ksz9477: MMD indirect access check fix

Instead of verifying MMD indirect access by checking
EEE ADVERTISEMENT register has value 0x6 (which may be changed),
the verification is done by write and check LED MODE register.
1. Backup current LED_MODE register value
2. Write value Single-LED mode (0x10) to LED_MODE
3. Read LED_MODE register and verify it returns value 0x10
4. Write original (backup) value to LED_MODE

drivers/net/ksz9477.c

@@ -617,6 +617,7 @@ int ksz9477_init(ksz9477_port_t master_port)
   int i;
   uint16_t regval16;
   uint32_t regval32;
+  uint16_t backup;
 
   /* Read the ID registers */
 
@@ -638,15 +639,58 @@ int ksz9477_init(ksz9477_port_t master_port)
     }
 
   /* Check that indirect access to PHY MMD works.
-   * Default value of MMD EEE ADVERTISEMENT REGISTER is 0x6
+   * Write LED mode to single-LED mode and verify access by
+   * reading back the value.
    */
 
+  /* 1. Read and backup the current LED MODE value */
+
+  ret = ksz9477_mmd_read_indirect(KSZ9477_PORT_PHY1,
+                                  KSZ9477_MMD_DEV_LED_MODE,
+                                  0x00, &backup, 1);
+
+  if (ret != OK)
+    {
+      nerr("MMD access failure. Failed to read LED_MODE "
+           "register value: ret %d\n", ret);
+      return ret ? ret : -EINVAL;
+    }
+
+  /* 2. Set LED mode to sigle-LED mode */
+
+  regval16 = 0x10;
+  ret = ksz9477_mmd_write_indirect(KSZ9477_PORT_PHY1,
+                                   KSZ9477_MMD_DEV_LED_MODE,
+                                   0x00, &regval16, 1);
+  if (ret != OK)
+    {
+      nerr("MMD access failure. Failed write 0x%04x to "
+           "LED_MODE register: ret %d\n", regval16, ret);
+      return ret ? ret : -EINVAL;
+    }
+
+  /* 3. Verify read returns Single-LED mode (0x10) */
+
   ret = ksz9477_mmd_read_indirect(KSZ9477_PORT_PHY1,
-                                  KSZ9477_MMD_DEV_EEE_ADVERTISEMENT,
-                                  0x3c, &regval16, 1);
-  if (ret != OK || regval16 != 0x6)
+                                  KSZ9477_MMD_DEV_LED_MODE,
+                                  0x00, &regval16, 1);
+
+  if (ret != OK || regval16 != 0x10)
+    {
+      nerr("MMD access failure. Failed to verify LED_MODE "
+           "register value 0x10: ret %d, regval %04x\n", ret, regval16);
+      return ret ? ret : -EINVAL;
+    }
+
+  /* 4. Set back to default value from backup */
+
+  ret = ksz9477_mmd_write_indirect(KSZ9477_PORT_PHY1,
+                                   KSZ9477_MMD_DEV_LED_MODE,
+                                   0x00, &backup, 1);
+  if (ret != OK)
     {
-      nerr("MMD access failure, ret %d\n", ret);
+      nerr("MMD access failure. Failed to write 0x%04x to "
+           "LED_MODE register: ret %d\n", backup, ret);
       return ret ? ret : -EINVAL;
     }