AST2700 dram size calculation is not back compatible AST2600.
According to the DDR capacity hardware behavior,
if users write the data to address which is beyond the ram size,
it would write the data to address 0.
For example:
a. sdram base address "0x4 00000000"
b. sdram size is 1 GiB
The available address range is from "0x4 00000000" to "0x4 40000000".
If users write 0xdeadbeef to address "0x6 00000000",
the value of DRAM address 0 (base address 0x4 00000000) should be 0xdeadbeef.

Adds aspeed_soc_ast2700_dram_init to calculation the dram size and add
memory I/O whose address range is from max_ram_size - ram_size to max_ram_size
and its read/write handler to emulate DDR capacity hardware behavior.

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
---
 hw/arm/aspeed_ast27x0.c     | 94 ++++++++++++++++++++++++++++++++++++-
 include/hw/arm/aspeed_soc.h |  1 +
 2 files changed, 94 insertions(+), 1 deletion(-)

diff --git a/hw/arm/aspeed_ast27x0.c b/hw/arm/aspeed_ast27x0.c
index c120994e5c..b78ae9369e 100644
--- a/hw/arm/aspeed_ast27x0.c
+++ b/hw/arm/aspeed_ast27x0.c
@@ -20,6 +20,7 @@
 #include "sysemu/sysemu.h"
 #include "hw/intc/arm_gicv3.h"
 #include "qapi/qmp/qlist.h"
+#include "qemu/log.h"
 
 static const hwaddr aspeed_soc_ast2700_memmap[] = {
     [ASPEED_DEV_SPI_BOOT]  =  0x400000000,
@@ -118,6 +119,97 @@ static qemu_irq aspeed_soc_ast2700_get_irq(AspeedSoCState *s, int dev)
     return qdev_get_gpio_in(a->intc.gic, sc->irqmap[dev]);
 }
 
+static uint64_t aspeed_ram_capacity_read(void *opaque, hwaddr addr,
+                                                    unsigned int size)
+{
+    qemu_log_mask(LOG_GUEST_ERROR,
+                  "%s: read @%" PRIx64 " out of ram size\n",
+                   __func__, addr);
+    return 0;
+}
+
+static void aspeed_ram_capacity_write(void *opaque, hwaddr addr, uint64_t data,
+                                                unsigned int size)
+{
+    AspeedSoCState *s = ASPEED_SOC(opaque);
+    uint32_t test_pattern = 0xdeadbeef;
+    bool invalid_pattern = true;
+    uint32_t *ram_ptr;
+    int sz;
+
+    ram_ptr = memory_region_get_ram_ptr(s->dram_mr);
+
+   /*
+    * Emulate ddr capacity hardware behavior.
+    * If writes the test_pattern to address which is beyond the ram size,
+    * it would write the test_pattern to address 0.
+    */
+    for (sz = 4; sz > 0 ; sz--) {
+        test_pattern = (test_pattern << 4) + sz;
+        if (data == test_pattern) {
+            ram_ptr[0] = test_pattern;
+            invalid_pattern = false;
+            break;
+        }
+    }
+
+    if (invalid_pattern) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: write invalid pattern @%" PRIx64
+                      " to addr @%" HWADDR_PRIx "]\n",
+                      __func__, data, addr);
+    }
+}
+
+static const MemoryRegionOps aspeed_ram_capacity_ops = {
+    .read = aspeed_ram_capacity_read,
+    .write = aspeed_ram_capacity_write,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+    .valid = {
+        .min_access_size = 1,
+        .max_access_size = 8,
+    },
+};
+
+/*
+ * SDMC should be realized first to get correct RAM size and max size
+ * values
+ */
+static bool aspeed_soc_ast2700_dram_init(DeviceState *dev, Error **errp)
+{
+    ram_addr_t ram_size, max_ram_size;
+    Aspeed27x0SoCState *a = ASPEED27X0_SOC(dev);
+    AspeedSoCState *s = ASPEED_SOC(dev);
+    AspeedSoCClass *sc = ASPEED_SOC_GET_CLASS(s);
+
+    ram_size = object_property_get_uint(OBJECT(&s->sdmc), "ram-size",
+                                        &error_abort);
+    max_ram_size = object_property_get_uint(OBJECT(&s->sdmc), "max-ram-size",
+                                            &error_abort);
+
+    memory_region_init(&s->dram_container, OBJECT(s), "ram-container",
+                       ram_size);
+    memory_region_add_subregion(&s->dram_container, 0, s->dram_mr);
+
+    /*
+     * Add a memory region beyond the RAM region to emulate
+     * ddr capacity hardware behavior.
+     */
+    if (ram_size < max_ram_size) {
+        memory_region_init_io(&a->dram_empty, OBJECT(s),
+                              &aspeed_ram_capacity_ops, s,
+                              "ram-empty", max_ram_size - ram_size);
+
+        memory_region_add_subregion(s->memory,
+                                    sc->memmap[ASPEED_DEV_SDRAM] + ram_size,
+                                    &a->dram_empty);
+    }
+
+    memory_region_add_subregion(s->memory,
+                      sc->memmap[ASPEED_DEV_SDRAM], &s->dram_container);
+    return true;
+}
+
 static void aspeed_soc_ast2700_init(Object *obj)
 {
     Aspeed27x0SoCState *a = ASPEED27X0_SOC(obj);
@@ -362,7 +454,7 @@ static void aspeed_soc_ast2700_realize(DeviceState *dev, Error **errp)
                     sc->memmap[ASPEED_DEV_SDMC]);
 
     /* RAM */
-    if (!aspeed_soc_dram_init(s, errp)) {
+    if (!aspeed_soc_ast2700_dram_init(dev, errp)) {
         return;
     }
 
diff --git a/include/hw/arm/aspeed_soc.h b/include/hw/arm/aspeed_soc.h
index fab5abff9e..fae49f205b 100644
--- a/include/hw/arm/aspeed_soc.h
+++ b/include/hw/arm/aspeed_soc.h
@@ -126,6 +126,7 @@ struct Aspeed27x0SoCState {
 
     ARMCPU cpu[ASPEED_CPUS_NUM];
     AspeedINTCState intc;
+    MemoryRegion dram_empty;
 };
 
 #define TYPE_ASPEED27X0_SOC "aspeed27x0-soc"
-- 
2.25.1