NFC: SHDLC implementation

Most NFC HCI chipsets actually use a simplified HDLC link layer to
carry HCI payloads.
This implementation registers itself as an HCI device on behalf of the
NFC driver.
Signed-off-by: Eric Lapuyade <eric.lapuyade@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/nfc/shdlc.h

+/*
+ * Copyright (C) 2012  Intel Corporation. All rights reserved.
+ *
+ * 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, USA.
+ */
+
+#ifndef __NFC_SHDLC_H
+#define __NFC_SHDLC_H
+
+struct nfc_shdlc;
+
+struct nfc_shdlc_ops {
+	int (*open) (struct nfc_shdlc *shdlc);
+	void (*close) (struct nfc_shdlc *shdlc);
+	int (*hci_ready) (struct nfc_shdlc *shdlc);
+	int (*xmit) (struct nfc_shdlc *shdlc, struct sk_buff *skb);
+	int (*start_poll) (struct nfc_shdlc *shdlc, u32 protocols);
+	int (*target_from_gate) (struct nfc_shdlc *shdlc, u8 gate,
+				 struct nfc_target *target);
+	int (*complete_target_discovered) (struct nfc_shdlc *shdlc, u8 gate,
+					   struct nfc_target *target);
+	int (*data_exchange) (struct nfc_shdlc *shdlc,
+			      struct nfc_target *target,
+			      struct sk_buff *skb, struct sk_buff **res_skb);
+};
+
+enum shdlc_state {
+	SHDLC_DISCONNECTED = 0,
+	SHDLC_CONNECTING = 1,
+	SHDLC_NEGOCIATING = 2,
+	SHDLC_CONNECTED = 3
+};
+
+struct nfc_shdlc {
+	struct mutex state_mutex;
+	enum shdlc_state state;
+	int hard_fault;
+
+	struct nfc_hci_dev *hdev;
+
+	wait_queue_head_t *connect_wq;
+	int connect_tries;
+	int connect_result;
+	struct timer_list connect_timer;/* aka T3 in spec 10.6.1 */
+
+	u8 w;				/* window size */
+	bool srej_support;
+
+	struct timer_list t1_timer;	/* send ack timeout */
+	bool t1_active;
+
+	struct timer_list t2_timer;	/* guard/retransmit timeout */
+	bool t2_active;
+
+	int ns;				/* next seq num for send */
+	int nr;				/* next expected seq num for receive */
+	int dnr;			/* oldest sent unacked seq num */
+
+	struct sk_buff_head rcv_q;
+
+	struct sk_buff_head send_q;
+	bool rnr;			/* other side is not ready to receive */
+
+	struct sk_buff_head ack_pending_q;
+
+	struct workqueue_struct *sm_wq;
+	struct work_struct sm_work;
+
+	struct nfc_shdlc_ops *ops;
+
+	int client_headroom;
+	int client_tailroom;
+
+	void *clientdata;
+};
+
+void nfc_shdlc_recv_frame(struct nfc_shdlc *shdlc, struct sk_buff *skb);
+
+struct nfc_shdlc *nfc_shdlc_allocate(struct nfc_shdlc_ops *ops,
+				     struct nfc_hci_init_data *init_data,
+				     u32 protocols,
+				     int tx_headroom, int tx_tailroom,
+				     int max_link_payload, const char *devname);
+
+void nfc_shdlc_free(struct nfc_shdlc *shdlc);
+
+void nfc_shdlc_set_clientdata(struct nfc_shdlc *shdlc, void *clientdata);
+void *nfc_shdlc_get_clientdata(struct nfc_shdlc *shdlc);
+struct nfc_hci_dev *nfc_shdlc_get_hci_dev(struct nfc_shdlc *shdlc);
+
+#endif /* __NFC_SHDLC_H */

net/nfc/hci/Kconfig

@@ -6,3 +6,11 @@ config NFC_HCI
 	 Say Y here if you want to build support for a kernel NFC HCI
 	 implementation. This is mostly needed for devices that only process
 	 HCI frames, like for example the NXP pn544.
+
+config NFC_SHDLC
+	depends on NFC_HCI
+	bool "SHDLC link layer for HCI based NFC drivers"
+	default n
+	---help---
+	  Say yes if you use an NFC HCI driver that requires SHDLC link layer.
+	  If unsure, say N here.

net/nfc/hci/Makefile

@@ -5,3 +5,4 @@
 obj-$(CONFIG_NFC_HCI) += hci.o
 
 hci-y			:= core.o hcp.o command.o
+hci-$(CONFIG_NFC_SHDLC)	+= shdlc.o

net/nfc/hci/shdlc.c

+/*
+ * Copyright (C) 2012  Intel Corporation. All rights reserved.
