Subject: Re: [PATCH] detour TTY driver - now ttyprintk
From: Samo Pogacnik <samo_pogacnik@t-2.net>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: linux-embedded <linux-embedded@vger.kernel.org>,
       linux kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <20100625120329.1303aa61@lxorguk.ukuu.org.uk>
References: <1273918658.2341.17.camel@itpsd6lap>
	 <1275171436.2122.29.camel@itpsd6lap>
	 <20100529235402.296406d9@lxorguk.ukuu.org.uk>
	 <1275175983.2122.42.camel@itpsd6lap> <1276123020.16010.97.camel@itpsd6lap>
	 <20100611134455.3fa7d563@lxorguk.ukuu.org.uk>
	 <1276291932.2154.45.camel@itpsd6lap>
	 <20100621153858.444695c0@lxorguk.ukuu.org.uk>
	 <1277244410.2195.47.camel@itpsd6lap>
	 <20100622232108.26752ff8@lxorguk.ukuu.org.uk>
	 <1277462596.2143.101.camel@itpsd6lap>
	 <20100625120329.1303aa61@lxorguk.ukuu.org.uk>
Content-Type: text/plain
Date: Sat, 26 Jun 2010 17:12:17 +0200
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> > +static int tpk_write_room(struct tty_struct *tty)
> > +{
> > +	int ret = tpk_space;
> > +
> > +	/* allow char by char under max pressure */
> > +	if (tpk_space == 0)
> > +		tpk_space = 1;
> 
> That won't do what you think, the ldisc will keep seeing progress and
> generate millions of 1 byte I/Os in a loop !
> 
Somewhat changed to slowly increase delay in write, if ratelimiting
doesn't end.

> Otherwise looks excellent.
> 
Thanks to you.

> 
> > +	switch (cmd) {
> > +	/* Stop TIOCCONS */
> > +	case TIOCCONS:
> > +		return -EOPNOTSUPP;
> 
> And I'll fix this bit up to work properly in the core code.
> 
> 
> With my devices.txt owner hat on I'll allocate the minor as you suggest
> (and double check this causes no problems), with my tty hat on can you
> send it to GregKH for merging into the tree.
I am not sure if i understand. Should i exclude devices.txt from patch
before sending it to GregKH?

Samo
---
Signed-off-by: Samo Pogacnik <samo_pogacnik@t-2.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..2fefa93
--- /dev/null
+++ b_linux/drivers/char/ttyprintk.c
@@ -0,0 +1,297 @@
+/*
+ *  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;
+
+#define TTY_PRINTK_STR_SIZE 508
+static int tpk_space = TTY_PRINTK_STR_SIZE;
+static const char *tpk_tag = "[U] "; /* U for User */
+
+/*
+ * 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).
+ */
+static struct ratelimit_state ttyprintk_rs = {
+	.interval = DEFAULT_RATELIMIT_INTERVAL,
+	.burst = DEFAULT_RATELIMIT_BURST,
+};
+
+static int tpk_ratelimiting;
+
+#define ttyprintk_printk(fmt, ...) \
+{ \
+	printk(KERN_INFO fmt, ##__VA_ARGS__); \
+}
+
+/*
+ * Our private ratelimit function, to suppress its printk warnings about
+ * missed callbacks, which are irrelevant in a flow control mechanism.
+ */
+static void ttyprintk_ratelimit(struct ratelimit_state *rs, int count)
+{
+	/* clear ratelimit missed callbacks counter */
+	rs->missed = 0;
+	if (__ratelimit(rs)) {
+		tpk_ratelimiting = 0;
+		tpk_space = TTY_PRINTK_STR_SIZE;
+		rs->burst = DEFAULT_RATELIMIT_BURST;
+	} else {
+		tpk_ratelimiting = 1;
+		if (TTY_PRINTK_STR_SIZE > count)
+			tpk_space = TTY_PRINTK_STR_SIZE - count;
+		else
+			tpk_space = 0;
+	}
+}
+
+/*
+ * 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
+ */
+static int tpk_printk(const unsigned char *buf, int count)
+{
+	static char tmp[TTY_PRINTK_STR_SIZE + 4];
+	static int curr;
+	int i = curr;
+
+	if (buf == NULL) {
+		/* flush tmp[] */
+		if (curr > 0) {
+			/* non nl or cr terminated message - add nl */
+			tmp[curr + 0] = '\n';
+			tmp[curr + 1] = '\0';
+			ttyprintk_printk("%s%s", tpk_tag, tmp);
+			curr = 0;
+		}
+		return i;
+	}
+
+	for (i = 0; i < count; i++) {
+		tmp[curr] = buf[i];
+		if (curr < TTY_PRINTK_STR_SIZE) {
+			switch (buf[i]) {
+			case '\r':
+				/* replace cr with nl */
+				tmp[curr + 0] = '\n';
+				tmp[curr + 1] = '\0';
+				ttyprintk_printk("%s%s", tpk_tag, tmp);
+				curr = 0;
+				if (buf[i + 1] == '\n')
+					i++;
+				break;
+			case '\n':
+				tmp[curr + 1] = '\0';
+				ttyprintk_printk("%s%s", tpk_tag, tmp);
+				curr = 0;
+				break;
+			default:
+				curr++;
+			}
+		} else {
+			/* end of tmp buffer reached: cut the message in two */
+			tmp[curr + 1] = '\\';
+			tmp[curr + 2] = '\n';
+			tmp[curr + 3] = '\0';
+			ttyprintk_printk("%s%s", tpk_tag, tmp);
+			curr = 0;
+		}
+	}
+	if ((tpk_ratelimiting == 0) && (curr > 0)) {
+		/* non nl or cr terminated message - add nl */
+		tmp[curr + 0] = '\n';
+		tmp[curr + 1] = '\0';
+		ttyprintk_printk("%s%s", tpk_tag, tmp);
+		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);
+	tpk_space = TTY_PRINTK_STR_SIZE;
+	tpk_ratelimiting = 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)
+{
+	static unsigned int tpk_write_delay;
+	struct ttyprintk_port *tpkp = tty->driver_data;
+	int ret;
+
+
+	/* exclusive use of tpk_printk within this tty */
+	mutex_lock(&tpkp->port_write_mutex);
+	ttyprintk_ratelimit(&ttyprintk_rs, count);
+	if (tpk_ratelimiting == 0) {
+		tpk_write_delay = 0;
+	} else {
+		/* increase delay under pressure upto 10 secs */
+		if (tpk_write_delay < 1000000)
+			tpk_write_delay++;
+		msleep_interruptible(tpk_write_delay / 100);
+
+		/* eliminate delay in ratelimiting */
+		ttyprintk_rs.burst = 1;
+		ttyprintk_rs.begin = 0;
+		__ratelimit(&ttyprintk_rs);
+	}
+
+	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)
+{
+	int ret = tpk_space;
+
+	/* just in case we reach zero space, let one char available */
+	if (tpk_space == 0)
+		tpk_space = 1;
+
+	return ret;
+}
+
+/*
+ * TTY operations ioctl function.
+ */
+static int tpk_ioctl(struct tty_struct *tty, struct file *file,
+			unsigned int cmd, unsigned long arg)
+{
+	struct ttyprintk_port *port;
+
+	port = tty->driver_data;
+
+	if (!port)
+		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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