[RFC PATCH v2 3/3] gnss: Add driver for AI2 protocol

[Andreas Kemnade]

Add a driver for the Air Independent Interface protocol used by some TI
Wilink combo chips. Per default, send out just NMEA to userspace and turn
on/off things at open()/close() but keep the door open for any
sophisticated development regarding the AI2 protocol by having a kernel
parameter to turn it into raw mode resembling /dev/tigps provided by some
TI vendor kernels.

Signed-off-by: Andreas Kemnade <andreas@xxxxxxxxxxxx>
---
 drivers/gnss/Kconfig  |  13 ++
 drivers/gnss/Makefile |   3 +
 drivers/gnss/ai2.c    | 523 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 539 insertions(+)
 create mode 100644 drivers/gnss/ai2.c

diff --git a/drivers/gnss/Kconfig b/drivers/gnss/Kconfig
index d7fe265c28696..3a20212dacc9e 100644
--- a/drivers/gnss/Kconfig
+++ b/drivers/gnss/Kconfig
@@ -65,4 +65,17 @@ config GNSS_USB
 
 	  If unsure, say N.
 
+config GNSS_AI2
+	tristate "TI AI2 procotol support"
+	depends on BT_HCIUART_LL
+	help
+	  Say Y here if you have a Texas Instruments Wilink combo chip
+	  contaning among other things a GNSS receiver speaking the
+	  Air Independent Interface (AI2) protocol.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called gnss-ai2.
+
+	  If unsure, say N.
+
 endif # GNSS
diff --git a/drivers/gnss/Makefile b/drivers/gnss/Makefile
index bb2cbada34359..bf6fefcb2e823 100644
--- a/drivers/gnss/Makefile
+++ b/drivers/gnss/Makefile
@@ -20,3 +20,6 @@ gnss-ubx-y := ubx.o
 
 obj-$(CONFIG_GNSS_USB)			+= gnss-usb.o
 gnss-usb-y := usb.o
+
+obj-$(CONFIG_GNSS_AI2)			+= gnss-ai2.o
+gnss-ai2-y := ai2.o
diff --git a/drivers/gnss/ai2.c b/drivers/gnss/ai2.c
new file mode 100644
index 0000000000000..673fbe8de7ef2
--- /dev/null
+++ b/drivers/gnss/ai2.c
@@ -0,0 +1,523 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Texas Instruments AI2 (Air independent interface) protocol device driver
+ * Used for some TI WLAN/Bluetooth/GNSS combo chips.
+ *
+ * Copyright (C) 2024 Andreas Kemnade <andreas@xxxxxxxxxxxx>
+ */
+#include <linux/errno.h>
+#include <linux/gnss.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mod_devicetable.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+#include <linux/ti_wilink_st.h>
+#include <net/bluetooth/bluetooth.h>
+#include <net/bluetooth/hci_core.h>
+
+/* Channel-9 details for GPS */
+#define GPS_CH9_PKT_NUMBER		0x9
+#define GPS_CH9_OP_WRITE		0x1
+#define GPS_CH9_OP_READ			0x2
+#define GPS_CH9_OP_COMPLETED_EVT	0x3
+
+/* arbitarily chosen, should fit everything seen in the past */
+#define MAX_AI2_FRAME_SIZE 2048
+
+#define AI2_ESCAPE 0x10 /* if sent as data, it is doubled */
+#define AI2_END_MARKER 0x3
+#define AI2_ACK 0x2
+
+/* reports */
+#define AI2_REPORT_NMEA 0xd3
+
+#define NMEA_HEADER_LEN 4
+
+/* commands */
+#define AI2_CMD_RECEIVER_STATE 2
+
+#define RECEIVER_STATE_OFF 1
+#define RECEIVER_STATE_IDLE 2
+#define RECEIVER_STATE_ON 3
+
+#define AI2_CMD_CONFIG_NMEA 0xe5
+#define NMEA_MASK_GGA (1 << 0)
+#define NMEA_MASK_GLL (1 << 1)
