openrisc: enable passing of flattened device tree pointer

This patch enables passing a fdt pointer to the kernel.

This makes for the kernel parameter API:

void kernel(unsigned int fdt);

which, in accordance with the OpenRISC ABI results in:
r3 = pointer to fdt

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Signed-off-by: Jonas Bonn <jonas@southpole.se>

arch/openrisc/kernel/head.S

@@ -26,6 +26,7 @@
 #include <asm/cache.h>
 #include <asm/spr_defs.h>
 #include <asm/asm-offsets.h>
+#include <linux/of_fdt.h>
 
 #define tophys(rd,rs)				\
 	l.movhi	rd,hi(-KERNELBASE)		;\
@@ -440,6 +441,9 @@ _dispatch_do_ipage_fault:
 	__HEAD
 	.global _start
 _start:
+	/* save kernel parameters */
+	l.or	r25,r0,r3	/* pointer to fdt */
+
 	/*
 	 * ensure a deterministic start
 	 */
@@ -471,7 +475,6 @@ _start:
 	CLEAR_GPR(r22)
 	CLEAR_GPR(r23)
 	CLEAR_GPR(r24)
-	CLEAR_GPR(r25)
 	CLEAR_GPR(r26)
 	CLEAR_GPR(r27)
 	CLEAR_GPR(r28)
@@ -565,6 +568,18 @@ enable_mmu:
 	// reset the simulation counters
 	l.nop 5
 
+	/* check fdt header magic word */
+	l.lwz	r3,0(r25)	/* load magic from fdt into r3 */
+	l.movhi	r4,hi(OF_DT_HEADER)
+	l.ori	r4,r4,lo(OF_DT_HEADER)
+	l.sfeq	r3,r4
+	l.bf	_fdt_found
+	 l.nop
+	/* magic number mismatch, set fdt pointer to null */
+	l.or	r25,r0,r0
+_fdt_found:
+	/* pass fdt pointer to or32_early_setup in r3 */
+	l.or	r3,r0,r25
 	LOAD_SYMBOL_2_GPR(r24, or32_early_setup)
 	l.jalr r24
 	 l.nop

arch/openrisc/kernel/setup.c

@@ -207,18 +207,18 @@ void __init setup_cpuinfo(void)
  * Handles the pointer to the device tree that this kernel is to use
  * for establishing the available platform devices.
  *
- * For now, this is limited to using the built-in device tree.  In the future,
- * it is intended that this function will take a pointer to the device tree
- * that is potentially built-in, but potentially also passed in by the
- * bootloader, or discovered by some equally clever means...
+ * Falls back on built-in device tree in case null pointer is passed.
  */
 
-void __init or32_early_setup(void)
+void __init or32_early_setup(unsigned int fdt)
 {
-
-	early_init_devtree(__dtb_start);
-
-	printk(KERN_INFO "Compiled-in FDT at 0x%p\n", __dtb_start);
+	if (fdt) {
+		early_init_devtree((void*) fdt);
+		printk(KERN_INFO "FDT at 0x%08x\n", fdt);
+	} else {
+		early_init_devtree(__dtb_start);
+		printk(KERN_INFO "Compiled-in FDT at %p\n", __dtb_start);
+	}
 }
 
 static int __init openrisc_device_probe(void)