[PATCH] powerpc: Make rtas console _much_ faster

Currently the hvc_rtas driver is painfully slow to use. Our "benchmark" is
ls -R /etc, which spits out about 27866 characters. The theoretical maximum
speed would be about 2.2 seconds, the current code takes ~50 seconds.

The core of the problem is that sometimes when the tty layer asks us to push
characters the firmware isn't able to handle some or all of them, and so
returns an error. The current code sees this and just returns to the tty code
with the buffer half sent.

The khvcd thread will eventually wake up and try to push more characters, which
will usually work because by then the firmware's had time to make room. But
the khvcd thread only wakes up every 10 milliseconds, which isn't fast enough.

So change the khvcd thread logic so that if there's an incomplete write we
yield() and then immediately try writing again. Doing so makes POLL_QUICK and
POLL_WRITE synonymous, so remove POLL_QUICK.

With this patch our "benchmark" takes ~2.8 seconds.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

drivers/char/hvc_console.c

@@ -553,7 +553,6 @@ static int hvc_chars_in_buffer(struct tty_struct *tty)
 
 #define HVC_POLL_READ	0x00000001
 #define HVC_POLL_WRITE	0x00000002
-#define HVC_POLL_QUICK	0x00000004
 
 static int hvc_poll(struct hvc_struct *hp)
 {
@@ -568,6 +567,7 @@ static int hvc_poll(struct hvc_struct *hp)
 	/* Push pending writes */
 	if (hp->n_outbuf > 0)
 		hvc_push(hp);
+
 	/* Reschedule us if still some write pending */
 	if (hp->n_outbuf > 0)
 		poll_mask |= HVC_POLL_WRITE;
@@ -680,7 +680,7 @@ int khvcd(void *unused)
 			poll_mask |= HVC_POLL_READ;
 		if (hvc_kicked)
 			continue;
-		if (poll_mask & HVC_POLL_QUICK) {
+		if (poll_mask & HVC_POLL_WRITE) {
 			yield();
 			continue;
 		}