[MPC52xx]Latency issue with DMA on FEC

[Jean-Michel Hautbois]

2010/12/1 David Laight <David.Laight at aculab.com>:
>
>> 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

Yes, I was also looking at in_be16 and out_be16, and my thoughts were
exactly the same.
I think the HW I am using is not a good one, but this is not
sufficient to explain the behaviour.
These instructions are called, for instance, from bcom_enable_task().