[MIPS] FLATMEM: introduce PHYS_OFFSET.

The old code was assuming that min_low_pfn was always 0. This
means that platforms having a big hole at their memory start
paid the price of wasting some memory for the allocation of
unused entries in mem_map[].

This patch prevents this waste.

It introduces PHYS_OFFSET define which is the start of the
physical memory and uses it wherever needed. Specially when
converting physical/virtual addresses into virtual/physical
ones.

Currently all platforms defines PHYS_OFFSET to 0.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

include/asm-mips/page.h

@@ -34,6 +34,20 @@
 
 #ifndef __ASSEMBLY__
 
+/*
+ * This gives the physical RAM offset.
+ */
+#ifndef PHYS_OFFSET
+#define PHYS_OFFSET		0UL
+#endif
+
+/*
+ * It's normally defined only for FLATMEM config but it's
+ * used in our early mem init code for all memory models.
+ * So always define it.
+ */
+#define ARCH_PFN_OFFSET		PFN_UP(PHYS_OFFSET)
+
 #include <linux/pfn.h>
 #include <asm/io.h>
 
@@ -132,20 +146,23 @@ typedef struct { unsigned long pgprot; } pgprot_t;
 /* to align the pointer to the (next) page boundary */
 #define PAGE_ALIGN(addr)	(((addr) + PAGE_SIZE - 1) & PAGE_MASK)
 
+/*
+ * __pa()/__va() should be used only during mem init.
+ */
 #if defined(CONFIG_64BIT) && !defined(CONFIG_BUILD_ELF64)
 #define __pa_page_offset(x)	((unsigned long)(x) < CKSEG0 ? PAGE_OFFSET : CKSEG0)
 #else
 #define __pa_page_offset(x)	PAGE_OFFSET
 #endif
-#define __pa(x)			((unsigned long)(x) - __pa_page_offset(x))
-#define __pa_symbol(x)		__pa(RELOC_HIDE((unsigned long)(x),0))
-#define __va(x)			((void *)((unsigned long)(x) + PAGE_OFFSET))
+#define __pa(x)		((unsigned long)(x) - __pa_page_offset(x) + PHYS_OFFSET)
+#define __va(x)		((void *)((unsigned long)(x) + PAGE_OFFSET - PHYS_OFFSET))
+#define __pa_symbol(x)	__pa(RELOC_HIDE((unsigned long)(x),0))
 
 #define pfn_to_kaddr(pfn)	__va((pfn) << PAGE_SHIFT)
 
 #ifdef CONFIG_FLATMEM
 
-#define pfn_valid(pfn)		((pfn) < max_mapnr)
+#define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
 
 #elif defined(CONFIG_SPARSEMEM)