LoongArch: Support mem=<size> kernel parameter

The LoongArch mem= parameter parser was previously limited to the
mem=<size>@<start> format. This was inconvenient for the common use
case of simply capping the total system memory, as it forced users to
manually specify a start address. It was also inconsistent with the
behavior on other architectures.

This patch enhances the parser in early_parse_mem() to also support the
more user-friendly mem=<size> format. The implementation now checks for
the presence of the '@' symbol to determine the user's intent:

- If mem=<size> is provided (no '@'), the kernel now calls
  memblock_enforce_memory_limit(). This trims memory from the top down
  to the specified size.
- If mem=<size>@<start> is provided, the original behavior is retained
  for backward compatibility. This allows for defining specific memory
  banks.

This change introduces an important usage rule reflected in the code's
comments: the mem=<size> format should only be specified once on the
kernel command line. It acts as a single, global cap on total memory. In
contrast, the mem=<size>@<start> format can be specified multiple times
to define several distinct memory regions.

Signed-off-by: Ming Wang <wangming01@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: Ming Wang

arch/loongarch/kernel/setup.c

@@ -191,6 +191,16 @@ static int __init early_parse_mem(char *p)
 		return -EINVAL;
 	}
 
+	start = 0;
+	size = memparse(p, &p);
+	if (*p == '@')	/* Every mem=... should contain '@' */
+		start = memparse(p + 1, &p);
+	else {		/* Only one mem=... is allowed if no '@' */
+		usermem = 1;
+		memblock_enforce_memory_limit(size);
+		return 0;
+	}
+
 	/*
 	 * If a user specifies memory size, we
 	 * blow away any automatically generated
@@ -201,14 +211,6 @@ static int __init early_parse_mem(char *p)
 		memblock_remove(memblock_start_of_DRAM(),
 			memblock_end_of_DRAM() - memblock_start_of_DRAM());
 	}
-	start = 0;
-	size = memparse(p, &p);
-	if (*p == '@')
-		start = memparse(p + 1, &p);
-	else {
-		pr_err("Invalid format!\n");
-		return -EINVAL;
-	}
 
 	if (!IS_ENABLED(CONFIG_NUMA))
 		memblock_add(start, size);