riscv: Allow NOMMU kernels to access all of RAM

NOMMU kernels currently cannot access memory below the kernel link
address. Remove this restriction by setting PAGE_OFFSET to the actual
start of RAM, as determined from the devicetree. The kernel link address
must be a constant, so keep using CONFIG_PAGE_OFFSET for that purpose.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Jesse Taube <mr.bossman075@gmail.com>
Link: https://lore.kernel.org/r/20241026171441.3047904-3-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: SiFiveHolland

arch/riscv/include/asm/page.h

@@ -24,12 +24,9 @@
  * When not using MMU this corresponds to the first free page in
  * physical memory (aligned on a page boundary).
  */
-#ifdef CONFIG_64BIT
 #ifdef CONFIG_MMU
+#ifdef CONFIG_64BIT
 #define PAGE_OFFSET		kernel_map.page_offset
-#else
-#define PAGE_OFFSET		_AC(CONFIG_PAGE_OFFSET, UL)
-#endif
 /*
  * By default, CONFIG_PAGE_OFFSET value corresponds to SV57 address space so
  * define the PAGE_OFFSET value for SV48 and SV39.
@@ -39,6 +36,9 @@
 #else
 #define PAGE_OFFSET		_AC(CONFIG_PAGE_OFFSET, UL)
 #endif /* CONFIG_64BIT */
+#else
+#define PAGE_OFFSET		((unsigned long)phys_ram_base)
+#endif /* CONFIG_MMU */
 
 #ifndef __ASSEMBLY__
 
@@ -95,11 +95,7 @@ typedef struct page *pgtable_t;
 #define MIN_MEMBLOCK_ADDR      0
 #endif
 
-#ifdef CONFIG_MMU
 #define ARCH_PFN_OFFSET		(PFN_DOWN((unsigned long)phys_ram_base))
-#else
-#define ARCH_PFN_OFFSET		(PAGE_OFFSET >> PAGE_SHIFT)
-#endif /* CONFIG_MMU */
 
 struct kernel_mapping {
 	unsigned long page_offset;

arch/riscv/include/asm/pgtable.h

@@ -12,7 +12,7 @@
 #include <asm/pgtable-bits.h>
 
 #ifndef CONFIG_MMU
-#define KERNEL_LINK_ADDR	PAGE_OFFSET
+#define KERNEL_LINK_ADDR	_AC(CONFIG_PAGE_OFFSET, UL)
 #define KERN_VIRT_SIZE		(UL(-1))
 #else