Patchwork [net-next-2.6] can: add slcan driver for serial/USB-serial CAN adapters

login
register
mail settings
Submitter Oliver Hartkopp
Date Nov. 29, 2010, 8:30 p.m.
Message ID <4CF40D75.8030203@hartkopp.net>
Download mbox | patch
Permalink /patch/73490/
State Superseded
Delegated to: David Miller
Headers show

Comments

Oliver Hartkopp - Nov. 29, 2010, 8:30 p.m.
This patch adds support for serial/USB-serial CAN adapters implementing the
LAWICEL ASCII protocol for CAN frame transport over serial lines.

The driver implements the SLCAN line discipline and is heavily based on the
slip.c driver. Therefore the code style remains similar to slip.c to be able
to apply changes of the SLIP driver to the SLCAN driver easily.

For more details see the slcan Kconfig entry.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>

---


--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Alan Cox - Nov. 29, 2010, 11:37 p.m.
On Mon, 29 Nov 2010 21:30:45 +0100
Oliver Hartkopp <socketcan@hartkopp.net> wrote:

> This patch adds support for serial/USB-serial CAN adapters implementing the
> LAWICEL ASCII protocol for CAN frame transport over serial lines.
> 
> The driver implements the SLCAN line discipline and is heavily based on the
> slip.c driver. Therefore the code style remains similar to slip.c to be able
> to apply changes of the SLIP driver to the SLCAN driver easily.

It looks almost identical. Would it not be better to either extract the
common code or put slip and slcann together as one, otherwise changes
will always get missed as they have to be done twice ?

How hard would it be to make the encap/decap pointers that can be
set to cleanly abstract both ?

Alan
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Oliver Hartkopp - Nov. 30, 2010, 4:40 p.m.
On 30.11.2010 00:37, Alan Cox wrote:
> On Mon, 29 Nov 2010 21:30:45 +0100
> Oliver Hartkopp <socketcan@hartkopp.net> wrote:
> 
>> This patch adds support for serial/USB-serial CAN adapters implementing the
>> LAWICEL ASCII protocol for CAN frame transport over serial lines.
>>
>> The driver implements the SLCAN line discipline and is heavily based on the
>> slip.c driver. Therefore the code style remains similar to slip.c to be able
>> to apply changes of the SLIP driver to the SLCAN driver easily.
> 
> It looks almost identical. Would it not be better to either extract the
> common code or put slip and slcann together as one, otherwise changes
> will always get missed as they have to be done twice ?
> 
> How hard would it be to make the encap/decap pointers that can be
> set to cleanly abstract both ?

My first approach was indeed to try to integrate the slcan ldisc into slip.
But due to the higher complexity with dynamic memory for the buffers, the
multiple #ifdef CONFIG_* blocks, the private statistics, the legacy ioctl()
and compat stuff, MTU change, i decided to cut all the unneeded code and make
an easy short driver from that ...

The things i did not really understand i preserved as-is 8-)
Therefore the code looks that similar in some cases.

I wonder whether the code in

- sl_open()
- sl_close()
- slip_write_wakeup()
- sl_xmit()
- sl_free_netdev()
- sl_sync()
- sl_alloc()
- slip_hangup()
- slip_exit()

could be shared e.g. in some separate module where also
drivers/net/hamradio/mkiss.c could participate?!?

This would need to unify the slip/slcan/mkiss structs.

Additionally mkiss.c does not have something like

    static struct net_device **slip_devs;

and creates the netdevices without parsing a fixed number of devices in
mkiss_open(). But as i'm not a tty specialist i better kept my hands off these
internals ...

Regards,
Oliver
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Oliver Hartkopp - Dec. 2, 2010, 8:57 p.m.
As there's obviously no better idea, i would suggest to commit the slcan
driver in it's current shape as it has been used successfully (out of tree)
for more than two years now.

If there are new ideas to clean up the code of slip.c the slcan code can
follow these easily.

I'll send a v2 patch, as i accidentally missed the Makefile modification in
the first post.

