[PATCH v2] LoongArch: mm: fix FLATMEM ARCH_PFN_OFFSET for non-zero DRAM base

[Ethan Yang <ethan.yang.kernel@gmail.com>]

With CONFIG_FLATMEM, pfn_to_page() is computed as:

  mem_map + (pfn - ARCH_PFN_OFFSET)

LoongArch currently derives ARCH_PFN_OFFSET from PFN_UP(PHYS_OFFSET),
while PHYS_OFFSET defaults to 0. On systems whose first DRAM range does
not start at physical address 0, ARCH_PFN_OFFSET becomes smaller than
the real RAM base PFN.

As a result, pfn_to_page() points past the intended mem_map base during
early memmap initialization. This causes struct page initialization to
touch wrong entries and can lead to early boot failures in the
memmap_init_range() path.

Fix this by deriving ARCH_PFN_OFFSET from min_low_pfn, and initialize/
recalculate min_low_pfn from memblock_start_of_DRAM() in LoongArch
memory init so PFN-to-page translation matches the actual DRAM base.

This change only fixes the FLATMEM PFN base and does not alter the
existing VA/PA linear mapping scheme.

Signed-off-by: Ethan Yang <ethan.yang.kernel@gmail.com>
---
v2:
  - Fixed tab-to-space formatting.
---
 arch/loongarch/include/asm/page.h | 3 ++-
 arch/loongarch/kernel/mem.c       | 1 +
 arch/loongarch/kernel/setup.c     | 1 +
 3 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/page.h b/arch/loongarch/include/asm/page.h
index 327bf0bc9..80aea8b51 100644
--- a/arch/loongarch/include/asm/page.h
+++ b/arch/loongarch/include/asm/page.h
@@ -25,7 +25,8 @@
  * used in our early mem init code for all memory models.
  * So always define it.
  */
-#define ARCH_PFN_OFFSET	PFN_UP(PHYS_OFFSET)
+extern unsigned long min_low_pfn;
+#define ARCH_PFN_OFFSET	min_low_pfn
 
 extern void clear_page(void *page);
 extern void copy_page(void *to, void *from);
diff --git a/arch/loongarch/kernel/mem.c b/arch/loongarch/kernel/mem.c
index 8ab1ffedc..03835f8cb 100644
--- a/arch/loongarch/kernel/mem.c
+++ b/arch/loongarch/kernel/mem.c
@@ -46,6 +46,7 @@ void __init memblock_init(void)
 		}
 	}
 
+	min_low_pfn = PFN_UP(memblock_start_of_DRAM());
 	max_pfn = PFN_DOWN(memblock_end_of_DRAM());
 	max_low_pfn = min(PFN_DOWN(HIGHMEM_START), max_pfn);
 	memblock_set_current_limit(PFN_PHYS(max_low_pfn));
diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
index 839b23ede..625be2494 100644
--- a/arch/loongarch/kernel/setup.c
+++ b/arch/loongarch/kernel/setup.c
@@ -394,6 +394,7 @@ static void __init check_kernel_sections_mem(void)
 static void __init arch_mem_init(char **cmdline_p)
 {
 	/* Recalculate max_low_pfn for "mem=xxx" */
+	min_low_pfn = PFN_UP(memblock_start_of_DRAM());
 	max_pfn = PFN_DOWN(memblock_end_of_DRAM());
 	max_low_pfn = min(PFN_DOWN(HIGHMEM_START), max_pfn);
 
-- 
2.47.3