+#define NMEA_MASK_GSA (1 << 2)
+#define NMEA_MASK_GSV (1 << 3)
+#define NMEA_MASK_RMC (1 << 4)
+#define NMEA_MASK_VTG (1 << 5)
+
+#define NMEA_MASK_ALL (NMEA_MASK_GGA | \
+		NMEA_MASK_GLL | \
+		NMEA_MASK_GSA | \
+		NMEA_MASK_GSV | \
+		NMEA_MASK_RMC | \
+		NMEA_MASK_VTG)
+
+
+static bool ai2raw;
+
+struct ai2_device {
+	struct mutex gdev_mutex;
+	bool gdev_open;
+	struct gnss_device *gdev;
+	struct device *dev;
+	struct sk_buff *recv_skb;
+	bool recv_esc;
+};
+
+static struct sk_buff *ai2_skb_alloc(unsigned int len, gfp_t how)
+{
+	struct sk_buff *skb;
+
+	skb = bt_skb_alloc(len + sizeof(struct gps_event_hdr), how);
+	if (skb)
+		skb_reserve(skb, sizeof(struct gps_event_hdr));
+
+	return skb;
+}
+
+static int ai2_send_frame(struct ai2_device *ai2dev,
+			  struct sk_buff *skb)
+{
+	int len;
+	struct gps_event_hdr *gnssdrv_hdr;
+	struct hci_dev *hdev;
+
+	if (skb->len >= U16_MAX)
+		return -EINVAL;
+
+	/*
+	 * note: fragmentation at this point not handled yet
+	 * not needed for simple config commands
+	 */
+	len = skb->len;
+	gnssdrv_hdr = skb_push(skb, sizeof(struct gps_event_hdr));
+	gnssdrv_hdr->opcode = GPS_CH9_OP_WRITE;
+	gnssdrv_hdr->plen = __cpu_to_le16(len);
+
+	hci_skb_pkt_type(skb) = GPS_CH9_PKT_NUMBER;
+	hdev = st_get_hci(ai2dev->dev->parent);
+	return hdev->send(hdev, skb);
+}
+
+static void ai2_put_escaped(struct sk_buff *skb, u8 d)
+{
+	skb_put_u8(skb, d);
+	if (d == 0x10)
+		skb_put_u8(skb, d);
+}
+
+static struct sk_buff *ai2_compose_frame(bool request_ack,
+					u8 cmd,
+					const u8 *data,
+					int len)
+{
+	u16 sum;
+	int i;
+	/* duplicate the length to have space for worst case escaping */
+	struct sk_buff *skb = ai2_skb_alloc(2 + len * 2 + 2 + 2, GFP_KERNEL);
+
+	skb_put_u8(skb, AI2_ESCAPE);
+	skb_put_u8(skb, request_ack ? 1 : 0);
+
+	sum = AI2_ESCAPE;
+	if (request_ack)
+		sum++;
+
+	ai2_put_escaped(skb, cmd);
+	sum += cmd;
+
+	ai2_put_escaped(skb, len & 0xff);
+	sum += len & 0xff;
+
+	ai2_put_escaped(skb, len >> 8);
+	sum += len >> 8;
+
+	for (i = 0; i < len; i++) {
+		sum += data[i];
+		ai2_put_escaped(skb, data[i]);
+	}
+
+	ai2_put_escaped(skb, sum & 0xFF);
+	ai2_put_escaped(skb, sum >> 8);
+	skb_put_u8(skb, AI2_ESCAPE);
+	skb_put_u8(skb, AI2_END_MARKER);
+
+	return skb;
+}
+
+static int ai2_set_receiver_state(struct ai2_device *ai2dev,
+					      uint8_t state)
+{
+	struct sk_buff *skb = ai2_compose_frame(true, AI2_CMD_RECEIVER_STATE,
+						&state, 1);
+	if (!skb)
+		return -ENOMEM;
+
+	return ai2_send_frame(ai2dev, skb);
+}
+
+static int ai2_config_nmea_reports(struct ai2_device *ai2dev,
+				   uint8_t mask)
+{
+	u8 buf[4] = {0};
+	struct sk_buff *skb;
+
+	buf[0] = mask;
+	skb = ai2_compose_frame(true, AI2_CMD_CONFIG_NMEA,
+				buf, sizeof(buf));
+	if (!skb)
+		return -ENOMEM;
+
+	return ai2_send_frame(ai2dev, skb);
+}
+
+/*
+ * Unknown commands, give some version information, must be sent
+ * once, not sure what undoes them besides resetting the whole
+ * bt part, but no sings of significant things being still
+ * turned on without undoing this.