Best regards,
Oliver


On 30.11.2010 17:40, Oliver Hartkopp wrote:
> On 30.11.2010 00:37, Alan Cox wrote:
>> On Mon, 29 Nov 2010 21:30:45 +0100
>> Oliver Hartkopp <socketcan@hartkopp.net> wrote:
>>
>>> This patch adds support for serial/USB-serial CAN adapters implementing the
>>> LAWICEL ASCII protocol for CAN frame transport over serial lines.
>>>
>>> The driver implements the SLCAN line discipline and is heavily based on the
>>> slip.c driver. Therefore the code style remains similar to slip.c to be able
>>> to apply changes of the SLIP driver to the SLCAN driver easily.
>>
>> It looks almost identical. Would it not be better to either extract the
>> common code or put slip and slcann together as one, otherwise changes
>> will always get missed as they have to be done twice ?
>>
>> How hard would it be to make the encap/decap pointers that can be
>> set to cleanly abstract both ?
> 
> My first approach was indeed to try to integrate the slcan ldisc into slip.
> But due to the higher complexity with dynamic memory for the buffers, the
> multiple #ifdef CONFIG_* blocks, the private statistics, the legacy ioctl()
> and compat stuff, MTU change, i decided to cut all the unneeded code and make
> an easy short driver from that ...
> 
> The things i did not really understand i preserved as-is 8-)
> Therefore the code looks that similar in some cases.
> 
> I wonder whether the code in
> 
> - sl_open()
> - sl_close()
> - slip_write_wakeup()
> - sl_xmit()
> - sl_free_netdev()
> - sl_sync()
> - sl_alloc()
> - slip_hangup()
> - slip_exit()
> 
> could be shared e.g. in some separate module where also
> drivers/net/hamradio/mkiss.c could participate?!?
> 
> This would need to unify the slip/slcan/mkiss structs.
> 
> Additionally mkiss.c does not have something like
> 
>     static struct net_device **slip_devs;
> 
> and creates the netdevices without parsing a fixed number of devices in
> mkiss_open(). But as i'm not a tty specialist i better kept my hands off these
> internals ...
> 
> Regards,
> Oliver

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Patch

diff -u -r -N a/drivers/net/can/Kconfig b/drivers/net/can/Kconfig
--- a/drivers/net/can/Kconfig	2010-11-29 20:36:31.000000000 +0100
+++ b/drivers/net/can/Kconfig	2010-11-29 20:48:47.000000000 +0100
@@ -12,6 +12,27 @@ 
 	  This driver can also be built as a module.  If so, the module
 	  will be called vcan.
 