+ *
+ * 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, USA.
+ */
+
+#define pr_fmt(fmt) "shdlc: %s: " fmt, __func__
+
+#include <linux/sched.h>
+#include <linux/export.h>
+#include <linux/wait.h>
+#include <linux/crc-ccitt.h>
+#include <linux/slab.h>
+#include <linux/skbuff.h>
+
+#include <net/nfc/hci.h>
+#include <net/nfc/shdlc.h>
+
+#define SHDLC_LLC_HEAD_ROOM	2
+#define SHDLC_LLC_TAIL_ROOM	2
+
+#define SHDLC_MAX_WINDOW	4
+#define SHDLC_SREJ_SUPPORT	false
+
+#define SHDLC_CONTROL_HEAD_MASK	0xe0
+#define SHDLC_CONTROL_HEAD_I	0x80
+#define SHDLC_CONTROL_HEAD_I2	0xa0
+#define SHDLC_CONTROL_HEAD_S	0xc0
+#define SHDLC_CONTROL_HEAD_U	0xe0
+
+#define SHDLC_CONTROL_NS_MASK	0x38
+#define SHDLC_CONTROL_NR_MASK	0x07
+#define SHDLC_CONTROL_TYPE_MASK	0x18
+
+#define SHDLC_CONTROL_M_MASK	0x1f
+
+enum sframe_type {
+	S_FRAME_RR = 0x00,
+	S_FRAME_REJ = 0x01,
+	S_FRAME_RNR = 0x02,
+	S_FRAME_SREJ = 0x03
+};
+
+enum uframe_modifier {
+	U_FRAME_UA = 0x06,
+	U_FRAME_RSET = 0x19
+};
+
+#define SHDLC_CONNECT_VALUE_MS	5
+#define SHDLC_T1_VALUE_MS(w)	((5 * w) / 4)
+#define SHDLC_T2_VALUE_MS	300
+
+#define SHDLC_DUMP_SKB(info, skb)				  \
+do {								  \
+	pr_debug("%s:\n", info);				  \
+	print_hex_dump(KERN_DEBUG, "shdlc: ", DUMP_PREFIX_OFFSET, \
+		       16, 1, skb->data, skb->len, 0);		  \
+} while (0)
+
+/* checks x < y <= z modulo 8 */
+static bool nfc_shdlc_x_lt_y_lteq_z(int x, int y, int z)
+{
+	if (x < z)
+		return ((x < y) && (y <= z)) ? true : false;
+	else
+		return ((y > x) || (y <= z)) ? true : false;
+}
+
+/* checks x <= y < z modulo 8 */
+static bool nfc_shdlc_x_lteq_y_lt_z(int x, int y, int z)
+{
+	if (x <= z)
+		return ((x <= y) && (y < z)) ? true : false;
+	else			/* x > z -> z+8 > x */
+		return ((y >= x) || (y < z)) ? true : false;
+}
+
+static struct sk_buff *nfc_shdlc_alloc_skb(struct nfc_shdlc *shdlc,
+					   int payload_len)
+{
+	struct sk_buff *skb;
+
+	skb = alloc_skb(shdlc->client_headroom + SHDLC_LLC_HEAD_ROOM +
+			shdlc->client_tailroom + SHDLC_LLC_TAIL_ROOM +
+			payload_len, GFP_KERNEL);
+	if (skb)
+		skb_reserve(skb, shdlc->client_headroom + SHDLC_LLC_HEAD_ROOM);
+
+	return skb;
+}
+
+static void nfc_shdlc_add_len_crc(struct sk_buff *skb)
+{
+	u16 crc;
+	int len;
+
+	len = skb->len + 2;
+	*skb_push(skb, 1) = len;
+
+	crc = crc_ccitt(0xffff, skb->data, skb->len);
+	crc = ~crc;
+	*skb_put(skb, 1) = crc & 0xff;
+	*skb_put(skb, 1) = crc >> 8;
+}
+
+/* immediately sends an S frame. */
+static int nfc_shdlc_send_s_frame(struct nfc_shdlc *shdlc,
+				  enum sframe_type sframe_type, int nr)
+{
+	int r;
+	struct sk_buff *skb;
+
+	pr_debug("sframe_type=%d nr=%d\n", sframe_type, nr);
+
+	skb = nfc_shdlc_alloc_skb(shdlc, 0);
+	if (skb == NULL)
+		return -ENOMEM;
+
+	*skb_push(skb, 1) = SHDLC_CONTROL_HEAD_S | (sframe_type << 3) | nr;
+
+	nfc_shdlc_add_len_crc(skb);
+
+	r = shdlc->ops->xmit(shdlc, skb);
+
+	kfree_skb(skb);
+
+	return r;
+}
+
+/* immediately sends an U frame. skb may contain optional payload */
+static int nfc_shdlc_send_u_frame(struct nfc_shdlc *shdlc,
+				  struct sk_buff *skb,
+				  enum uframe_modifier uframe_modifier)