+ */
+static int gnss_ai2_init(struct ai2_device *ai2dev)
+{
+	int ret;
+	u8 d = 0x01;
+	struct sk_buff *skb = ai2_compose_frame(true, 0xf5, &d, 1);
+
+	if (!skb)
+		return -ENOMEM;
+
+	ret = ai2_send_frame(ai2dev, skb);
+	if (ret)
+		return ret;
+
+	msleep(200);
+	d = 5;
+	skb = ai2_compose_frame(true, 0xf1, &d, 1);
+	if (!skb)
+		return -ENOMEM;
+
+	return ai2_send_frame(ai2dev, skb);
+}
+
+static int gnss_ai2_open(struct gnss_device *gdev)
+{
+	struct ai2_device *ai2dev = gnss_get_drvdata(gdev);
+	int ret;
+
+	mutex_lock(&ai2dev->gdev_mutex);
+	ai2dev->gdev_open = true;
+	mutex_unlock(&ai2dev->gdev_mutex);
+	if (ai2raw)
+		return 0;
+
+	ret = gnss_ai2_init(ai2dev);
+	if (ret)
+		goto err;
+
+	/* TODO: find out on what kind of ack we should wait */
+	msleep(200);
+	ret = ai2_set_receiver_state(ai2dev, RECEIVER_STATE_IDLE);
+	if (ret)
+		goto err;
+
+	msleep(200);
+	ret = ai2_config_nmea_reports(ai2dev, NMEA_MASK_ALL);
+	if (ret)
+		goto err;
+
+	msleep(200);
+	ret = ai2_set_receiver_state(ai2dev, RECEIVER_STATE_ON);
+	if (ret)
+		goto err;
+
+	return 0;
+err:
+	mutex_lock(&ai2dev->gdev_mutex);
+	ai2dev->gdev_open = false;
+	if (ai2dev->recv_skb)
+		kfree_skb(ai2dev->recv_skb);
+
+	ai2dev->recv_skb = NULL;
+	mutex_unlock(&ai2dev->gdev_mutex);
+	return ret;
+}
+
+static void gnss_ai2_close(struct gnss_device *gdev)
+{
+	struct ai2_device *ai2dev = gnss_get_drvdata(gdev);
+
+	/* TODO: find out on what kind of ack we should wait */
+	if (!ai2raw) {
+		msleep(200);
+		ai2_set_receiver_state(ai2dev, RECEIVER_STATE_IDLE);
+		msleep(200);
+		ai2_set_receiver_state(ai2dev, RECEIVER_STATE_OFF);
+		msleep(200);
+	}
+
+	mutex_lock(&ai2dev->gdev_mutex);
+	ai2dev->gdev_open = false;
+	if (ai2dev->recv_skb)
+		kfree_skb(ai2dev->recv_skb);
+
+	ai2dev->recv_skb = NULL;
+	mutex_unlock(&ai2dev->gdev_mutex);
+}
+
+
+static int gnss_ai2_write_raw(struct gnss_device *gdev,
+		const unsigned char *buf, size_t count)
+{
+	struct ai2_device *ai2dev = gnss_get_drvdata(gdev);
+	int err = 0;
+	struct sk_buff *skb = NULL;
+
+	if (!ai2raw)
+		return -EPERM;
+
+	/* allocate packet */
+	skb = ai2_skb_alloc(count, GFP_KERNEL);
+	if (!skb) {
+		BT_ERR("cannot allocate memory for HCILL packet");
+		err = -ENOMEM;
+		goto out;
+	}
+
+	skb_put_data(skb, buf, count);
+
+	err = ai2_send_frame(ai2dev, skb);
+	if (err)
+		goto out;
+
+	return count;
+out:
+	return err;
+}
+
+static const struct gnss_operations gnss_ai2_ops = {
+	.open		= gnss_ai2_open,
+	.close		= gnss_ai2_close,
+	.write_raw	= gnss_ai2_write_raw,
+};
+