+config CAN_SLCAN
+	tristate "Serial / USB serial CAN Adaptors (slcan)"
+	depends on CAN
+	default N
+	---help---
+	  CAN driver for several 'low cost' CAN interfaces that are attached
+	  via serial lines or via USB-to-serial adapters using the LAWICEL
+	  ASCII protocol. The driver implements the tty linediscipline N_SLCAN.
+
+	  As only the sending and receiving of CAN frames is implemented, this
+	  driver should work with the (serial/USB) CAN hardware from:
+	  www.canusb.com / www.can232.com / www.mictronic.com / www.canhack.de
+
+	  Userspace tools to attach the SLCAN line discipline (slcan_attach,
+	  slcand) can be found in the can-utils at the SocketCAN SVN, see
+	  http://developer.berlios.de/projects/socketcan for details.
+
+	  The slcan driver supports up to 10 CAN netdevices by default which
+	  can be changed by the 'maxdev=xx' module option. This driver can
+	  also be built as a module. If so, the module will be called slcan.
+
 config CAN_DEV
 	tristate "Platform CAN drivers with Netlink support"
 	depends on CAN
diff -u -r -N a/drivers/net/can/slcan.c b/drivers/net/can/slcan.c
--- a/drivers/net/can/slcan.c	1970-01-01 01:00:00.000000000 +0100
+++ b/drivers/net/can/slcan.c	2010-11-29 20:59:59.000000000 +0100
@@ -0,0 +1,755 @@ 
+/*
+ * slcan.c - serial line CAN interface driver (using tty line discipline)
+ *
+ * This file is derived from linux/drivers/net/slip.c
+ *
+ * slip.c Authors  : Laurence Culhane <loz@holmes.demon.co.uk>
+ *                   Fred N. van Kempen <waltje@uwalt.nl.mugnet.org>
+ * slcan.c Author  : Oliver Hartkopp <socketcan@hartkopp.net>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307. You can also get it
+ * at http://www.gnu.org/licenses/gpl.html
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+ * DAMAGE.
+ *
+ * Send feedback to <socketcan-users@lists.berlios.de>
+ *
+ */
+
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+
+#include <asm/system.h>
+#include <linux/uaccess.h>
+#include <linux/bitops.h>
+#include <linux/string.h>
+#include <linux/tty.h>
+#include <linux/errno.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <linux/rtnetlink.h>
+#include <linux/if_arp.h>
+#include <linux/if_ether.h>
+#include <linux/delay.h>
+#include <linux/init.h>
+#include <linux/can.h>
+
+static __initdata const char banner[] =
+	KERN_INFO "slcan: serial line CAN interface driver\n";
+
+MODULE_ALIAS_LDISC(N_SLCAN);
+MODULE_DESCRIPTION("serial line CAN interface");
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Oliver Hartkopp <socketcan@hartkopp.net>");
+
+#define SLCAN_MAGIC 0x53CA
+
+static int maxdev = 10;		/* MAX number of SLCAN channels;
+				   This can be overridden with
+				   insmod slcan.ko maxdev=nnn	*/
+module_param(maxdev, int, 0);
+MODULE_PARM_DESC(maxdev, "Maximum number of slcan interfaces");
+
+/* maximum rx buffer len: extended CAN frame with timestamp */
+#define SLC_MTU (sizeof("T1111222281122334455667788EA5F\r")+1)
+
+struct slcan {
+	int			magic;
+
+	/* Various fields. */
+	struct tty_struct	*tty;		/* ptr to TTY structure	     */
+	struct net_device	*dev;		/* easy for intr handling    */
+	spinlock_t		lock;
+
+	/* These are pointers to the malloc()ed frame buffers. */
+	unsigned char		rbuff[SLC_MTU];	/* receiver buffer	     */
+	int			rcount;         /* received chars counter    */
