ppc: Add CFG_MEM_TOP_HIDE option to hide memory area that doesn't get "touched"

[J-u-boot.git] / README

diff --git a/README b/README
index 5cbe7c1e80daabeee8c43247db7041b999919025..46e6a33bc122da646e435e3ebd9bc1047468ab28 100644 (file)
--- a/README
+++ b/README
@@ -1929,6 +1929,27 @@ Configuration Settings:
                Scratch address used by the alternate memory test
                You only need to set this if address zero isn't writeable
 
+- CFG_MEM_TOP_HIDE (PPC only):
+               If CFG_MEM_TOP_HIDE is defined in the board config header,
+               this specified memory area will get subtracted from the top
+               (end) of ram and won't get "touched" at all by U-Boot. By
+               fixing up gd->ram_size the Linux kernel should gets passed
+               the now "corrected" memory size and won't touch it either.
+               This should work for arch/ppc and arch/powerpc. Only Linux
+               board ports in arch/powerpc with bootwrapper support, that
+               recalculate the memory size from the SDRAM controller setup
+               will have to get fixed.
+
+               This option can be used as a workaround for the 440EPx/GRx
+               CHIP 11 errata where the last 256 bytes in SDRAM shouldn't
+               be touched.
+
+               WARNING: Please make sure that this value is a multiple of
+               the Linux page size (normally 4k). If this is not the case,
+               then the end address of the Linux memory will be located at a
+               non page size aligned address and this could cause major
+               problems.
+
 - CFG_TFTP_LOADADDR:
                Default load address for network file downloads