openrisc: update DTLB-miss handler last

The self-modifying code that updates the TLB handler at start-up has
a subtle ordering requirement:  the DTLB handler must be the last thing
changed.

What I was seeing was the following:

i)   The DTLB handler was updated
ii)  The following printk caused a TLB miss and the look-up resulted
     in the page containing itlb_vector (0xc0000a00) being bounced from
     the TLB.
iii) The subsequent access to itlb_vector caused a TLB miss and reload
     of the page containing itlb_vector from the page tables.
iv)  But this reload of the page in iii) was being done by the "new"
     DTLB-miss handler which resulted (correctly) in the page flags being
     set to read-only; the subsequent write-access to itlb_vector thus
     resulted in a page (access) fault.

This is easily remedied if we ensure that the boot-time DTLB-miss handler
continues running until the very last bit of self-modifying code has been
executed.  This patch should ensure that the very last thing updated is the
DTLB-handler itself.
Signed-off-by: Jonas Bonn <jonas@southpole.se>
Acked-by: Julius Baxter <juliusbaxter@gmail.com>
Tested-by: Sebastian Macke <sebastian@macke.de>

arch/openrisc/mm/init.c

@@ -167,15 +167,26 @@ void __init paging_init(void)
 		unsigned long *dtlb_vector = __va(0x900);
 		unsigned long *itlb_vector = __va(0xa00);
 
+		printk(KERN_INFO "itlb_miss_handler %p\n", &itlb_miss_handler);
+		*itlb_vector = ((unsigned long)&itlb_miss_handler -
+				(unsigned long)itlb_vector) >> 2;
+
+		/* Soft ordering constraint to ensure that dtlb_vector is
+		 * the last thing updated
+		 */
+		barrier();
+
 		printk(KERN_INFO "dtlb_miss_handler %p\n", &dtlb_miss_handler);
 		*dtlb_vector = ((unsigned long)&dtlb_miss_handler -
 				(unsigned long)dtlb_vector) >> 2;
 
-		printk(KERN_INFO "itlb_miss_handler %p\n", &itlb_miss_handler);
-		*itlb_vector = ((unsigned long)&itlb_miss_handler -
-				(unsigned long)itlb_vector) >> 2;
 	}
 
+	/* Soft ordering constraint to ensure that cache invalidation and
+	 * TLB flush really happen _after_ code has been modified.
+	 */
+	barrier();
+
 	/* Invalidate instruction caches after code modification */
 	mtspr(SPR_ICBIR, 0x900);
 	mtspr(SPR_ICBIR, 0xa00);