[U-Boot-Users] Some RFCs about U-Boot's the generic FPGA support

[Bruce_Leonard at selinc.com]

Hi Laurent,

Laurent Pinchart <laurentp at cse-semaphore.com> wrote on 08/22/2007 12:28:36 
AM:

> Hi Bruce,
> 
> ordering. UG322 sheds some light on the issue. D0-D7 are indeed MSB-LSB, 

> making the SelectMAP interface use the big-endian PPC bit ordering 
> convention.

I'm with Matthaias, I've learned to refer to them as MSB/LSB on the list 
:).  Avoids confusion.

> to D7-D0 on 
> the FPGA. I'll go scold our hardware engineer :-)

Don't scold him to hard.  It's an easy mistake to make especially if 
you're used to everything being little endian.  (I can say this because I 
am a HW engineer and I've made this mistake!)

> 
> With a properly connected bus between the processor and the FPGA, 
> bit reversal 
> is thus not needed when loading a .bit file (fpga loadb) or a non 
mirrored 
> PROM file (fpga load).
> 
> For hardware with bit mirroring on the bus, I suggest either using 
amirrored 
> PROM file or mirroring the bit in the data write callback. I thus vote 
for 
> removing bit mirroring in fpga_loadbitstream() and adding an explanation 
of 
> this issue (either in the code or in the documentation) to prevent 
hardware 
> mistakes in future designs.

I agree with all of this with the exception of totally removing the bit 
mirroring in fpga_loadbitstream().  If this were going in only a single 
product or it was being done for the first timeI would be all over that, 
however we know for a fact that there's at least one board out in the wild 
that relies on that bit mirroring to take place (another un-enlightened HW 
engineer no doubt ;)  ).  Flat out removing the code will break that board 
and I'm not inclined to face the wrath of the maintainer.  If we can 
figure out who depends on that and get them to push the bit mirroring down 
to the call back function where it rightfully belongs than I agree remove 
the code.  Otherwise I think it needs to conditionally stay.

Bruce