MIPS: ralink: mt7621: do memory detection on KSEG1

It's reported that current memory detection code occasionally detects
larger memory under some bootloaders.
Current memory detection code tests whether address space wraps around
on KSEG0, which is unreliable because it's cached.

Rewrite memory size detection to perform the same test on KSEG1 instead.
While at it, this patch also does the following two things:
1. use a fixed pattern instead of a random function pointer as the magic
   value.
2. add an additional memory write and a second comparison as part of the
   test to prevent possible smaller memory detection result due to
   leftover values in memory.

Fixes: 139c949f MIPS: ("ralink: mt7621: add memory detection support")
Reported-by: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Tested-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Tested-by: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/ralink/mt7621.c

@@ -22,7 +22,9 @@
 
 #include "common.h"
 
-static void *detect_magic __initdata = detect_memory_region;
+#define MT7621_MEM_TEST_PATTERN         0xaa5555aa
+
+static u32 detect_magic __initdata;
 
 int pcibios_root_bridge_prepare(struct pci_host_bridge *bridge)
 {
@@ -58,24 +60,32 @@ phys_addr_t mips_cpc_default_phys_base(void)
 	panic("Cannot detect cpc address");
 }
 
+static bool __init mt7621_addr_wraparound_test(phys_addr_t size)
+{
+	void *dm = (void *)KSEG1ADDR(&detect_magic);
+
+	if (CPHYSADDR(dm + size) >= MT7621_LOWMEM_MAX_SIZE)
+		return true;
+	__raw_writel(MT7621_MEM_TEST_PATTERN, dm);
+	if (__raw_readl(dm) != __raw_readl(dm + size))
+		return false;
+	__raw_writel(!MT7621_MEM_TEST_PATTERN, dm);
+	return __raw_readl(dm) == __raw_readl(dm + size);
+}
+
 static void __init mt7621_memory_detect(void)
 {
-	void *dm = &detect_magic;
 	phys_addr_t size;
 
-	for (size = 32 * SZ_1M; size < 256 * SZ_1M; size <<= 1) {
-		if (!__builtin_memcmp(dm, dm + size, sizeof(detect_magic)))
-			break;
+	for (size = 32 * SZ_1M; size <= 256 * SZ_1M; size <<= 1) {
+		if (mt7621_addr_wraparound_test(size)) {
+			memblock_add(MT7621_LOWMEM_BASE, size);
+			return;
+		}
 	}
 
-	if ((size == 256 * SZ_1M) &&
-	    (CPHYSADDR(dm + size) < MT7621_LOWMEM_MAX_SIZE) &&
-	    __builtin_memcmp(dm, dm + size, sizeof(detect_magic))) {
-		memblock_add(MT7621_LOWMEM_BASE, MT7621_LOWMEM_MAX_SIZE);
-		memblock_add(MT7621_HIGHMEM_BASE, MT7621_HIGHMEM_SIZE);
-	} else {
-		memblock_add(MT7621_LOWMEM_BASE, size);
-	}
+	memblock_add(MT7621_LOWMEM_BASE, MT7621_LOWMEM_MAX_SIZE);
+	memblock_add(MT7621_HIGHMEM_BASE, MT7621_HIGHMEM_SIZE);
 }
 
 void __init ralink_of_remap(void)