MIPS: Allow auto-dection of ARCH_PFN_OFFSET & PHYS_OFFSET

On systems where physical memory begins at a non-zero address, defining
PHYS_OFFSET (which influences ARCH_PFN_OFFSET) can save us time & memory
by avoiding book-keeping for pages from address zero to the start of
memory.

Some MIPS platforms already make use of this, but with the definition of
PHYS_OFFSET being compile-time constant it hasn't been possible to
enable this optimization for a kernel which may run on systems with
varying physical memory base addresses.

Introduce a new Kconfig option CONFIG_MIPS_AUTO_PFN_OFFSET which, when
enabled, makes ARCH_PFN_OFFSET a variable & detects it from the boot
memory map (which for example may have been populated from DT). The
relationship with PHYS_OFFSET is reversed, with PHYS_OFFSET now being
based on ARCH_PFN_OFFSET. This is because ARCH_PFN_OFFSET is used far
more often, so avoiding the need for runtime calculation gives us a
smaller impact on kernel text size (0.1% rather than 0.15% for
64r6el_defconfig).

Signed-off-by: Paul Burton <paul.burton@mips.com>
Suggested-by: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
Patchwork: https://patchwork.linux-mips.org/patch/20048/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org

arch/mips/Kconfig

@@ -2985,6 +2985,9 @@ config PGTABLE_LEVELS
 	default 3 if 64BIT && !PAGE_SIZE_64KB
 	default 2
 
+config MIPS_AUTO_PFN_OFFSET
+	bool
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"

arch/mips/include/asm/mach-generic/spaces.h

@@ -17,9 +17,13 @@
 /*
  * This gives the physical RAM offset.
  */
-#ifndef PHYS_OFFSET
-#define PHYS_OFFSET		_AC(0, UL)
-#endif
+#ifndef __ASSEMBLY__
+# if defined(CONFIG_MIPS_AUTO_PFN_OFFSET)
+#  define PHYS_OFFSET		((unsigned long)PFN_PHYS(ARCH_PFN_OFFSET))
+# elif !defined(PHYS_OFFSET)
+#  define PHYS_OFFSET		_AC(0, UL)
+# endif
+#endif /* __ASSEMBLY__ */
 
 #ifdef CONFIG_32BIT
 #ifdef CONFIG_KVM_GUEST

arch/mips/include/asm/page.h

@@ -80,7 +80,12 @@ extern void build_copy_page(void);
  * used in our early mem init code for all memory models.
  * So always define it.
  */
-#define ARCH_PFN_OFFSET		PFN_UP(PHYS_OFFSET)
+#ifdef CONFIG_MIPS_AUTO_PFN_OFFSET
+extern unsigned long ARCH_PFN_OFFSET;
+# define ARCH_PFN_OFFSET	ARCH_PFN_OFFSET
+#else
+# define ARCH_PFN_OFFSET	PFN_UP(PHYS_OFFSET)
+#endif
 
 extern void clear_page(void * page);
 extern void copy_page(void * to, void * from);

arch/mips/kernel/setup.c

@@ -85,6 +85,11 @@ static struct resource bss_resource = { .name = "Kernel bss", };
 
 static void *detect_magic __initdata = detect_memory_region;
 
+#ifdef CONFIG_MIPS_AUTO_PFN_OFFSET
+unsigned long ARCH_PFN_OFFSET;
+EXPORT_SYMBOL(ARCH_PFN_OFFSET);
+#endif
+
 void __init add_memory_region(phys_addr_t start, phys_addr_t size, long type)
 {
 	int x = boot_mem_map.nr_map;
@@ -442,6 +447,12 @@ static void __init bootmem_init(void)
 		mapstart = max(reserved_end, start);
 	}
 
+	if (min_low_pfn >= max_low_pfn)
+		panic("Incorrect memory mapping !!!");
+
+#ifdef CONFIG_MIPS_AUTO_PFN_OFFSET
+	ARCH_PFN_OFFSET = PFN_UP(ramstart);
+#else
 	/*
 	 * Reserve any memory between the start of RAM and PHYS_OFFSET
 	 */
@@ -449,8 +460,6 @@ static void __init bootmem_init(void)
 		add_memory_region(PHYS_OFFSET, ramstart - PHYS_OFFSET,
 				  BOOT_MEM_RESERVED);
 
-	if (min_low_pfn >= max_low_pfn)
-		panic("Incorrect memory mapping !!!");
 	if (min_low_pfn > ARCH_PFN_OFFSET) {
 		pr_info("Wasting %lu bytes for tracking %lu unused pages\n",
 			(min_low_pfn - ARCH_PFN_OFFSET) * sizeof(struct page),
@@ -460,6 +469,7 @@ static void __init bootmem_init(void)
 			ARCH_PFN_OFFSET - min_low_pfn);
 	}
 	min_low_pfn = ARCH_PFN_OFFSET;
+#endif
 
 	/*
 	 * Determine low and high memory ranges