[U-Boot] PPC405EX CHIP_21 erratum

[Stefan Roese <sr@denx.de>]

Hi Steve,

On Friday 29 April 2011 18:54:02 Steven A. Falco wrote:
> We've been bitten by the PPC405EX CHIP_21 erratum.

How did it affect you exactly? Was an incorrect PVR detected? Did this result 
in some problems (Linux etc)?

> I've looked through the
> U-Boot code, but it doesn't appear that there is a work-around for this
> one.

Correct, there currently is no workaround.
 
> The following patch is my adaptation of AMCC's suggestion as to the fix. 
> But I have to say that I don't care for it, if for no other reason that it
> will break if a rev E comes out.
> 
> Also, I made it #ifdef'd on CONFIG_405EX, but it will not work on a 405EXr.
> And I have seen a 405EX report its PVR as if it were a 405EXr.  It's not
> looking good for the suggested work-around.
>
> AMCC claims this can be fixed in hardware, by always doing a double reset.
> Naturally, that is hard to implement and would mean massive rework of
> existing boards.

Yes, there are many 405EX(r) boards in the field already. Such an hardware 
workaround should really be avoided if possible.

> So I am contemplating a different work-around, whereby software always
> resets the board on a cold boot (if such a thing can be reliably
> detected).  That would hopefully be the equivalent of the hardware double
> reset, and would not be dependent on specific PVR values, making it more
> "future proof".
> 
> Has anyone else run across this?  Do you have similar concerns about the
> patch?

No, I have not seen this problem before. And yes, I would also prefer your 
"alternative suggestion", with the always reboot after powerup optionally 
built into the U-Boot image. But as you already mentioned, we would have to 
find a way to reliably detect the powerup reset, so that we don't end in an 
reset-loop.

Cheers,
Stefan

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