Message-Id: <1278184911.2187.25.camel@itpsd6lap>
Date:	Sat, 03 Jul 2010 21:21:51 +0200
From:	Samo Pogacnik <samo_pogacnik@....net>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-embedded <linux-embedded@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] detour TTY driver - now ttyprintk

Hi,

Please bear with me for a bit longer, because i got slightly modified
perspective of some things i did within the driver.
I.e. somewhere down the road i forgot that my pre-formatting function
does not really limit output buffer size with its "one-line" buffer
size. So now 4K output size is assumed and always provided by
write_room(). The other thing is that i tried to resolve two things
being in conflict. I tried to fit driver to existing distro
initialization, which required immediate output also for not line
terminated strings. And on the other hand not line terminated strings
could have been provided by line discipline all the time. So i finally
removed immediate output code and let user takes care by terminating
lines if immediate output is required.

There is additional possibility to fine tune ratelimiting and printk
delaying parameters via defines also with more comments added.

Alan, if you agree, i would send this one to GregKH?

regards, Samo
---
Signed-off-by: Samo Pogacnik <samo_pogacnik@....net>
diff --git a_linux/Documentation/devices.txt b_linux/Documentation/devices.txt
index 53d64d3..71aef33 100644
--- a_linux/Documentation/devices.txt
+++ b_linux/Documentation/devices.txt
@@ -239,6 +239,7 @@ Your cooperation is appreciated.
 		  0 = /dev/tty		Current TTY device
 		  1 = /dev/console	System console
 		  2 = /dev/ptmx		PTY master multiplex
+		  3 = /dev/ttyprintk	User messages via printk TTY device
 		 64 = /dev/cua0		Callout device for ttyS0
 		    ...
 		255 = /dev/cua191	Callout device for ttyS191
diff --git a_linux/drivers/char/Kconfig b_linux/drivers/char/Kconfig
index 3141dd3..5c38a06 100644
--- a_linux/drivers/char/Kconfig
+++ b_linux/drivers/char/Kconfig
@@ -485,6 +485,20 @@ config LEGACY_PTY_COUNT
 	  When not in use, each legacy PTY occupies 12 bytes on 32-bit
 	  architectures and 24 bytes on 64-bit architectures.
 
+config TTY_PRINTK
+	bool "TTY driver to output user messages via printk"
+	default n
+	---help---
+	  If you say Y here, the support for writing user messages (i.e.
+	  console messages) via printk is available.
+
+	  The feature is useful to inline user messages with kernel
+	  messages.
+	  In order to use this feature, you should output user messages
+	  to /dev/ttyprintk or redirect console to this TTY.
+
+	  If unsure, say N.
+
 config BRIQ_PANEL
 	tristate 'Total Impact briQ front panel driver'
 	depends on PPC_CHRP
diff --git a_linux/drivers/char/Makefile b_linux/drivers/char/Makefile
index f957edf..ed60f45 100644
--- a_linux/drivers/char/Makefile
+++ b_linux/drivers/char/Makefile
@@ -11,6 +11,7 @@ obj-y	 += mem.o random.o tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o tty_buffer.o t
 
