[PATCH] MIPS: Refactor early_parse_mem() to fix mem= parameter

[Tiezhu Yang <yangtiezhu@loongson.cn>]

According to Documentation/admin-guide/kernel-parameters.txt,
the kernel command-line parameter mem= means "Force usage of
a specific amount of memory", but when add "mem=3G" to the
command-line, kernel boot hangs in sparse_init().

This commit is similar with the implementation of the other
archs such as arm64, powerpc and riscv, refactor the function
early_parse_mem() and then use memblock_enforce_memory_limit()
to limit the memory size.

With this patch, when add "mem=3G" to the command-line, the
kernel boots successfully, we can see the following messages:

  [    0.000000] Memory limited to 3072MB
  ...
  [    0.000000] Memory: 2991952K/3145728K available (...)

After login, the output of free command is consistent with the
above log.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
 arch/mips/kernel/setup.c | 25 ++++++++-----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index f979adf..2917412 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -339,27 +339,15 @@ static void __init bootmem_init(void)
 #endif	/* CONFIG_SGI_IP27 */
 
 static int usermem __initdata;
+static phys_addr_t memory_limit;
 
 static int __init early_parse_mem(char *p)
 {
-	phys_addr_t start, size;
-
-	/*
-	 * If a user specifies memory size, we
-	 * blow away any automatically generated
-	 * size.
-	 */
-	if (usermem == 0) {
-		usermem = 1;
-		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);
+	if (!p)
+		return 1;
 
-	memblock_add(start, size);
+	memory_limit = memparse(p, &p) & PAGE_MASK;
+	pr_notice("Memory limited to %lldMB\n", memory_limit >> 20);
 
 	return 0;
 }
@@ -633,6 +621,9 @@ static void __init arch_mem_init(char **cmdline_p)
 
 	parse_early_param();
 
+	/* Limit the memory size via mem= command-line parameter */
+	memblock_enforce_memory_limit(memory_limit);
+
 	if (usermem)
 		pr_info("User-defined physical RAM map overwrite\n");
 
-- 
2.1.0