+	unsigned char		xbuff[SLC_MTU];	/* transmitter buffer	     */
+	unsigned char		*xhead;         /* pointer to next XMIT byte */
+	int			xleft;          /* bytes left in XMIT queue  */
+
+	unsigned long		flags;		/* Flag values/ mode etc     */
+#define SLF_INUSE		0		/* Channel in use            */
+#define SLF_ERROR		1               /* Parity, etc. error        */
+
+	unsigned char		leased;
+	dev_t			line;
+	pid_t			pid;
+};
+
+static struct net_device **slcan_devs;
+
+ /************************************************************************
+  *			SLCAN ENCAPSULATION FORMAT			 *
+  ************************************************************************/
+
+/*
+ * A CAN frame has a can_id (11 bit standard frame format OR 29 bit extended
+ * frame format) a data length code (can_dlc) which can be from 0 to 8
+ * and up to <can_dlc> data bytes as payload.
+ * Additionally a CAN frame may become a remote transmission frame if the
+ * RTR-bit is set. This causes another ECU to send a CAN frame with the
+ * given can_id.
+ *
+ * The SLCAN ASCII representation of these different frame types is:
+ * <type> <id> <dlc> <data>*
+ *
+ * Extended frames (29 bit) are defined by capital characters in the type.
+ * RTR frames are defined as 'r' types - normal frames have 't' type:
+ * t => 11 bit data frame
+ * r => 11 bit RTR frame
+ * T => 29 bit data frame
+ * R => 29 bit RTR frame
+ *
+ * The <id> is 3 (standard) or 8 (extended) bytes in ASCII Hex (base64).
+ * The <dlc> is a one byte ASCII number ('0' - '8')
+ * The <data> section has at much ASCII Hex bytes as defined by the <dlc>
+ *
+ * Examples:
+ *
+ * t1230 : can_id 0x123, can_dlc 0, no data
+ * t4563112233 : can_id 0x456, can_dlc 3, data 0x11 0x22 0x33
+ * T12ABCDEF2AA55 : extended can_id 0x12ABCDEF, can_dlc 2, data 0xAA 0x55
+ * r1230 : can_id 0x123, can_dlc 0, no data, remote transmission request
+ *
+ */
+
+ /************************************************************************
+  *			STANDARD SLCAN DECAPSULATION			 *
+  ************************************************************************/
+
+static int asc2nibble(char c)
+{
+
+	if ((c >= '0') && (c <= '9'))
+		return c - '0';
+
+	if ((c >= 'A') && (c <= 'F'))
+		return c - 'A' + 10;
+
+	if ((c >= 'a') && (c <= 'f'))
+		return c - 'a' + 10;
+
+	return 16; /* error */
+}
+
+/* Send one completely decapsulated can_frame to the network layer */
+static void slc_bump(struct slcan *sl)
+{
+	struct sk_buff *skb;
+	struct can_frame cf;
+	int i, dlc_pos, tmp;
+	unsigned long ultmp;
+	char cmd = sl->rbuff[0];
+
+	if ((cmd != 't') && (cmd != 'T') && (cmd != 'r') && (cmd != 'R'))
+		return;
+
+	if (cmd & 0x20) /* tiny chars 'r' 't' => standard frame format */
+		dlc_pos = 4; /* dlc position tiiid */
+	else
+		dlc_pos = 9; /* dlc position Tiiiiiiiid */
+
+	if (!((sl->rbuff[dlc_pos] >= '0') && (sl->rbuff[dlc_pos] < '9')))
+		return;
+
+	cf.can_dlc = sl->rbuff[dlc_pos] - '0'; /* get can_dlc from ASCII val */
+
+	sl->rbuff[dlc_pos] = 0; /* terminate can_id string */
+
+	if (strict_strtoul(sl->rbuff+1, 16, &ultmp))
+		return;
+
+	cf.can_id = ultmp;
+
+	if (!(cmd & 0x20)) /* NO tiny chars => extended frame format */
+		cf.can_id |= CAN_EFF_FLAG;
+
+	if ((cmd | 0x20) == 'r') /* RTR frame */
+		cf.can_id |= CAN_RTR_FLAG;
+
+	*(u64 *) (&cf.data) = 0; /* clear payload */
