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

[Laurent Pinchart]

Hi Matthias,

On Wednesday 22 August 2007 09:59, Matthias Fuchs wrote:
> Hi Laurent,
>
> you cannot imagine how many grey hair this bit naming and numbering
> convention cost me. I learned to never use D0/7 when meaning MSB/LSB:-)
>
> On Wednesday 22 August 2007 09:28, Laurent Pinchart wrote:
> > > I must respectfully disagree.  In my design, if I mirror in the code
> > > the FPGA does NOT load.  In a PPC, D0 is the MSB, NOT the LSB.  In a
> > > Spartan 3 FPGA, D0 is the MSB, NOT the LSB.  Therefore if you connect
> > > your bus straight across between the PPC and the Spartan 3 FPGA, AND do
> > > mirroring in the code, your FPGA will not load.
> >
> > I got a similar design, except that D0-D7 on the PPC are connected to
> > D7-D0 on the FPGA. I'll go scold our hardware engineer :-)
>
> Sometimes it's even worth as softwa/orkers to review schematics when you
> know that you will get the board on your desk for writing software :-)
> (But do not sign anything or the hardware engineer will scold you
> afterwards.)
>
> > 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).
>
> BTW, you can even load a .bit file using fpga load because the FPGA will
> discard the header.

Except when the bitstream start sequence is present in the header. "fpga load" 
with a .bit file shouldn't be adviced, especially if we remove the bit 
mirroring from fpga_loadbitstream.

> > For hardware with bit mirroring on the bus, I suggest either using a
> > mirrored PROM file or mirroring the bit in the data write callback. I
> > thus vote for
>
> Ack
>
> > removing bit mirroring in fpga_loadbitstream() and adding an explanation
> > of
>
> Ack. So all the malloc and the timeconsuming loop code will disappear.
> Yippi!
>
> > this issue (either in the code or in the documentation) to prevent
> > hardware mistakes in future designs.
>
> Good idea. But do you think that hardware engineers will read through the
> cmd_fpga.c file?

Probably not, that's why a more visible place would be better.

> I will cleanup my changes and submit a patch soon.

Best regards,

-- 
Laurent Pinchart
CSE Semaphore Belgium

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