+{
+	int r;
+
+	pr_debug("uframe_modifier=%d\n", uframe_modifier);
+
+	*skb_push(skb, 1) = SHDLC_CONTROL_HEAD_U | uframe_modifier;
+
+	nfc_shdlc_add_len_crc(skb);
+
+	r = shdlc->ops->xmit(shdlc, skb);
+
+	kfree_skb(skb);
+
+	return r;
+}
+
+/*
+ * Free ack_pending frames until y_nr - 1, and reset t2 according to
+ * the remaining oldest ack_pending frame sent time
+ */
+static void nfc_shdlc_reset_t2(struct nfc_shdlc *shdlc, int y_nr)
+{
+	struct sk_buff *skb;
+	int dnr = shdlc->dnr;	/* MUST initially be < y_nr */
+
+	pr_debug("release ack pending up to frame %d excluded\n", y_nr);
+
+	while (dnr != y_nr) {
+		pr_debug("release ack pending frame %d\n", dnr);
+
+		skb = skb_dequeue(&shdlc->ack_pending_q);
+		kfree_skb(skb);
+
+		dnr = (dnr + 1) % 8;
+	}
+
+	if (skb_queue_empty(&shdlc->ack_pending_q)) {
+		if (shdlc->t2_active) {
+			del_timer_sync(&shdlc->t2_timer);
+			shdlc->t2_active = false;
+
+			pr_debug
+			    ("All sent frames acked. Stopped T2(retransmit)\n");
+		}
+	} else {
+		skb = skb_peek(&shdlc->ack_pending_q);
+
+		mod_timer(&shdlc->t2_timer, *(unsigned long *)skb->cb +
+			  msecs_to_jiffies(SHDLC_T2_VALUE_MS));
+		shdlc->t2_active = true;
+
+		pr_debug
+		    ("Start T2(retransmit) for remaining unacked sent frames\n");
+	}
+}
+
+/*
+ * Receive validated frames from lower layer. skb contains HCI payload only.
+ * Handle according to algorithm at spec:10.8.2
+ */
+static void nfc_shdlc_rcv_i_frame(struct nfc_shdlc *shdlc,
+				  struct sk_buff *skb, int ns, int nr)
+{
+	int x_ns = ns;
+	int y_nr = nr;
+
+	pr_debug("recvd I-frame %d, remote waiting frame %d\n", ns, nr);
+
+	if (shdlc->state != SHDLC_CONNECTED)
+		goto exit;
+
+	if (x_ns != shdlc->nr) {
+		nfc_shdlc_send_s_frame(shdlc, S_FRAME_REJ, shdlc->nr);
+		goto exit;
+	}
+
+	if (shdlc->t1_active == false) {
+		shdlc->t1_active = true;
+		mod_timer(&shdlc->t1_timer,
+			  msecs_to_jiffies(SHDLC_T1_VALUE_MS(shdlc->w)));
+		pr_debug("(re)Start T1(send ack)\n");
+	}
+
+	if (skb->len) {
+		nfc_hci_recv_frame(shdlc->hdev, skb);
+		skb = NULL;
+	}
+
+	shdlc->nr = (shdlc->nr + 1) % 8;
+
+	if (nfc_shdlc_x_lt_y_lteq_z(shdlc->dnr, y_nr, shdlc->ns)) {
+		nfc_shdlc_reset_t2(shdlc, y_nr);
+
+		shdlc->dnr = y_nr;
+	}
+
+exit:
+	if (skb)
+		kfree_skb(skb);
+}
+
+static void nfc_shdlc_rcv_ack(struct nfc_shdlc *shdlc, int y_nr)
+{
+	pr_debug("remote acked up to frame %d excluded\n", y_nr);
+
+	if (nfc_shdlc_x_lt_y_lteq_z(shdlc->dnr, y_nr, shdlc->ns)) {
+		nfc_shdlc_reset_t2(shdlc, y_nr);
+		shdlc->dnr = y_nr;
+	}
+}
+
+static void nfc_shdlc_requeue_ack_pending(struct nfc_shdlc *shdlc)
+{
+	struct sk_buff *skb;
+
+	pr_debug("ns reset to %d\n", shdlc->dnr);
+
+	while ((skb = skb_dequeue_tail(&shdlc->ack_pending_q))) {
+		skb_pull(skb, 2);	/* remove len+control */
+		skb_trim(skb, skb->len - 2);	/* remove crc */
+		skb_queue_head(&shdlc->send_q, skb);
+	}
+	shdlc->ns = shdlc->dnr;
+}
+
+static void nfc_shdlc_rcv_rej(struct nfc_shdlc *shdlc, int y_nr)
+{
+	struct sk_buff *skb;
+
+	pr_debug("remote asks retransmition from frame %d\n", y_nr);
+
+	if (nfc_shdlc_x_lteq_y_lt_z(shdlc->dnr, y_nr, shdlc->ns)) {