+
+	for (i = 0, dlc_pos++; i < cf.can_dlc; i++) {
+
+		tmp = asc2nibble(sl->rbuff[dlc_pos++]);
+		if (tmp > 0x0F)
+			return;
+		cf.data[i] = (tmp << 4);
+		tmp = asc2nibble(sl->rbuff[dlc_pos++]);
+		if (tmp > 0x0F)
+			return;
+		cf.data[i] |= tmp;
+	}
+
+
+	skb = dev_alloc_skb(sizeof(struct can_frame));
+	if (!skb)
+		return;
+
+	skb->dev = sl->dev;
+	skb->protocol = htons(ETH_P_CAN);
+	skb->pkt_type = PACKET_BROADCAST;
+	skb->ip_summed = CHECKSUM_UNNECESSARY;
+	memcpy(skb_put(skb, sizeof(struct can_frame)),
+	       &cf, sizeof(struct can_frame));
+	netif_rx(skb);
+
+	sl->dev->stats.rx_packets++;
+	sl->dev->stats.rx_bytes += cf.can_dlc;
+}
+
+/* parse tty input stream */
+static void slcan_unesc(struct slcan *sl, unsigned char s)
+{
+
+	if ((s == '\r') || (s == '\a')) { /* CR or BEL ends the pdu */
+		if (!test_and_clear_bit(SLF_ERROR, &sl->flags) &&
+		    (sl->rcount > 4))  {
+			slc_bump(sl);
+		}
+		sl->rcount = 0;
+	} else {
+		if (!test_bit(SLF_ERROR, &sl->flags))  {
+			if (sl->rcount < SLC_MTU)  {
+				sl->rbuff[sl->rcount++] = s;
+				return;
+			} else {
+				sl->dev->stats.rx_over_errors++;
+				set_bit(SLF_ERROR, &sl->flags);
+			}
+		}
+	}
+}
+
+ /************************************************************************
+  *			STANDARD SLCAN ENCAPSULATION			 *
+  ************************************************************************/
+
+/* Encapsulate one can_frame and stuff into a TTY queue. */
+static void slc_encaps(struct slcan *sl, struct can_frame *cf)
+{
+	int actual, idx, i;
+	char cmd;
+
+	if (cf->can_id & CAN_RTR_FLAG)
+		cmd = 'R'; /* becomes 'r' in standard frame format */
+	else
+		cmd = 'T'; /* becomes 't' in standard frame format */
+
+	if (cf->can_id & CAN_EFF_FLAG)
+		sprintf(sl->xbuff, "%c%08X%d", cmd,
+			cf->can_id & CAN_EFF_MASK, cf->can_dlc);
+	else
+		sprintf(sl->xbuff, "%c%03X%d", cmd | 0x20,
+			cf->can_id & CAN_SFF_MASK, cf->can_dlc);
+
+	idx = strlen(sl->xbuff);
+
+	for (i = 0; i < cf->can_dlc; i++)
+		sprintf(&sl->xbuff[idx + 2*i], "%02X", cf->data[i]);
+
+	strcat(sl->xbuff, "\r"); /* add terminating character */
+
+	/* Order of next two lines is *very* important.
+	 * When we are sending a little amount of data,
+	 * the transfer may be completed inside the ops->write()
+	 * routine, because it's running with interrupts enabled.
+	 * In this case we *never* got WRITE_WAKEUP event,
+	 * if we did not request it before write operation.
+	 *       14 Oct 1994  Dmitry Gorodchanin.
+	 */
+	set_bit(TTY_DO_WRITE_WAKEUP, &sl->tty->flags);
+	actual = sl->tty->ops->write(sl->tty, sl->xbuff, strlen(sl->xbuff));
+	sl->xleft = strlen(sl->xbuff) - actual;
+	sl->xhead = sl->xbuff + actual;
+	sl->dev->stats.tx_bytes += cf->can_dlc;
+}
+
+/*
+ * Called by the driver when there's room for more data.  If we have
+ * more packets to send, we send them here.
+ */
+static void slcan_write_wakeup(struct tty_struct *tty)
+{
+	int actual;
+	struct slcan *sl = (struct slcan *) tty->disc_data;
+
+	/* First make sure we're connected. */
+	if (!sl || sl->magic != SLCAN_MAGIC || !netif_running(sl->dev))
+		return;
+
+	if (sl->xleft <= 0)  {
+		/* Now serial buffer is almost free & we can start
+		 * transmission of another packet */
+		sl->dev->stats.tx_packets++;
+		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+		netif_wake_queue(sl->dev);
