linux-kernel - [PATCH 1/6] n_tty: Start making use of -EAGAIN returned from process_output

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Date:   Tue, 21 Sep 2021 13:33:41 +0300
From:   Tony Lindgren <tony@...mide.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Johan Hovold <johan@...nel.org>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-serial@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/6] n_tty: Start making use of -EAGAIN returned from process_output_block()

We check for -EAGAIN in n_tty_write() but never currently get it from
process_output_block(). Let's add -EAGAIN handling and break out with 0
bytes processed. Note that if we return -EAGAIN from n_tty_write(), it
will be treated as error by the caller rather than a retry.

Looking at the patch description for commit 9ef8927f45f2 ("n_tty: check
for negative and zero space return from tty_write_room") it looks like we
have not made use of -EGAIN from process_output_block() so far, so this
does not seem like it's currently needed as a fix.

We can use -EAGAIN for serial layer power management changes as we now
will make use of write_room() returning 0 for an idled serial port.

Signed-off-by: Tony Lindgren <tony@...mide.com>
---
 drivers/tty/n_tty.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -549,6 +549,8 @@ static ssize_t process_output_block(struct tty_struct *tty,
 	space = tty_write_room(tty);
 	if (space <= 0) {
 		mutex_unlock(&ldata->output_lock);
+		if (!space)
+			space = -EAGAIN;
 		return space;
 	}
 	if (nr > space)
@@ -2287,8 +2289,10 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
 			while (nr > 0) {
 				ssize_t num = process_output_block(tty, b, nr);
 				if (num < 0) {
-					if (num == -EAGAIN)
-						break;
+					if (num == -EAGAIN) {
+						retval = 0;
+						goto break_out;
+					}
 					retval = num;
 					goto break_out;
 				}
-- 
2.33.0