+		if (shdlc->t2_active) {
+			del_timer_sync(&shdlc->t2_timer);
+			shdlc->t2_active = false;
+			pr_debug("Stopped T2(retransmit)\n");
+		}
+
+		if (shdlc->dnr != y_nr) {
+			while ((shdlc->dnr = ((shdlc->dnr + 1) % 8)) != y_nr) {
+				skb = skb_dequeue(&shdlc->ack_pending_q);
+				kfree_skb(skb);
+			}
+		}
+
+		nfc_shdlc_requeue_ack_pending(shdlc);
+	}
+}
+
+/* See spec RR:10.8.3 REJ:10.8.4 */
+static void nfc_shdlc_rcv_s_frame(struct nfc_shdlc *shdlc,
+				  enum sframe_type s_frame_type, int nr)
+{
+	struct sk_buff *skb;
+
+	if (shdlc->state != SHDLC_CONNECTED)
+		return;
+
+	switch (s_frame_type) {
+	case S_FRAME_RR:
+		nfc_shdlc_rcv_ack(shdlc, nr);
+		if (shdlc->rnr == true) {	/* see SHDLC 10.7.7 */
+			shdlc->rnr = false;
+			if (shdlc->send_q.qlen == 0) {
+				skb = nfc_shdlc_alloc_skb(shdlc, 0);
+				if (skb)
+					skb_queue_tail(&shdlc->send_q, skb);
+			}
+		}
+		break;
+	case S_FRAME_REJ:
+		nfc_shdlc_rcv_rej(shdlc, nr);
+		break;
+	case S_FRAME_RNR:
+		nfc_shdlc_rcv_ack(shdlc, nr);
+		shdlc->rnr = true;
+		break;
+	default:
+		break;
+	}
+}
+
+static void nfc_shdlc_connect_complete(struct nfc_shdlc *shdlc, int r)
+{
+	pr_debug("result=%d\n", r);
+
+	del_timer_sync(&shdlc->connect_timer);
+
+	if (r == 0) {
+		shdlc->ns = 0;
+		shdlc->nr = 0;
+		shdlc->dnr = 0;
+
+		shdlc->state = SHDLC_CONNECTED;
+	} else {
+		shdlc->state = SHDLC_DISCONNECTED;
+
+		/*
+		 * TODO: Could it be possible that there are pending
+		 * executing commands that are waiting for connect to complete
+		 * before they can be carried? As connect is a blocking
+		 * operation, it would require that the userspace process can
+		 * send commands on the same device from a second thread before
+		 * the device is up. I don't think that is possible, is it?
+		 */
+	}
+
+	shdlc->connect_result = r;
+
+	wake_up(shdlc->connect_wq);
+}
+
+static int nfc_shdlc_connect_initiate(struct nfc_shdlc *shdlc)
+{
+	struct sk_buff *skb;
+
+	pr_debug("\n");
+
+	skb = nfc_shdlc_alloc_skb(shdlc, 2);
+	if (skb == NULL)
+		return -ENOMEM;
+
+	*skb_put(skb, 1) = SHDLC_MAX_WINDOW;
+	*skb_put(skb, 1) = SHDLC_SREJ_SUPPORT ? 1 : 0;
+
+	return nfc_shdlc_send_u_frame(shdlc, skb, U_FRAME_RSET);
+}
+
+static int nfc_shdlc_connect_send_ua(struct nfc_shdlc *shdlc)
+{
+	struct sk_buff *skb;
+
+	pr_debug("\n");
+
+	skb = nfc_shdlc_alloc_skb(shdlc, 0);
+	if (skb == NULL)
+		return -ENOMEM;
+
+	return nfc_shdlc_send_u_frame(shdlc, skb, U_FRAME_UA);
+}
+
+static void nfc_shdlc_rcv_u_frame(struct nfc_shdlc *shdlc,
+				  struct sk_buff *skb,
+				  enum uframe_modifier u_frame_modifier)
+{
+	u8 w = SHDLC_MAX_WINDOW;
+	bool srej_support = SHDLC_SREJ_SUPPORT;
+	int r;
+
+	pr_debug("u_frame_modifier=%d\n", u_frame_modifier);
+
+	switch (u_frame_modifier) {
+	case U_FRAME_RSET:
+		if (shdlc->state == SHDLC_NEGOCIATING) {
+			/* we sent RSET, but chip wants to negociate */
+			if (skb->len > 0)
+				w = skb->data[0];
+
+			if (skb->len > 1)
+				srej_support = skb->data[1] & 0x01 ? true :
+					       false;
+
+			if ((w <= SHDLC_MAX_WINDOW) &&
+			    (SHDLC_SREJ_SUPPORT || (srej_support == false))) {
