirmed by dumping zImage.elf
with objdump) that the cache is being invalidated with "dccci".
"dccci" is being called, as expected, and therefore everything looks
good in the code.

I am completely perplexed.  The cache appears to be initialized
properly.  I don't have any trouble in real mode.  But as soon the
kernel switches into virtual mode (in head_4xx.S), then this memory
aliasing problem happens.  I wrote a completely standalone assembler
script that initializes the cache, TLBs, and virtual mode in the same
way as embed_config and head_4xx.S and so I am able to reproduce the
problem outside of the kernel.  I can attach that code if someone
thinks it will be useful - just let me know.

Does anyone have an idea about what might be going wrong?  I can send
more information or test anything that you want me to.


More information about the Linuxppc-embedded mailing list