+		return;
+	}
+
+	actual = tty->ops->write(tty, sl->xhead, sl->xleft);
+	sl->xleft -= actual;
+	sl->xhead += actual;
+}
+
+/* Send a can_frame to a TTY queue. */
+static netdev_tx_t slc_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+	struct slcan *sl = netdev_priv(dev);
+
+	if (skb->len != sizeof(struct can_frame))
+		goto out;
+
+	spin_lock(&sl->lock);
+	if (!netif_running(dev))  {
+		spin_unlock(&sl->lock);
+		printk(KERN_WARNING "%s: xmit: iface is down\n", dev->name);
+		goto out;
+	}
+	if (sl->tty == NULL) {
+		spin_unlock(&sl->lock);
+		goto out;
+	}
+
+	netif_stop_queue(sl->dev);
+	slc_encaps(sl, (struct can_frame *) skb->data); /* encaps & send */
+	spin_unlock(&sl->lock);
+
+out:
+	kfree_skb(skb);
+	return NETDEV_TX_OK;
+}
+
+
+/******************************************
+ *   Routines looking at netdevice side.
+ ******************************************/
+
+/* Netdevice UP -> DOWN routine */
+static int slc_close(struct net_device *dev)
+{
+	struct slcan *sl = netdev_priv(dev);
+
+	spin_lock_bh(&sl->lock);
+	if (sl->tty) {
+		/* TTY discipline is running. */
+		clear_bit(TTY_DO_WRITE_WAKEUP, &sl->tty->flags);
+	}
+	netif_stop_queue(dev);
+	sl->rcount   = 0;
+	sl->xleft    = 0;
+	spin_unlock_bh(&sl->lock);
+
+	return 0;
+}
+
+/* Netdevice DOWN -> UP routine */
+static int slc_open(struct net_device *dev)
+{
+	struct slcan *sl = netdev_priv(dev);
+
+	if (sl->tty == NULL)
+		return -ENODEV;
+
+	sl->flags &= (1 << SLF_INUSE);
+	netif_start_queue(dev);
+	return 0;
+}
+
+/* Hook the destructor so we can free slcan devs at the right point in time */
+static void slc_free_netdev(struct net_device *dev)
+{
+	int i = dev->base_addr;
+	free_netdev(dev);
+	slcan_devs[i] = NULL;
+}
+
+static const struct net_device_ops slc_netdev_ops = {
+	.ndo_open               = slc_open,
+	.ndo_stop               = slc_close,
+	.ndo_start_xmit         = slc_xmit,
+};
+
+static void slc_setup(struct net_device *dev)
+{
+	dev->netdev_ops		= &slc_netdev_ops;
+	dev->destructor		= slc_free_netdev;
+
+	dev->hard_header_len	= 0;
+	dev->addr_len		= 0;
+	dev->tx_queue_len	= 10;
+
+	dev->mtu		= sizeof(struct can_frame);
+	dev->type		= ARPHRD_CAN;
+
+	/* New-style flags. */
+	dev->flags		= IFF_NOARP;
+	dev->features           = NETIF_F_NO_CSUM;
+}
+
+/******************************************
+  Routines looking at TTY side.
+ ******************************************/
+
+/*
+ * Handle the 'receiver data ready' interrupt.
+ * This function is called by the 'tty_io' module in the kernel when
+ * a block of SLCAN data has been received, which can now be decapsulated
+ * and sent on to some IP layer for further processing. This will not
+ * be re-entered while running but other ldisc functions may be called
+ * in parallel
+ */
+
+static void slcan_receive_buf(struct tty_struct *tty,
+			      const unsigned char *cp, char *fp, int count)
+{
+	struct slcan *sl = (struct slcan *) tty->disc_data;
+
+	if (!sl || sl->magic != SLCAN_MAGIC || !netif_running(sl->dev))
+		return;
+
+	/* Read the characters out of the buffer */
+	while (count--) {
+		if (fp && *fp++) {
+			if (!test_and_set_bit(SLF_ERROR, &sl->flags))
+				sl->dev->stats.rx_errors++;
+			cp++;
+			continue;
+		}
+		slcan_unesc(sl, *cp++);
+	}
+}
+
+/************************************
+ *  slcan_open helper routines.