+				shdlc->w = w;
+				shdlc->srej_support = srej_support;
+				r = nfc_shdlc_connect_send_ua(shdlc);
+				nfc_shdlc_connect_complete(shdlc, r);
+			}
+		} else if (shdlc->state > SHDLC_NEGOCIATING) {
+			/*
+			 * TODO: Chip wants to reset link
+			 * send ua, empty skb lists, reset counters
+			 * propagate info to HCI layer
+			 */
+		}
+		break;
+	case U_FRAME_UA:
+		if ((shdlc->state == SHDLC_CONNECTING &&
+		     shdlc->connect_tries > 0) ||
+		    (shdlc->state == SHDLC_NEGOCIATING))
+			nfc_shdlc_connect_complete(shdlc, 0);
+		break;
+	default:
+		break;
+	}
+
+	kfree_skb(skb);
+}
+
+static void nfc_shdlc_handle_rcv_queue(struct nfc_shdlc *shdlc)
+{
+	struct sk_buff *skb;
+	u8 control;
+	int nr;
+	int ns;
+	enum sframe_type s_frame_type;
+	enum uframe_modifier u_frame_modifier;
+
+	if (shdlc->rcv_q.qlen)
+		pr_debug("rcvQlen=%d\n", shdlc->rcv_q.qlen);
+
+	while ((skb = skb_dequeue(&shdlc->rcv_q)) != NULL) {
+		control = skb->data[0];
+		skb_pull(skb, 1);
+		switch (control & SHDLC_CONTROL_HEAD_MASK) {
+		case SHDLC_CONTROL_HEAD_I:
+		case SHDLC_CONTROL_HEAD_I2:
+			ns = (control & SHDLC_CONTROL_NS_MASK) >> 3;
+			nr = control & SHDLC_CONTROL_NR_MASK;
+			nfc_shdlc_rcv_i_frame(shdlc, skb, ns, nr);
+			break;
+		case SHDLC_CONTROL_HEAD_S:
+			s_frame_type = (control & SHDLC_CONTROL_TYPE_MASK) >> 3;
+			nr = control & SHDLC_CONTROL_NR_MASK;
+			nfc_shdlc_rcv_s_frame(shdlc, s_frame_type, nr);
+			kfree_skb(skb);
+			break;
+		case SHDLC_CONTROL_HEAD_U:
+			u_frame_modifier = control & SHDLC_CONTROL_M_MASK;
+			nfc_shdlc_rcv_u_frame(shdlc, skb, u_frame_modifier);
+			break;
+		default:
+			pr_err("UNKNOWN Control=%d\n", control);
+			kfree_skb(skb);
+			break;
+		}
+	}
+}
+
+static int nfc_shdlc_w_used(int ns, int dnr)
+{
+	int unack_count;
+
+	if (dnr <= ns)
+		unack_count = ns - dnr;
+	else
+		unack_count = 8 - dnr + ns;
+
+	return unack_count;
+}
+
+/* Send frames according to algorithm at spec:10.8.1 */
+static void nfc_shdlc_handle_send_queue(struct nfc_shdlc *shdlc)
+{
+	struct sk_buff *skb;
+	int r;
+	unsigned long time_sent;
+
+	if (shdlc->send_q.qlen)
+		pr_debug
+		    ("sendQlen=%d ns=%d dnr=%d rnr=%s w_room=%d unackQlen=%d\n",
+		     shdlc->send_q.qlen, shdlc->ns, shdlc->dnr,
+		     shdlc->rnr == false ? "false" : "true",
+		     shdlc->w - nfc_shdlc_w_used(shdlc->ns, shdlc->dnr),
+		     shdlc->ack_pending_q.qlen);
+
+	while (shdlc->send_q.qlen && shdlc->ack_pending_q.qlen < shdlc->w &&
+	       (shdlc->rnr == false)) {
+
+		if (shdlc->t1_active) {
+			del_timer_sync(&shdlc->t1_timer);
+			shdlc->t1_active = false;
+			pr_debug("Stopped T1(send ack)\n");
+		}
+
+		skb = skb_dequeue(&shdlc->send_q);
+
+		*skb_push(skb, 1) = SHDLC_CONTROL_HEAD_I | (shdlc->ns << 3) |
+				    shdlc->nr;
+
+		pr_debug("Sending I-Frame %d, waiting to rcv %d\n", shdlc->ns,
+			 shdlc->nr);
+	/*	SHDLC_DUMP_SKB("shdlc frame written", skb); */
+
+		nfc_shdlc_add_len_crc(skb);
+
+		r = shdlc->ops->xmit(shdlc, skb);
+		if (r < 0) {
+			/*
+			 * TODO: Cannot send, shdlc machine is dead, we
+			 * must propagate the information up to HCI.