 obj-$(CONFIG_LEGACY_PTYS)	+= pty.o
 obj-$(CONFIG_UNIX98_PTYS)	+= pty.o
+obj-$(CONFIG_TTY_PRINTK)	+= ttyprintk.o
 obj-y				+= misc.o
 obj-$(CONFIG_VT)		+= vt_ioctl.o vc_screen.o selection.o keyboard.o
 obj-$(CONFIG_BFIN_JTAG_COMM)	+= bfin_jtag_comm.o
diff --git a_linux/drivers/char/ttyprintk.c b_linux/drivers/char/ttyprintk.c
new file mode 100644
index 0000000..22da614
--- /dev/null
+++ b_linux/drivers/char/ttyprintk.c
@@ -0,0 +1,283 @@
+/*
+ *  linux/drivers/char/ttyprintk.c
+ *
+ *  Copyright (C) 2010  Samo Pogacnik
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the smems of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ */
+
+/*
+ * This pseudo device allows user to make printk messages. It is possible
+ * to store "console" messages inline with kernel messages for better analyses
+ * of the boot process, for example.
+ */
+
+#include <linux/sched.h>
+#include <linux/delay.h>
+#include <linux/ratelimit.h>
+#include <linux/device.h>
+#include <linux/serial.h>
+#include <linux/tty.h>
+
+struct ttyprintk_port {
+	struct tty_port port;
+	struct mutex port_write_mutex;
+};
+
+static struct ttyprintk_port tpk_port;
+
+/*
+ * Ratelimiting support to handle to much output to this device,
+ * because of explicit writes or because of unintentional loop
+ * setup (caught printks again sent to this device).
+ * Max TPK...BURST printks per TPK...INTERVAL
+ */
+#define TPK_RATELIMIT_INTERVAL (1 * HZ)
+#define TPK_RATELIMIT_BURST 10
+static struct ratelimit_state ttyprintk_rs = {
+	.interval = TPK_RATELIMIT_INTERVAL,
+	.burst = TPK_RATELIMIT_BURST,
+};
+
+static int tpk_ratelimiting;
+
+/*
+ * Our private ratelimit function, to suppress its printk warnings about
+ * missed callbacks, which doesn't really happen (not discarding messages).
+ * Calls final printk at the end.
+ * Raise delay on every TPK_SCALE_DELAY ratelimited printk for another
+ * milisecond and can not exceed TPK_MAX_DELAY.
+ * Delay is reset allways, when ratelimiting ends.
+ */
+#define TPK_SCALE_DELAY 10
+#define TPK_MAX_DELAY 2000 /* 1 secs */
+static void ttyprintk_ratelimit(const char *tag, const char *buf)
+{
+	static unsigned int tpk_write_delay;
+
+	/* clear ratelimit missed callbacks counter */
+	ttyprintk_rs.missed = 0;
+	if (__ratelimit(&ttyprintk_rs)) {
+		tpk_ratelimiting = 0;
+		ttyprintk_rs.burst = TPK_RATELIMIT_BURST;
+	} else
+		tpk_ratelimiting = 1;
+
+	if (tpk_ratelimiting == 0) {
+		tpk_write_delay = 0;
+	} else {
+		/* increase delay under pressure */
+		if (tpk_write_delay < (TPK_MAX_DELAY * TPK_SCALE_DELAY))
+			tpk_write_delay++;
+		msleep_interruptible(tpk_write_delay / TPK_SCALE_DELAY);
+
+		/* eliminate delay in ratelimiting */
+		ttyprintk_rs.burst = 1;
+		ttyprintk_rs.begin = 0;
+		__ratelimit(&ttyprintk_rs);
+	}
+
+	/* send the string down the drain */
+	printk(KERN_INFO "%s%s", tag, buf);
+}
+
+/*
+ * Our simple preformatting supports transparent output of (time-stamped)
+ * printk messages (also suitable for logging service):
+ * - any cr is replaced by nl
+ * - adds a ttyprintk source tag in front of each line
+ * - too long message is fragmeted, with '\'nl between fragments
+ * - TPK_STR_SIZE isn't really the write_room limiting factor, bcause
+ *   it is emptied on the fly during preformatting.
+ */
+#define TPK_STR_SIZE 508 /* should be bigger then max expected line length */
+#define TPK_MAX_ROOM 4096 /* we could assume 4K for instance */
+static const char *tpk_tag = "[U] "; /* U for User */
+static int tpk_curr;
+
+static int tpk_printk(const unsigned char *buf, int count)
+{
+	static char tmp[TPK_STR_SIZE + 4];
+	int i = tpk_curr;
+
+	if (buf == NULL) {
+		/* flush tmp[] */
+		if (tpk_curr > 0) {
+			/* non nl or cr terminated message - add nl */
+			tmp[tpk_curr + 0] = '\n';
+			tmp[tpk_curr + 1] = '\0';
+			ttyprintk_ratelimit(tpk_tag, tmp);
+			tpk_curr = 0;
+		}
+		return i;
+	}
+
+	for (i = 0; i < count; i++) {
