[PATCH] arm64: mm: drop redundant remap of FDT first page

[PATCH] arm64: mm: drop redundant remap of FDT first page

[Sang-Heon Jeon]

fixmap_remap_fdt() calls create_mapping_noalloc() to map the first
page of the FDT to read its magic and totalsize from the header. If
the FDT does not fit in a single page, it calls create_mapping_noalloc()
again to map the rest.

The second mapping redundantly covers the first page that was just
mapped by the first mapping.

Start the second mapping at dt_phys_base + PAGE_SIZE so it only covers
the pages that have not been mapped yet. No functional change.

Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
---
QEMU-based test results

$ qemu-system-aarch64 -M virt -singlestep -d nochain,exec ...

Count log lines from fixmap_remap_fdt() entry to return
- entry PC : address of the <fixmap_remap_fdt> symbol
- return PC : next address of each bl ... <fixmap_remap_fdt>

1) AS-IS
  - 1st call (KERNEL) : 4935
  - 2nd call (KERNEL_RO) : 14151
2) TO-BE
  - 1st call : 4888
  - 2nd call : 14104

---
Hello,

While looking into boot information, I found a minor cleanup point.
If I misunderstood anything, please feel free to let me know.

Thank you for taking valuable time to review this work.

Best Regards,
Sang-Heon Jeon
---
 arch/arm64/mm/fixmap.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/fixmap.c b/arch/arm64/mm/fixmap.c
index c5c5425791da..f8aea5572f7c 100644
--- a/arch/arm64/mm/fixmap.c
+++ b/arch/arm64/mm/fixmap.c
@@ -167,8 +167,9 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
 		return NULL;
 
 	if (offset + *size > PAGE_SIZE) {
-		create_mapping_noalloc(dt_phys_base, dt_virt_base,
-				       offset + *size, prot);
+		create_mapping_noalloc(dt_phys_base + PAGE_SIZE,
+			dt_virt_base + PAGE_SIZE,
+			offset + *size - PAGE_SIZE, prot);
 	}
 
 	return dt_virt;
-- 
2.43.0