> A mb() is usually used if you do a write to device and read from it.
> With out it, the CPU could perform the read before the write, which
> would give you an incorrect result. There's no other way around that.
Possibly the synchronisation functions are doing significantly
more work than is required.
I was looking at the in_le32() and out_le32() functions for the
ppc e300 (and maybe others).
The out_le32() contains a 'sync' instruction - this may only
be needed after a series of writes (eg just before a command).
The iosync() function just adds a 'sync' and can be used as needed.
The in_le32() not only contains the unwanted 'sync', but also
a 'twi' (trap immediate - NFI exactly what this does) and 'isync'.
The 'isync' is particularly horrid and unnecessary (aborts
the instruction queue and refeches the opcode bytes).
The very slow in_le32() might be there to give semi-synchronous
traps on address fault - but unless the hardware is being probed
that really isn't necessary.
I did find st_le32() and ld_le32() in arch/powerpc/include/asm/swab.h
but had difficulty #including that version of swab.h!
#include <../arch/powerpc/include/asm/swab.h>
worked - but isn't that nice.
David
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