northbridge/amd/{lx,gx2}: remove immediate accesses of 0

gcc doesn't like these because they're undefined behavior, so use
zeroptr instead. For the loop that just does a number of writes (0..4),
use zeroptr + i.

Checked the disassembly (AMD_RUMBA and PCENGINES_ALIX2D) to not contain
ud2 anymore and to look reasonable where zeroptr was used.

Change-Id: I4a58220ec9a10c465909ca4ecbe5366d0a8cc0df
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/14345
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)

src/northbridge/amd/gx2/northbridgeinit.c

@@ -656,7 +656,7 @@ void northbridge_init_early(void)
 
 	/* Now that the descriptor to memory is set up. */
 	/* The memory controller needs one read to synch its lines before it can be used. */
-	i = *(volatile int *) 0;
+	read32(zeroptr);
 
 	GeodeLinkPriority();
 

src/northbridge/amd/gx2/raminit.c

@@ -16,6 +16,7 @@
 
 #include <cpu/amd/gx2def.h>
 #include <spd.h>
+#include <stddef.h>
 
 static const unsigned char NumColAddr[] = {
 	0x00, 0x10, 0x11, 0x00, 0x00, 0x00, 0x00, 0x07,
@@ -596,10 +597,8 @@ static void sdram_enable(int controllers, const struct mem_controller *ctrl)
 
 	/* The RAM dll needs a write to lock on so generate a few dummy writes */
 	/* Note: The descriptor needs to be enabled to point at memory */
-	volatile unsigned long *ptr;
 	for (i = 0; i < 5; i++) {
-		ptr = (void *)i;
-		*ptr = (unsigned long)i;
+		write32(zeroptr + i, i);
 	}
 
 	printk(BIOS_INFO, "RAM DLL lock\n");

src/northbridge/amd/lx/northbridgeinit.c

@@ -738,7 +738,7 @@ void northbridge_init_early(void)
 
 	/*  Now that the descriptor to memory is set up. */
 	/*  The memory controller needs one read to synch its lines before it can be used. */
-	i = *(volatile int *)0;
+	read32(zeroptr);
 
 	GeodeLinkPriority();
 

src/northbridge/amd/lx/raminit.c

@@ -18,6 +18,7 @@
 #include <cpu/amd/lxdef.h>
 #include <arch/io.h>
 #include <spd.h>
+#include <stddef.h>
 #include "southbridge/amd/cs5536/cs5536.h"
 #include "raminit.h"
 #include "northbridge.h"
@@ -747,10 +748,8 @@ void sdram_enable(int controllers, const struct mem_controller *ctrl)
 
 	/* The RAM dll needs a write to lock on so generate a few dummy writes */
 	/* Note: The descriptor needs to be enabled to point at memory */
-	volatile unsigned long *ptr;
 	for (i = 0; i < 5; i++) {
-		ptr = (void *)i;
-		*ptr = (unsigned long)i;
+		write32(zeroptr + i, i);
 	}
 	/* SWAPSiF for PBZ 4112 (Errata 34) */
 	/* check for failed DLL settings now that we have done a memory write. */