linux-kernel - [034/107] hvc_console: Improve tty/console put

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Message-Id: <20111102221453.222863192@clark.kroah.org>
Date:	Wed, 02 Nov 2011 15:14:00 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>,
	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [034/107] hvc_console: Improve tty/console put_chars handling

2.6.32-longterm review patch.  If anyone has any objections, please let us know.

------------------

From: Hendrik Brueckner <brueckner@...ux.vnet.ibm.com>

commit 8c2381af0d3ef62a681dac5a141b6dabb27bf2e1 upstream.

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@...ux.vnet.ibm.com>
Acked-by: Anton Blanchard <anton@...ba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/char/hvc_console.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/drivers/char/hvc_console.c
+++ b/drivers/char/hvc_console.c
@@ -162,8 +162,10 @@ static void hvc_console_print(struct con
 		} 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)
@@ -447,7 +449,7 @@ static int hvc_push(struct hvc_struct *h
 
 	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;
 		}


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