hwrng: use rng source with best quality

This patch rewoks the hwrng to always use the
rng source with best entropy quality.

On registation and unregistration the hwrng now
tries to choose the best (= highest quality value)
rng source. The handling of the internal list
of registered rng sources is now always sorted
by quality and the top most rng chosen.
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/core.c

@@ -29,6 +29,7 @@
 
 static struct hwrng *current_rng;
 static struct task_struct *hwrng_fill;
+/* list of registered rngs, sorted decending by quality */
 static LIST_HEAD(rng_list);
 /* Protects rng_list and current_rng */
 static DEFINE_MUTEX(rng_mutex);
@@ -417,6 +418,7 @@ int hwrng_register(struct hwrng *rng)
 {
 	int err = -EINVAL;
 	struct hwrng *old_rng, *tmp;
+	struct list_head *rng_list_ptr;
 
 	if (!rng->name || (!rng->data_read && !rng->read))
 		goto out;
@@ -432,14 +434,25 @@ int hwrng_register(struct hwrng *rng)
 	init_completion(&rng->cleanup_done);
 	complete(&rng->cleanup_done);
 
+	/* rng_list is sorted by decreasing quality */
+	list_for_each(rng_list_ptr, &rng_list) {
+		tmp = list_entry(rng_list_ptr, struct hwrng, list);
+		if (tmp->quality < rng->quality)
+			break;
+	}
+	list_add_tail(&rng->list, rng_list_ptr);
+
 	old_rng = current_rng;
 	err = 0;
-	if (!old_rng) {
+	if (!old_rng || (rng->quality > old_rng->quality)) {
+		/*
+		 * Set new rng as current as the new rng source
+		 * provides better entropy quality.
+		 */
 		err = set_current_rng(rng);
 		if (err)
 			goto out_unlock;
 	}
-	list_add_tail(&rng->list, &rng_list);
 
 	if (old_rng && !rng->init) {
 		/*
@@ -466,12 +479,12 @@ void hwrng_unregister(struct hwrng *rng)
 	list_del(&rng->list);
 	if (current_rng == rng) {
 		drop_current_rng();
+		/* rng_list is sorted by quality, use the best (=first) one */
 		if (!list_empty(&rng_list)) {
-			struct hwrng *tail;
-
-			tail = list_entry(rng_list.prev, struct hwrng, list);
+			struct hwrng *new_rng;
 
-			set_current_rng(tail);
+			new_rng = list_entry(rng_list.next, struct hwrng, list);
+			set_current_rng(new_rng);
 		}
 	}