[MPC52xx]Latency issue with DMA on FEC

[David Laight]

> 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