RISC-V: relocate DTB if it's outside memory region

In case the DTB provided by the bootloader/BootROM is before the kernel
image or outside /memory, we won't be able to access it through the
linear mapping, and get a segfault on setup_arch(). Currently OpenSBI
relocates DTB but that's not always the case (e.g. if FW_JUMP_FDT_ADDR
is not specified), and it's also not the most portable approach since
the default FW_JUMP_FDT_ADDR of the generic platform relocates the DTB
at a specific offset that may not be available. To avoid this situation
copy DTB so that it's visible through the linear mapping.

Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Link: https://lore.kernel.org/r/20220322132839.3653682-1-mick@ics.forth.gr
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Fixes: f105aa9 ("riscv: add BUILTIN_DTB support for MMU-enabled targets")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: mickflemm

arch/riscv/mm/init.c

@@ -208,8 +208,25 @@ static void __init setup_bootmem(void)
 	 * early_init_fdt_reserve_self() since __pa() does
 	 * not work for DTB pointers that are fixmap addresses
 	 */
-	if (!IS_ENABLED(CONFIG_BUILTIN_DTB))
-		memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
+	if (!IS_ENABLED(CONFIG_BUILTIN_DTB)) {
+		/*
+		 * In case the DTB is not located in a memory region we won't
+		 * be able to locate it later on via the linear mapping and
+		 * get a segfault when accessing it via __va(dtb_early_pa).
+		 * To avoid this situation copy DTB to a memory region.
+		 * Note that memblock_phys_alloc will also reserve DTB region.
+		 */
+		if (!memblock_is_memory(dtb_early_pa)) {
+			size_t fdt_size = fdt_totalsize(dtb_early_va);
+			phys_addr_t new_dtb_early_pa = memblock_phys_alloc(fdt_size, PAGE_SIZE);
+			void *new_dtb_early_va = early_memremap(new_dtb_early_pa, fdt_size);
+
+			memcpy(new_dtb_early_va, dtb_early_va, fdt_size);
+			early_memunmap(new_dtb_early_va, fdt_size);
+			_dtb_early_pa = new_dtb_early_pa;
+		} else
+			memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
+	}
 
 	early_init_fdt_scan_reserved_mem();
 	dma_contiguous_reserve(dma32_phys_limit);