+			 */
+			shdlc->hard_fault = r;
+			break;
+		}
+
+		shdlc->ns = (shdlc->ns + 1) % 8;
+
+		time_sent = jiffies;
+		*(unsigned long *)skb->cb = time_sent;
+
+		skb_queue_tail(&shdlc->ack_pending_q, skb);
+
+		if (shdlc->t2_active == false) {
+			shdlc->t2_active = true;
+			mod_timer(&shdlc->t2_timer, time_sent +
+				  msecs_to_jiffies(SHDLC_T2_VALUE_MS));
+			pr_debug("Started T2 (retransmit)\n");
+		}
+	}
+}
+
+static void nfc_shdlc_connect_timeout(unsigned long data)
+{
+	struct nfc_shdlc *shdlc = (struct nfc_shdlc *)data;
+
+	pr_debug("\n");
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+}
+
+static void nfc_shdlc_t1_timeout(unsigned long data)
+{
+	struct nfc_shdlc *shdlc = (struct nfc_shdlc *)data;
+
+	pr_debug("SoftIRQ: need to send ack\n");
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+}
+
+static void nfc_shdlc_t2_timeout(unsigned long data)
+{
+	struct nfc_shdlc *shdlc = (struct nfc_shdlc *)data;
+
+	pr_debug("SoftIRQ: need to retransmit\n");
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+}
+
+static void nfc_shdlc_sm_work(struct work_struct *work)
+{
+	struct nfc_shdlc *shdlc = container_of(work, struct nfc_shdlc, sm_work);
+	int r;
+
+	pr_debug("\n");
+
+	mutex_lock(&shdlc->state_mutex);
+
+	switch (shdlc->state) {
+	case SHDLC_DISCONNECTED:
+		skb_queue_purge(&shdlc->rcv_q);
+		skb_queue_purge(&shdlc->send_q);
+		skb_queue_purge(&shdlc->ack_pending_q);
+		break;
+	case SHDLC_CONNECTING:
+		if (shdlc->connect_tries++ < 5)
+			r = nfc_shdlc_connect_initiate(shdlc);
+		else
+			r = -ETIME;
+		if (r < 0)
+			nfc_shdlc_connect_complete(shdlc, r);
+		else {
+			mod_timer(&shdlc->connect_timer, jiffies +
+				  msecs_to_jiffies(SHDLC_CONNECT_VALUE_MS));
+
+			shdlc->state = SHDLC_NEGOCIATING;
+		}
+		break;
+	case SHDLC_NEGOCIATING:
+		if (timer_pending(&shdlc->connect_timer) == 0) {
+			shdlc->state = SHDLC_CONNECTING;
+			queue_work(shdlc->sm_wq, &shdlc->sm_work);
+		}
+
+		nfc_shdlc_handle_rcv_queue(shdlc);
+		break;
+	case SHDLC_CONNECTED:
+		nfc_shdlc_handle_rcv_queue(shdlc);
+		nfc_shdlc_handle_send_queue(shdlc);
+
+		if (shdlc->t1_active && timer_pending(&shdlc->t1_timer) == 0) {
+			pr_debug
+			    ("Handle T1(send ack) elapsed (T1 now inactive)\n");
+
+			shdlc->t1_active = false;
+			r = nfc_shdlc_send_s_frame(shdlc, S_FRAME_RR,
+						   shdlc->nr);
+			if (r < 0)
+				shdlc->hard_fault = r;
+		}
+
+		if (shdlc->t2_active && timer_pending(&shdlc->t2_timer) == 0) {
+			pr_debug
+			    ("Handle T2(retransmit) elapsed (T2 inactive)\n");
+
+			shdlc->t2_active = false;
+
+			nfc_shdlc_requeue_ack_pending(shdlc);
+			nfc_shdlc_handle_send_queue(shdlc);
+		}
+
+		if (shdlc->hard_fault) {
+			/*
+			 * TODO: Handle hard_fault that occured during
+			 * this invocation of the shdlc worker
+			 */
+		}
+		break;
+	default:
+		break;
+	}
+	mutex_unlock(&shdlc->state_mutex);
+}
+
+/*
+ * Called from syscall context to establish shdlc link. Sleeps until
+ * link is ready or failure.
