hvc_console: Improve tty/console put_chars handling

Currently, the hvc_console_print() function drops console output if the
hvc backend's put_chars() returns 0.  This patch changes this behavior
to allow a retry through returning -EAGAIN.

This change also affects the hvc_push() function.  Both functions are
changed to handle -EAGAIN and to retry the put_chars() operation.

If a hvc backend returns -EAGAIN, the retry handling differs:

  - hvc_console_print() spins to write the complete console output.
  - hvc_push() behaves the same way as for returning 0.

Now hvc backends can indirectly control the way how console output is
handled through the hvc console layer.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Acked-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/tty/hvc/hvc_console.c

@@ -163,8 +163,10 @@ static void hvc_console_print(struct console *co, const char *b,
 		} else {
 			r = cons_ops[index]->put_chars(vtermnos[index], c, i);
 			if (r <= 0) {
-				/* throw away chars on error */
-				i = 0;
+				/* throw away characters on error
+				 * but spin in case of -EAGAIN */
+				if (r != -EAGAIN)
+					i = 0;
 			} else if (r > 0) {
 				i -= r;
 				if (i > 0)
@@ -448,7 +450,7 @@ static int hvc_push(struct hvc_struct *hp)
 
 	n = hp->ops->put_chars(hp->vtermno, hp->outbuf, hp->n_outbuf);
 	if (n <= 0) {
-		if (n == 0) {
+		if (n == 0 || n == -EAGAIN) {
 			hp->do_wakeup = 1;
 			return 0;
 		}