[U-Boot] armv7 DMA and cache mangement functions

[Markus Niebel <list-09_u-boot@tqsc.de>]

Hello,

I'm not an expert in the low level details of this area. So please sorry if there are
wrong assumptions in this post post.

Hardware: i.MX6 Solo (TQMa6 on custom Mainboard)
U-Boot: 2014.10
gcc: 4.8.3 

We see an error using TFTP on i.MX6 that seems to triggered, if the code / data size goes
over a limit. Code changes have nothing to do with network stack, network drivers, 
memory mangement. TFTP will completely unusable: device sees frequently erroneous packages 
with different of wierd errors. If code stays below this size all works fine.

Up to now we checked a lot of things. The following brought us to the assumption, that this
could be cache related:

dynamically disable data cache before doing TFTP: 	TFTP works well again
running with disabled L2 cache (data cache enabled):	TFTP works well again

Looking at the code in drivers/net/fec_mxc.c, function fec_recv we see a call to
invalidate_dcache_range before accessing the received ethernet data. When looking at
the code for invalidate_dcache_range in arch/arm/cpu/armv7/cache_v7.c an comparing
how the things done in linux and barebox we noticed that the order of L2 chache / data cache
invalidation is just swapped there. Applying this to the receive code for fec_mxc,
TFTP will work again.

Question: is the order of cache invalidation important?

Thanks

Markus