+ */
+static int nfc_shdlc_connect(struct nfc_shdlc *shdlc)
+{
+	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(connect_wq);
+
+	pr_debug("\n");
+
+	mutex_lock(&shdlc->state_mutex);
+
+	shdlc->state = SHDLC_CONNECTING;
+	shdlc->connect_wq = &connect_wq;
+	shdlc->connect_tries = 0;
+	shdlc->connect_result = 1;
+
+	mutex_unlock(&shdlc->state_mutex);
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+
+	wait_event(connect_wq, shdlc->connect_result != 1);
+
+	return shdlc->connect_result;
+}
+
+static void nfc_shdlc_disconnect(struct nfc_shdlc *shdlc)
+{
+	pr_debug("\n");
+
+	mutex_lock(&shdlc->state_mutex);
+
+	shdlc->state = SHDLC_DISCONNECTED;
+
+	mutex_unlock(&shdlc->state_mutex);
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+}
+
+/*
+ * Receive an incoming shdlc frame. Frame has already been crc-validated.
+ * skb contains only LLC header and payload.
+ * If skb == NULL, it is a notification that the link below is dead.
+ */
+void nfc_shdlc_recv_frame(struct nfc_shdlc *shdlc, struct sk_buff *skb)
+{
+	if (skb == NULL) {
+		pr_err("NULL Frame -> link is dead\n");
+		shdlc->hard_fault = -EREMOTEIO;
+	} else {
+		SHDLC_DUMP_SKB("incoming frame", skb);
+		skb_queue_tail(&shdlc->rcv_q, skb);
+	}
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+}
+EXPORT_SYMBOL(nfc_shdlc_recv_frame);
+
+static int nfc_shdlc_open(struct nfc_hci_dev *hdev)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+	int r;
+
+	pr_debug("\n");
+
+	if (shdlc->ops->open) {
+		r = shdlc->ops->open(shdlc);
+		if (r < 0)
+			return r;
+	}
+
+	r = nfc_shdlc_connect(shdlc);
+	if (r < 0 && shdlc->ops->close)
+		shdlc->ops->close(shdlc);
+
+	return r;
+}
+
+static void nfc_shdlc_close(struct nfc_hci_dev *hdev)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+
+	pr_debug("\n");
+
+	nfc_shdlc_disconnect(shdlc);
+
+	if (shdlc->ops->close)
+		shdlc->ops->close(shdlc);
+}
+
+static int nfc_shdlc_hci_ready(struct nfc_hci_dev *hdev)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+	int r = 0;
+
+	pr_debug("\n");
+
+	if (shdlc->ops->hci_ready)
+		r = shdlc->ops->hci_ready(shdlc);
+
+	return r;
+}
+
+static int nfc_shdlc_xmit(struct nfc_hci_dev *hdev, struct sk_buff *skb)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+
+	SHDLC_DUMP_SKB("queuing HCP packet to shdlc", skb);
+
+	skb_queue_tail(&shdlc->send_q, skb);
+
+	queue_work(shdlc->sm_wq, &shdlc->sm_work);
+
+	return 0;
+}
+
+static int nfc_shdlc_start_poll(struct nfc_hci_dev *hdev, u32 protocols)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+
+	pr_debug("\n");
+
+	if (shdlc->ops->start_poll)
+		return shdlc->ops->start_poll(shdlc, protocols);
+
+	return 0;
+}
+
+static int nfc_shdlc_target_from_gate(struct nfc_hci_dev *hdev, u8 gate,
+				      struct nfc_target *target)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+
+	if (shdlc->ops->target_from_gate)
+		return shdlc->ops->target_from_gate(shdlc, gate, target);
+
+	return -EPERM;
+}
+
+static int nfc_shdlc_complete_target_discovered(struct nfc_hci_dev *hdev,
+						u8 gate,
+						struct nfc_target *target)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+
+	pr_debug("\n");
+
+	if (shdlc->ops->complete_target_discovered)
+		return shdlc->ops->complete_target_discovered(shdlc, gate,
+							      target);
+
+	return 0;
+}
+
+static int nfc_shdlc_data_exchange(struct nfc_hci_dev *hdev,
+				   struct nfc_target *target,
+				   struct sk_buff *skb,
+				   struct sk_buff **res_skb)
+{
