[U-Boot-Users] [PATCH] ppc4xx: Don't use last 256 bytes of SDRAM, workaround for 440EPx CHIP 11 errata

[Stefan Roese <sr@denx.de>]

On Thursday 27 March 2008, Larry Johnson wrote:
> Yes, we normally use ECC modules in our testing.  I've been looking at a
> patch for "initdram()" Denali SPD, but I've been waiting to see how your
> "CFG_MEM_TOP_HIDE" patch would turn out.
>
> As things stand now, can I assume that boards using the Denali SPD will
> also define "CFG_MEM_TOP_HIDE", and therefore initdram() should continue
> to return the full size of the memory?

Correct.

> The only place that the last 256 bytes of memory are touched is when
> "dflush()" is called to zero the SDRAM.

To be more specific, in the dcbz_area() call.

> This does not cause a Machine 
> Check interrupt.  I am guessing that all the writes from "dflush()" are
> aligned, and therefore there are no burst that access beyond the end of
> of the SDRAM memory space.  If so, then my inclination is not to change
> this part of the code.  Does this make sense?

I recommend to change it this way:

        /* Zero the memory */
        debug("Zeroing SDRAM...");
-       dcbz_area(CFG_SDRAM_BASE, dram_size);
+       dcbz_area(CFG_SDRAM_BASE, dram_size - CFG_MEM_TOP_HIDE);
        dflush();

And perhaps put something like:

#ifndef CFG_MEM_TOP_HIDE
#error "Please define CFG_MEM_TOP_HIDE (see README) in your board config 
file!"
#endif

at the top of the file. What do you think?

Best regards,
Stefan

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