[U-Boot] AMCC 405ex memory size issue

Hi all, we are using u-boot 2009.01, linux-2.6.25-rc2, 1GB DDR2 memory(2Gbit * 4, 1 rank), AMCC powerpc 405ex, both 1G and 512MB memory works fine under u-boot, but linux boot fails in 1G memory, if we limit mem=512M, linux could boot. Could anyone pls help us how to find the root cause? thanks