The MIPS32 EJTAG debug capability includes a set of memory mapped
registers from 0xff200000 to 0xff3ffff, which overlay kernel mapped memory
ONLY if the processor is in a Debug state. If that's the way it's been
done, then one is constrained in one's ability to debug kernel code/data in
that region using EJTAG (it can still be done by slight-of-hand) but it
won't
otherwise interfere with the kernel's use of the region.
That should be the case for all "true" MIPS32 processors, but EJTAG
pre-dates the MIPS32 architecture specification, and I know that some
processors were done in the late 1990's where this "dseg" overlay of kseg2
was larger and/or permanently mapped. If you've got one of the later,
you may have to play games with FIXADDR_TOP etc.
Regards,
Kevin K.
Pelton, Dave wrote:
David VomLehn wrote:
Pelton, Dave wrote:
<snip>
Normally kseg2 is in the address range 0xC0000000-0xFFFFFFFF.
However,
on the BMIPS3300 (the embedded MIPS32 core used on my SOC), there is
a
range of addresses within kseg2 that are reserved
(0xFF200000-0xFFFEFFFF).
This means that the TLB entry that kmap_coherent creates will not
work
if it falls within the reserved range. The virtual address space
used
by kmap_coherent is controlled by FIXADDR_TOP in
include/asm-mips/fixmap.h.
To fix my issue, I changed FIXADDR_TOP to avoid the reserved address
space.
<snip>
Is your range of addresses reserved on the BMIPS3300 because it is
being used as
dseg, i.e. because it is being used for debugging? If I read the
documentation on
the processor I am using, the 24Kc, it has a similar issue. I don't
need to be
able to use kmap_coherent because the 24K hardware takes care of data
coherence
issues, i.e. cpu_has_dc_aliases is false, but I'm sort of thinking we
might just
generally want to change FIXADDR_TOP to avoid address in the dseg
range for all
MIPS32 processors. As we work our way through some of the cache
flushing issues
related to using high memory, I'd like to be able to develop code that
works on
processors for which cpu_has_dc_aliases is true. If my kmap_coherent
is working I
can check things out for those processors and then simply use
kmap_atomic for
processors where cpu_has_dc_aliases is false.
Apologies in advance for what my plain-text impaired mail client is
likely to do to this message. The reserved range on the BMIPS3300 is
used by memory mapped registers. I am not a memory management expert,
so I am not sure what implications there would be to moving FIXADDR_TOP
as a general fix. The impression I have from general MIPS documentation
is that kseg2 should not be used for memory mapped i/o and hence
platforms that do this should be treated as an exception case, rather
than moving the general case to deal with this.
- David Pelton