+ ************************************/
+
+/* Collect hanged up channels */
+static void slc_sync(void)
+{
+	int i;
+	struct net_device *dev;
+	struct slcan	  *sl;
+
+	for (i = 0; i < maxdev; i++) {
+		dev = slcan_devs[i];
+		if (dev == NULL)
+			break;
+
+		sl = netdev_priv(dev);
+		if (sl->tty || sl->leased)
+			continue;
+		if (dev->flags & IFF_UP)
+			dev_close(dev);
+	}
+}
+
+/* Find a free SLCAN channel, and link in this `tty' line. */
+static struct slcan *slc_alloc(dev_t line)
+{
+	int i;
+	struct net_device *dev = NULL;
+	struct slcan       *sl;
+
+	if (slcan_devs == NULL)
+		return NULL;	/* Master array missing ! */
+
+	for (i = 0; i < maxdev; i++) {
+		dev = slcan_devs[i];
+		if (dev == NULL)
+			break;
+
+	}
+
+	/* Sorry, too many, all slots in use */
+	if (i >= maxdev)
+		return NULL;
+
+	if (dev) {
+		sl = netdev_priv(dev);
+		if (test_bit(SLF_INUSE, &sl->flags)) {
+			unregister_netdevice(dev);
+			dev = NULL;
+			slcan_devs[i] = NULL;
+		}
+	}
+
+	if (!dev) {
+		char name[IFNAMSIZ];
+		sprintf(name, "slcan%d", i);
+
+		dev = alloc_netdev(sizeof(*sl), name, slc_setup);
+		if (!dev)
+			return NULL;
+		dev->base_addr  = i;
+	}
+
+	sl = netdev_priv(dev);
+
+	/* Initialize channel control data */
+	sl->magic = SLCAN_MAGIC;
+	sl->dev	= dev;
+	spin_lock_init(&sl->lock);
+	slcan_devs[i] = dev;
+
+	return sl;
+}
+
+/*
+ * Open the high-level part of the SLCAN channel.
+ * This function is called by the TTY module when the
+ * SLCAN line discipline is called for.  Because we are
+ * sure the tty line exists, we only have to link it to
+ * a free SLCAN channel...
+ *
+ * Called in process context serialized from other ldisc calls.
+ */
+
+static int slcan_open(struct tty_struct *tty)
+{
+	struct slcan *sl;
+	int err;
+
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
+	if (tty->ops->write == NULL)
+		return -EOPNOTSUPP;
+
+	/* RTnetlink lock is misused here to serialize concurrent
+	   opens of slcan channels. There are better ways, but it is
+	   the simplest one.
+	 */
+	rtnl_lock();
+
+	/* Collect hanged up channels. */
+	slc_sync();
+
+	sl = tty->disc_data;
+
+	err = -EEXIST;
+	/* First make sure we're not already connected. */
+	if (sl && sl->magic == SLCAN_MAGIC)
+		goto err_exit;
+
+	/* OK.  Find a free SLCAN channel to use. */
+	err = -ENFILE;
+	sl = slc_alloc(tty_devnum(tty));
+	if (sl == NULL)
+		goto err_exit;
+
+	sl->tty = tty;
+	tty->disc_data = sl;
+	sl->line = tty_devnum(tty);
+	sl->pid = current->pid;
+
+	if (!test_bit(SLF_INUSE, &sl->flags)) {
+		/* Perform the low-level SLCAN initialization. */
+		sl->rcount   = 0;
+		sl->xleft    = 0;
+
+		set_bit(SLF_INUSE, &sl->flags);
+
+		err = register_netdevice(sl->dev);
+		if (err)
+			goto err_free_chan;
+	}
+
+	/* Done.  We have linked the TTY line to a channel. */
+	rtnl_unlock();
+	tty->receive_room = 65536;	/* We don't flow control */
+	return sl->dev->base_addr;
+
+err_free_chan:
+	sl->tty = NULL;
+	tty->disc_data = NULL;
+	clear_bit(SLF_INUSE, &sl->flags);
+
+err_exit:
+	rtnl_unlock();
+
+	/* Count references from TTY module */
+	return err;
+}
+
+/*
+ * Close down a SLCAN channel.
+ * This means flushing out any pending queues, and then returning. This
+ * call is serialized against other ldisc functions.
+ *
+ * We also use this method for a hangup event.
+ */
+