+		tmp[tpk_curr] = buf[i];
+		if (tpk_curr < TPK_STR_SIZE) {
+			switch (buf[i]) {
+			case '\r':
+				/* replace cr with nl */
+				tmp[tpk_curr + 0] = '\n';
+				tmp[tpk_curr + 1] = '\0';
+				ttyprintk_ratelimit(tpk_tag, tmp);
+				tpk_curr = 0;
+				if (buf[i + 1] == '\n')
+					i++;
+				break;
+			case '\n':
+				tmp[tpk_curr + 1] = '\0';
+				ttyprintk_ratelimit(tpk_tag, tmp);
+				tpk_curr = 0;
+				break;
+			default:
+				tpk_curr++;
+			}
+		} else {
+			/* end of tmp buffer reached: cut the message in two */
+			tmp[tpk_curr + 1] = '\\';
+			tmp[tpk_curr + 2] = '\n';
+			tmp[tpk_curr + 3] = '\0';
+			ttyprintk_ratelimit(tpk_tag, tmp);
+			tpk_curr = 0;
+		}
+	}
+
+	return count;
+}
+
+/*
+ * TTY operations open function.
+ */
+static int tpk_open(struct tty_struct *tty, struct file *filp)
+{
+	tty->driver_data = &tpk_port;
+
+	return tty_port_open(&tpk_port.port, tty, filp);
+}
+
+/*
+ * TTY operations close function.
+ */
+static void tpk_close(struct tty_struct *tty, struct file *filp)
+{
+	struct ttyprintk_port *tpkp = tty->driver_data;
+
+	mutex_lock(&tpkp->port_write_mutex);
+	/* flush tpk_printk buffer */
+	tpk_printk(NULL, 0);
+	mutex_unlock(&tpkp->port_write_mutex);
+
+	tty_port_close(&tpkp->port, tty, filp);
+}
+
+/*
+ * TTY operations write function.
+ */
+static int tpk_write(struct tty_struct *tty,
+		const unsigned char *buf, int count)
+{
+	struct ttyprintk_port *tpkp = tty->driver_data;
+	int ret;
+
+
+	/* exclusive use of tpk_printk within this tty */
+	mutex_lock(&tpkp->port_write_mutex);
+	ret = tpk_printk(buf, count);
+	mutex_unlock(&tpkp->port_write_mutex);
+
+	return ret;
+}
+
+/*
+ * TTY operations write_room function.
+ */
+static int tpk_write_room(struct tty_struct *tty)
+{
+	return TPK_MAX_ROOM;
+}
+
+/*
+ * TTY operations ioctl function.
+ */
+static int tpk_ioctl(struct tty_struct *tty, struct file *file,
+			unsigned int cmd, unsigned long arg)
+{
+	struct ttyprintk_port *tpkp = tty->driver_data;
+
+	if (!tpkp)
+		return -EINVAL;
+
+	switch (cmd) {
+	/* Stop TIOCCONS */
+	case TIOCCONS:
+		return -EOPNOTSUPP;
+	default:
+		return -ENOIOCTLCMD;
+	}
+	return 0;
+}
+
+static const struct tty_operations ttyprintk_ops = {
+	.open = tpk_open,
+	.close = tpk_close,
+	.write = tpk_write,
+	.write_room = tpk_write_room,
+	.ioctl = tpk_ioctl,
+};
+
+struct tty_port_operations null_ops = { };
+
+static struct tty_driver *ttyprintk_driver;
+
+static int __init ttyprintk_init(void)
+{
+	int ret = -ENOMEM;
+	void *rp;
+
+	ttyprintk_driver = alloc_tty_driver(1);
+	if (!ttyprintk_driver)
+		return ret;
+
+	ttyprintk_driver->owner = THIS_MODULE;
+	ttyprintk_driver->driver_name = "ttyprintk";
+	ttyprintk_driver->name = "ttyprintk";
+	ttyprintk_driver->major = TTYAUX_MAJOR;
+	ttyprintk_driver->minor_start = 3;
+	ttyprintk_driver->num = 1;
+	ttyprintk_driver->type = TTY_DRIVER_TYPE_CONSOLE;
+	ttyprintk_driver->init_termios = tty_std_termios;
+	ttyprintk_driver->init_termios.c_oflag = OPOST | OCRNL | ONOCR | ONLRET;
+	ttyprintk_driver->flags = TTY_DRIVER_RESET_TERMIOS |
+		TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;
+	tty_set_operations(ttyprintk_driver, &ttyprintk_ops);
+
+	ret = tty_register_driver(ttyprintk_driver);
+	if (ret < 0) {
+		printk(KERN_ERR "Couldn't register ttyprintk driver\n");
+		goto error;
+	}
+
+	/* create our unnumbered device */
+	rp = device_create(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 3), NULL,
+				ttyprintk_driver->name);
+	if (IS_ERR(rp)) {
+		printk(KERN_ERR "Couldn't create ttyprintk device\n");
+		ret = PTR_ERR(rp);
+		goto error;
+	}
+
+	tty_port_init(&tpk_port.port);
+	tpk_port.port.ops = &null_ops;
+	mutex_init(&tpk_port.port_write_mutex);
+
+	return 0;
+
+error:
+	put_tty_driver(ttyprintk_driver);
+	ttyprintk_driver = NULL;
+	return ret;
+}
+module_init(ttyprintk_init);


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