MIPS: ralink: mt7621: add memory detection support

mt7621 has the following memory map:
0x0-0x1c000000: lower 448m memory
0x1c000000-0x2000000: peripheral registers
0x20000000-0x2400000: higher 64m memory

detect_memory_region in arch/mips/kernel/setup.c only adds the first
memory region and isn't suitable for 512m memory detection because
it may accidentally read the memory area for peripheral registers.

This commit adds memory detection capability for mt7621:
  1. Add the highmem area when 512m is detected.
  2. Guard memcmp from accessing peripheral registers:
     This only happens when a user decided to change kernel load address
     to 256m or higher address. Since this is a quite unusual case, we
     just skip 512m testing and return 256m as memory size.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
[Minor commit message reword, make mt7621_memory_detect static]
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/mach-ralink/mt7621.h

@@ -24,9 +24,10 @@
 #define CHIP_REV_VER_SHIFT		8
 #define CHIP_REV_ECO_MASK		0xf
 
-#define MT7621_DRAM_BASE                0x0
-#define MT7621_DDR2_SIZE_MIN		32
-#define MT7621_DDR2_SIZE_MAX		256
+#define MT7621_LOWMEM_BASE		0x0
+#define MT7621_LOWMEM_MAX_SIZE		0x1C000000
+#define MT7621_HIGHMEM_BASE		0x20000000
+#define MT7621_HIGHMEM_SIZE		0x4000000
 
 #define MT7621_CHIP_NAME0		0x3637544D
 #define MT7621_CHIP_NAME1		0x20203132

arch/mips/ralink/common.h

@@ -17,6 +17,7 @@ struct ralink_soc_info {
 	unsigned long mem_size;
 	unsigned long mem_size_min;
 	unsigned long mem_size_max;
+	void (*mem_detect)(void);
 };
 extern struct ralink_soc_info soc_info;
 

arch/mips/ralink/mt7621.c

@@ -9,7 +9,9 @@
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/sys_soc.h>
+#include <linux/memblock.h>
 
+#include <asm/bootinfo.h>
 #include <asm/mipsregs.h>
 #include <asm/smp-ops.h>
 #include <asm/mips-cps.h>
@@ -49,6 +51,8 @@
 #define MT7621_GPIO_MODE_SDHCI_SHIFT	18
 #define MT7621_GPIO_MODE_SDHCI_GPIO	1
 
+static void *detect_magic __initdata = detect_memory_region;
+
 static struct rt2880_pmx_func uart1_grp[] =  { FUNC("uart1", 0, 1, 2) };
 static struct rt2880_pmx_func i2c_grp[] =  { FUNC("i2c", 0, 3, 2) };
 static struct rt2880_pmx_func uart3_grp[] = {
@@ -110,6 +114,26 @@ phys_addr_t mips_cpc_default_phys_base(void)
 	panic("Cannot detect cpc address");
 }
 
+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;
+	}
+
+	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);
+	}
+}
+
 void __init ralink_of_remap(void)
 {
 	rt_sysc_membase = plat_of_remap_node("mtk,mt7621-sysc");
@@ -194,10 +218,7 @@ void __init prom_soc_init(struct ralink_soc_info *soc_info)
 		(rev >> CHIP_REV_VER_SHIFT) & CHIP_REV_VER_MASK,
 		(rev & CHIP_REV_ECO_MASK));
 
-	soc_info->mem_size_min = MT7621_DDR2_SIZE_MIN;
-	soc_info->mem_size_max = MT7621_DDR2_SIZE_MAX;
-	soc_info->mem_base = MT7621_DRAM_BASE;
-
+	soc_info->mem_detect = mt7621_memory_detect;
 	rt2880_pinmux_data = mt7621_pinmux_data;
 
 	soc_dev_init(soc_info, rev);

arch/mips/ralink/of.c

@@ -78,6 +78,8 @@ void __init plat_mem_setup(void)
 	of_scan_flat_dt(early_init_dt_find_memory, NULL);
 	if (memory_dtb)
 		of_scan_flat_dt(early_init_dt_scan_memory, NULL);
+	else if (soc_info.mem_detect)
+		soc_info.mem_detect();
 	else if (soc_info.mem_size)
 		memblock_add(soc_info.mem_base, soc_info.mem_size * SZ_1M);
 	else