+static void slcan_close(struct tty_struct *tty)
+{
+	struct slcan *sl = (struct slcan *) tty->disc_data;
+
+	/* First make sure we're connected. */
+	if (!sl || sl->magic != SLCAN_MAGIC || sl->tty != tty)
+		return;
+
+	tty->disc_data = NULL;
+	sl->tty = NULL;
+	if (!sl->leased)
+		sl->line = 0;
+
+	/* Flush network side */
+	unregister_netdev(sl->dev);
+	/* This will complete via sl_free_netdev */
+}
+
+static int slcan_hangup(struct tty_struct *tty)
+{
+	slcan_close(tty);
+	return 0;
+}
+
+/* Perform I/O control on an active SLCAN channel. */
+static int slcan_ioctl(struct tty_struct *tty, struct file *file,
+		       unsigned int cmd, unsigned long arg)
+{
+	struct slcan *sl = (struct slcan *) tty->disc_data;
+	unsigned int tmp;
+
+	/* First make sure we're connected. */
+	if (!sl || sl->magic != SLCAN_MAGIC)
+		return -EINVAL;
+
+	switch (cmd) {
+	case SIOCGIFNAME:
+		tmp = strlen(sl->dev->name) + 1;
+		if (copy_to_user((void __user *)arg, sl->dev->name, tmp))
+			return -EFAULT;
+		return 0;
+
+	case SIOCSIFHWADDR:
+		return -EINVAL;
+
+	default:
+		return tty_mode_ioctl(tty, file, cmd, arg);
+	}
+}
+
+static struct tty_ldisc_ops slc_ldisc = {
+	.owner		= THIS_MODULE,
+	.magic		= TTY_LDISC_MAGIC,
+	.name		= "slcan",
+	.open		= slcan_open,
+	.close		= slcan_close,
+	.hangup		= slcan_hangup,
+	.ioctl		= slcan_ioctl,
+	.receive_buf	= slcan_receive_buf,
+	.write_wakeup	= slcan_write_wakeup,
+};
+
+static int __init slcan_init(void)
+{
+	int status;
+
+	if (maxdev < 4)
+		maxdev = 4; /* Sanity */
+
+	printk(banner);
+	printk(KERN_INFO "slcan: %d dynamic interface channels.\n", maxdev);
+
+	slcan_devs = kzalloc(sizeof(struct net_device *)*maxdev, GFP_KERNEL);
+	if (!slcan_devs) {
+		printk(KERN_ERR "slcan: can't allocate slcan device array!\n");
+		return -ENOMEM;
+	}
+
+	/* Fill in our line protocol discipline, and register it */
+	status = tty_register_ldisc(N_SLCAN, &slc_ldisc);
+	if (status)  {
+		printk(KERN_ERR "slcan: can't register line discipline\n");
+		kfree(slcan_devs);
+	}
+	return status;
+}
+
+static void __exit slcan_exit(void)
+{
+	int i;
+	struct net_device *dev;
+	struct slcan *sl;
+	unsigned long timeout = jiffies + HZ;
+	int busy = 0;
+
+	if (slcan_devs == NULL)
+		return;
+
+	/* First of all: check for active disciplines and hangup them.
+	 */
+	do {
+		if (busy)
+			msleep_interruptible(100);
+
+		busy = 0;
+		for (i = 0; i < maxdev; i++) {
+			dev = slcan_devs[i];
+			if (!dev)
+				continue;
+			sl = netdev_priv(dev);
+			spin_lock_bh(&sl->lock);
+			if (sl->tty) {
+				busy++;
+				tty_hangup(sl->tty);
+			}
+			spin_unlock_bh(&sl->lock);
+		}
+	} while (busy && time_before(jiffies, timeout));
+
+	/* FIXME: hangup is async so we should wait when doing this second
+	   phase */
+
+	for (i = 0; i < maxdev; i++) {
+		dev = slcan_devs[i];
+		if (!dev)
+			continue;
+		slcan_devs[i] = NULL;
+
+		sl = netdev_priv(dev);
+		if (sl->tty) {
+			printk(KERN_ERR "%s: tty discipline still running\n",
+			       dev->name);
+			/* Intentionally leak the control block. */
+			dev->destructor = NULL;
+		}
+
+		unregister_netdev(dev);
+	}
+
+	kfree(slcan_devs);
+	slcan_devs = NULL;
+
+	i = tty_unregister_ldisc(N_SLCAN);
+	if (i)
+		printk(KERN_ERR "slcan: can't unregister ldisc (err %d)\n", i);
+}
+
+module_init(slcan_init);
+module_exit(slcan_exit);