+static void process_ai2_packet(struct ai2_device *ai2dev,
+			       u8 cmd, u8 *data, u16 len)
+{
+	if (cmd != AI2_REPORT_NMEA)
+		return;
+
+	if (len <= NMEA_HEADER_LEN)
+		return;
+
+	len -= NMEA_HEADER_LEN;
+	data += NMEA_HEADER_LEN;
+
+	gnss_insert_raw(ai2dev->gdev, data, len);
+}
+
+/* do some sanity checks and split frame into packets */
+static void process_ai2_frame(struct ai2_device *ai2dev)
+{
+	u16 sum;
+	int i;
+	u8 *head;
+	u8 *data;
+
+	sum = 0;
+	data = ai2dev->recv_skb->data;
+	for (i = 0; i < ai2dev->recv_skb->len - 2; i++)
+		sum += data[i];
+
+	print_hex_dump_bytes("ai2 frame: ", DUMP_PREFIX_OFFSET, data, ai2dev->recv_skb->len);
+
+	if (get_unaligned_le16(data + i) != sum) {
+		dev_dbg(ai2dev->dev,
+			"checksum error in reception, dropping frame\n");
+		return;
+	}
+
+	/* reached if byte 1 in the command packet is set to 1 */
+	if (data[1] == AI2_ACK)
+		return;
+
+	head = skb_pull(ai2dev->recv_skb, 2); /* drop frame start marker */
+	while (head && (ai2dev->recv_skb->len >= 3)) {
+		u8 cmd;
+		u16 pktlen;
+
+		cmd = head[0];
+		pktlen = get_unaligned_le16(head + 1);
+		data = skb_pull(ai2dev->recv_skb, 3);
+		if (!data)
+			break;
+
+		if (pktlen > ai2dev->recv_skb->len)
+			break;
+
+		head = skb_pull(ai2dev->recv_skb, pktlen);
+
+		process_ai2_packet(ai2dev, cmd, data, pktlen);
+	}
+}
+
+static void process_ai2_data(struct ai2_device *ai2dev,
+			     u8 *data, int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++) {
+		if (!ai2dev->recv_skb) {
+			ai2dev->recv_esc = false;
+			if (data[i] != AI2_ESCAPE) {
+				dev_dbg(ai2dev->dev, "dropping data, trying to resync\n");
+				continue;
+			}
+			ai2dev->recv_skb = alloc_skb(MAX_AI2_FRAME_SIZE, GFP_KERNEL);
+			if (!ai2dev->recv_skb)
+				return;
+
+			dev_dbg(ai2dev->dev, "starting packet\n");
+
+			/* this initial AI2_ESCAPE is part of checksum computation */
+			skb_put_u8(ai2dev->recv_skb, data[i]);
+			continue;
+		}
+		if (ai2dev->recv_skb->len == 1) {
+			if (data[i] == AI2_END_MARKER) {
+				dev_dbg(ai2dev->dev, "unexpected end of frame received\n");
+				kfree_skb(ai2dev->recv_skb);
+				ai2dev->recv_skb = NULL;
+				continue;
+			}
+			skb_put_u8(ai2dev->recv_skb, data[i]);
+		} else {
+			/* drop one of two AI2_ESCAPE */
+			if ((!ai2dev->recv_esc) &&
+			   (data[i] == AI2_ESCAPE)) {
+				ai2dev->recv_esc = true;
+				continue;
+			}
+
+			if (ai2dev->recv_esc &&
+			    (data[i] == AI2_END_MARKER)) {
+				process_ai2_frame(ai2dev);
+				kfree_skb(ai2dev->recv_skb);
+				ai2dev->recv_skb = NULL;
