[PATCH] POLL_OUT for tty drivers

[Sapan J . Bhatia]

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Hello,

While fixing a flow control bug that truncated output to a terminal in
User Mode Linux, I noticed that the tty drivers only send POLL_IN on new
data being available but not POLL_OUT when the device is ready for new
data. This is quite important for processes that use ASYNC IO, since if
they're using POLLIN already, it doesn't make sense to employ an alternative
method for POLLOUT

This patch fixes the bug in the line discipline and the pty driver.

Also, there's another minor bug in n_tty.c where write_chan returns
on a (retvalue < 0) unconditionally. This is a problem, since the type of
IO (BLOCKING / NON_BLOCKING) is stored in the tty, and if the console driver
returns a -EAGAIN (eg. in UML on getting an EAGAIN from the host kernel),
write_chan returns even in the case of a blocking write, which is wrong
since the process doesn't expect it.

Regards,
Sapan

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--- /tmp/work/linux/drivers/char/pty.c	Fri Dec 21 23:11:54 2001
+++ /usr/src/linux-2.4.18/drivers/char/pty.c	Thu Mar 21 20:26:01 2002
@@ -5,6 +5,10 @@
  *
  *  Added support for a Unix98-style ptmx device.
  *    -- C. Scott Ananian <[e-mail]>, 14-Jan-1998
+ *  Added TTY_DO_WRITE_WAKEUP to enable n_tty to send POLL_OUT to
+ *      waiting writers -- Sapan Bhatia <[e-mail]>
+ *
+ *
  */
 
 #include <linux/config.h>
@@ -331,6 +335,8 @@
 	clear_bit(TTY_OTHER_CLOSED, &tty->link->flags);
 	wake_up_interruptible(&pty->open_wait);
 	set_bit(TTY_THROTTLED, &tty->flags);
+	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+
 	/*  Register a slave for the master  */
 	if (tty->driver.major == PTY_MASTER_MAJOR)
 		tty_register_devfs(&tty->link->driver,
--- /tmp/work/linux/drivers/char/n_tty.c	Fri Apr  6 23:12:55 2001
+++ /usr/src/linux-2.4.18/drivers/char/n_tty.c	Thu Mar 21 07:22:44 2002
@@ -23,6 +23,13 @@
  * 2000/01/20   Fixed SMP locking on put_tty_queue using bits of 
  *		the patch by Andrew J. Kroll <[e-mail]>
  *		who actually finally proved there really was a race.
+ *
+ * 2002/03/18   Implemented n_tty_wakeup to send SIGIO POLL_OUTs to
+ *		waiting writing processes-Sapan Bhatia <[e-mail]>
+ *
+ * 2002/03/19   Fixed write_chan to stay put if console driver returns
+ *              EAGAIN and not return since it returns an EAGAIN in a 
+ *		non-blocking operation-Sapan Bhatia <[e-mail]>
  */
 
 #include <linux/types.h>
@@ -711,6 +718,22 @@
 	return 0;
 }
 
+/*
+ * Required for the ptys, serial driver etc. since processes
+ * that attach themselves to the master and rely on ASYNC
+ * IO must be woken up
+ */
+
+static void n_tty_write_wakeup(struct tty_struct *tty)
+{
+	if (tty->fasync)
+	{
+ 		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+		kill_fasync(&tty->fasync, SIGIO, POLL_OUT);
+	}
+	return;
+}
+
 static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 			      char *fp, int count)
 {
@@ -1156,7 +1179,7 @@
 		if (O_OPOST(tty) && !(test_bit(TTY_HW_COOK_OUT, &tty->flags))) {
 			while (nr > 0) {
 				ssize_t num = opost_block(tty, b, nr);
-				if (num < 0) {
+				if (num < 0 && num != -EAGAIN) {
 					retval = num;
 					goto break_out;
 				}
@@ -1236,6 +1259,6 @@
 	normal_poll,		/* poll */
 	n_tty_receive_buf,	/* receive_buf */
 	n_tty_receive_room,	/* receive_room */
-	0			/* write_wakeup */
+	n_tty_write_wakeup	/* write_wakeup */
 };
 

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