[U-Boot-Users] Some RFCs about U-Boot's the generic FPGA support
Bruce_Leonard at selinc.com
Bruce_Leonard at selinc.com
Wed Aug 22 19:09:17 CEST 2007
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
More information about the U-Boot
mailing list