+	struct nfc_shdlc *shdlc = nfc_hci_get_clientdata(hdev);
+
+	if (shdlc->ops->data_exchange)
+		return shdlc->ops->data_exchange(shdlc, target, skb, res_skb);
+
+	return -EPERM;
+}
+
+static struct nfc_hci_ops shdlc_ops = {
+	.open = nfc_shdlc_open,
+	.close = nfc_shdlc_close,
+	.hci_ready = nfc_shdlc_hci_ready,
+	.xmit = nfc_shdlc_xmit,
+	.start_poll = nfc_shdlc_start_poll,
+	.target_from_gate = nfc_shdlc_target_from_gate,
+	.complete_target_discovered = nfc_shdlc_complete_target_discovered,
+	.data_exchange = nfc_shdlc_data_exchange,
+};
+
+struct nfc_shdlc *nfc_shdlc_allocate(struct nfc_shdlc_ops *ops,
+				     struct nfc_hci_init_data *init_data,
+				     u32 protocols,
+				     int tx_headroom, int tx_tailroom,
+				     int max_link_payload, const char *devname)
+{
+	struct nfc_shdlc *shdlc;
+	int r;
+	char name[32];
+
+	if (ops->xmit == NULL)
+		return NULL;
+
+	shdlc = kzalloc(sizeof(struct nfc_shdlc), GFP_KERNEL);
+	if (shdlc == NULL)
+		return NULL;
+
+	mutex_init(&shdlc->state_mutex);
+	shdlc->ops = ops;
+	shdlc->state = SHDLC_DISCONNECTED;
+
+	init_timer(&shdlc->connect_timer);
+	shdlc->connect_timer.data = (unsigned long)shdlc;
+	shdlc->connect_timer.function = nfc_shdlc_connect_timeout;
+
+	init_timer(&shdlc->t1_timer);
+	shdlc->t1_timer.data = (unsigned long)shdlc;
+	shdlc->t1_timer.function = nfc_shdlc_t1_timeout;
+
+	init_timer(&shdlc->t2_timer);
+	shdlc->t2_timer.data = (unsigned long)shdlc;
+	shdlc->t2_timer.function = nfc_shdlc_t2_timeout;
+
+	shdlc->w = SHDLC_MAX_WINDOW;
+	shdlc->srej_support = SHDLC_SREJ_SUPPORT;
+
+	skb_queue_head_init(&shdlc->rcv_q);
+	skb_queue_head_init(&shdlc->send_q);
+	skb_queue_head_init(&shdlc->ack_pending_q);
+
+	INIT_WORK(&shdlc->sm_work, nfc_shdlc_sm_work);
+	snprintf(name, sizeof(name), "%s_shdlc_sm_wq", devname);
+	shdlc->sm_wq = alloc_workqueue(name, WQ_NON_REENTRANT | WQ_UNBOUND |
+				       WQ_MEM_RECLAIM, 1);
+	if (shdlc->sm_wq == NULL)
+		goto err_allocwq;
+
+	shdlc->client_headroom = tx_headroom;
+	shdlc->client_tailroom = tx_tailroom;
+
+	shdlc->hdev = nfc_hci_allocate_device(&shdlc_ops, init_data, protocols,
+					      tx_headroom + SHDLC_LLC_HEAD_ROOM,
+					      tx_tailroom + SHDLC_LLC_TAIL_ROOM,
+					      max_link_payload);
+	if (shdlc->hdev == NULL)
+		goto err_allocdev;
+
+	nfc_hci_set_clientdata(shdlc->hdev, shdlc);
+
+	r = nfc_hci_register_device(shdlc->hdev);
+	if (r < 0)
+		goto err_regdev;
+
+	return shdlc;
+
+err_regdev:
+	nfc_hci_free_device(shdlc->hdev);
+
+err_allocdev:
+	destroy_workqueue(shdlc->sm_wq);
+
+err_allocwq:
+	kfree(shdlc);
+
+	return NULL;
+}
+EXPORT_SYMBOL(nfc_shdlc_allocate);
+
+void nfc_shdlc_free(struct nfc_shdlc *shdlc)
+{
+	pr_debug("\n");
+
+	/* TODO: Check that this cannot be called while still in use */
+
+	nfc_hci_unregister_device(shdlc->hdev);
+	nfc_hci_free_device(shdlc->hdev);
+
+	destroy_workqueue(shdlc->sm_wq);
+
+	skb_queue_purge(&shdlc->rcv_q);
+	skb_queue_purge(&shdlc->send_q);
+	skb_queue_purge(&shdlc->ack_pending_q);
+
+	kfree(shdlc);
+}
+EXPORT_SYMBOL(nfc_shdlc_free);
+
+void nfc_shdlc_set_clientdata(struct nfc_shdlc *shdlc, void *clientdata)
+{
+	pr_debug("\n");
+
+	shdlc->clientdata = clientdata;
+}
+EXPORT_SYMBOL(nfc_shdlc_set_clientdata);
+
+void *nfc_shdlc_get_clientdata(struct nfc_shdlc *shdlc)
+{
+	return shdlc->clientdata;
+}
+EXPORT_SYMBOL(nfc_shdlc_get_clientdata);
+
+struct nfc_hci_dev *nfc_shdlc_get_hci_dev(struct nfc_shdlc *shdlc)
+{
+	return shdlc->hdev;
+}
+EXPORT_SYMBOL(nfc_shdlc_get_hci_dev);