riscv: Add support for memtest

The riscv [rv32_]defconfig enabled CONFIG_MEMTEST,
but memtest feature is not supported in RISCV.

Add early_memtest() to support for memtest.
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -2794,7 +2794,7 @@
 			seconds.  Use this parameter to check at some
 			other rate.  0 disables periodic checking.
 
-	memtest=	[KNL,X86,ARM,PPC] Enable memtest
+	memtest=	[KNL,X86,ARM,PPC,RISCV] Enable memtest
 			Format: <integer>
 			default : 0 <disable>
 			Specifies the number of memtest passes to be

arch/riscv/mm/init.c

@@ -128,8 +128,9 @@ void __init setup_bootmem(void)
 	if (max_mapped_addr == (dram_end - 1))
 		memblock_set_current_limit(max_mapped_addr - 4096);
 
-	max_pfn = PFN_DOWN(dram_end);
-	max_low_pfn = max_pfn;
+	min_low_pfn = PFN_UP(memblock_start_of_DRAM());
+	max_low_pfn = max_pfn = PFN_DOWN(dram_end);
+
 	dma32_phys_limit = min(4UL * SZ_1G, (unsigned long)PFN_PHYS(max_low_pfn));
 	set_max_mapnr(max_low_pfn - ARCH_PFN_OFFSET);
 
@@ -593,6 +594,7 @@ void __init paging_init(void)
 
 void __init misc_mem_init(void)
 {
+	early_memtest(min_low_pfn << PAGE_SHIFT, max_low_pfn << PAGE_SHIFT);
 	arch_numa_init();
 	sparse_init();
 	zone_sizes_init();