[U-Boot] [PATCH] boston: Ensure DDR address calcuations don't overflow

[Paul Burton]

When constraining the highest DDR address that U-Boot will use for its
data & relocated self, we need to handle the common case in which a 32
bit system with 2GB DDR will have a zero gd->ram_top, due to the
addition of 2GB (0x80000000) to the base address of kseg0 (also
0x80000000) which overflows & wraps to 0.

We originally had a check for this case, but it was lost in commit
78edb25729ce ("boston: Provide physical CONFIG_SYS_SDRAM_BASE") causing
problems for the affected 32 bit systems.

Signed-off-by: Paul Burton <paul.burton at mips.com>
Fixes: 78edb25729ce ("boston: Provide physical CONFIG_SYS_SDRAM_BASE")
Cc: Daniel Schwierzeck <daniel.schwierzeck at gmail.com>

---
Feel free to fold this into 78edb25729ce ("boston: Provide physical
CONFIG_SYS_SDRAM_BASE") if you prefer Daniel.

 board/imgtec/boston/ddr.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/board/imgtec/boston/ddr.c b/board/imgtec/boston/ddr.c
index 00428496bd..892bb1754c 100644
--- a/board/imgtec/boston/ddr.c
+++ b/board/imgtec/boston/ddr.c
@@ -26,6 +26,15 @@ int dram_init(void)
 ulong board_get_usable_ram_top(ulong total_size)
 {
 	DECLARE_GLOBAL_DATA_PTR;
+	ulong max_top;
 
-	return min_t(ulong, gd->ram_top, (ulong)phys_to_virt(SZ_256M));
+	/* Limit memory use to the top of (c)kseg0 */
+	max_top = (ulong)phys_to_virt(SZ_256M);
+
+	if (gd->ram_top < (ulong)phys_to_virt(0)) {
+		/* >= 2GB RAM on a 32 bit system wrapped around to 0 */
+		return max_top;
+	}
+
+	return min_t(ulong, gd->ram_top, max_top);
 }
-- 
2.15.1