+				continue;
+			}
+			skb_put_u8(ai2dev->recv_skb, data[i]);
+		}
+	}
+}
+
+static void gnss_recv_frame(struct device *dev, struct sk_buff *skb)
+{
+	struct ai2_device *ai2dev = dev_get_drvdata(dev);
+	struct gps_event_hdr *gnss_hdr;
+	u8 *data;
+
+	if (!ai2dev->gdev) {
+		kfree_skb(skb);
+		return;
+	}
+
+	gnss_hdr = (struct gps_event_hdr *)skb->data;
+
+	data = skb_pull(skb, sizeof(*gnss_hdr));
+	/*
+	 * REVISIT: maybe do something with the completed
+	 * event
+	 */
+	if (gnss_hdr->opcode ==	GPS_CH9_OP_READ) {
+		mutex_lock(&ai2dev->gdev_mutex);
+		if (ai2dev->gdev_open) {
+			if (ai2raw)
+				gnss_insert_raw(ai2dev->gdev, data, skb->len);
+			else
+				process_ai2_data(ai2dev, data, skb->len);
+		} else {
+			dev_dbg(ai2dev->dev,
+				"receiving data while chip should be off\n");
+		}
+		mutex_unlock(&ai2dev->gdev_mutex);
+	}
+	kfree_skb(skb);
+}
+
+static int gnss_ai2_probe(struct platform_device *pdev)
+{
+	struct gnss_device *gdev;
+	struct ai2_device *ai2dev;
+	int ret;
+
+	ai2dev = devm_kzalloc(&pdev->dev, sizeof(*ai2dev), GFP_KERNEL);
+	if (!ai2dev)
+		return -ENOMEM;
+
+	ai2dev->dev = &pdev->dev;
+	gdev = gnss_allocate_device(&pdev->dev);
+	if (!gdev)
+		return -ENOMEM;
+
+	gdev->ops = &gnss_ai2_ops;
+	gdev->type = ai2raw ? GNSS_TYPE_AI2 : GNSS_TYPE_NMEA;
+	gnss_set_drvdata(gdev, ai2dev);
+	platform_set_drvdata(pdev, ai2dev);
+	st_set_gnss_recv_func(pdev->dev.parent, gnss_recv_frame);
+	mutex_init(&ai2dev->gdev_mutex);
+
+	ret = gnss_register_device(gdev);
+	if (ret)
+		goto err;
+
+	ai2dev->gdev = gdev;
+	return 0;
+
+err:
+	st_set_gnss_recv_func(pdev->dev.parent, NULL);
+
+	if (ai2dev->recv_skb)
+		kfree_skb(ai2dev->recv_skb);
+
+	gnss_put_device(gdev);
+	return ret;
+}
+
+static void gnss_ai2_remove(struct platform_device *pdev)
+{
+	struct ai2_device *ai2dev =  platform_get_drvdata(pdev);
+
+	st_set_gnss_recv_func(pdev->dev.parent, NULL);
+	gnss_deregister_device(ai2dev->gdev);
+	gnss_put_device(ai2dev->gdev);
+	if (ai2dev->recv_skb)
+		kfree_skb(ai2dev->recv_skb);
+}
+
+static const struct platform_device_id gnss_ai2_id[] = {
+	{
+		.name = "ti-ai2-gnss"
+	}, {
+		/* sentinel */
+	}
+};
+MODULE_DEVICE_TABLE(platform, gnss_ai2_id);
+
+static struct platform_driver gnss_ai2_driver = {
+	.driver = {
+		.name = "gnss-ai2",
+	},
+	.probe		= gnss_ai2_probe,
+	.remove_new	= gnss_ai2_remove,
+	.id_table	= gnss_ai2_id,
+};
+module_platform_driver(gnss_ai2_driver);
+
+module_param(ai2raw, bool, 0600);
+MODULE_DESCRIPTION("AI2 GNSS driver");
+MODULE_LICENSE("GPL");
-- 
2.39.2