[v2 09/10] iio: imu: add BNO055 serdev driver

[Andrea Merello <andrea.merello@gmail.com>]

This path adds a serdev driver for communicating to a BNO055 IMU via
serial bus, and it enables the BNO055 core driver to work in this
scenario.

Signed-off-by: Andrea Merello <andrea.merello@iit.it>
---
 drivers/iio/imu/bno055/Kconfig     |   5 +
 drivers/iio/imu/bno055/Makefile    |   1 +
 drivers/iio/imu/bno055/bno055_sl.c | 568 +++++++++++++++++++++++++++++
 3 files changed, 574 insertions(+)
 create mode 100644 drivers/iio/imu/bno055/bno055_sl.c

diff --git a/drivers/iio/imu/bno055/Kconfig b/drivers/iio/imu/bno055/Kconfig
index d197310661af..941e43f0368d 100644
--- a/drivers/iio/imu/bno055/Kconfig
+++ b/drivers/iio/imu/bno055/Kconfig
@@ -2,3 +2,8 @@
 
 config BOSH_BNO055_IIO
 	tristate
+
+config BOSH_BNO055_SERIAL
+	tristate "Bosh BNO055 attached via serial bus"
+	depends on SERIAL_DEV_BUS
+	select BOSH_BNO055_IIO
diff --git a/drivers/iio/imu/bno055/Makefile b/drivers/iio/imu/bno055/Makefile
index c55741d0e96f..7285ade2f4b9 100644
--- a/drivers/iio/imu/bno055/Makefile
+++ b/drivers/iio/imu/bno055/Makefile
@@ -1,3 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0
 
 obj-$(CONFIG_BOSH_BNO055_IIO) += bno055.o
+obj-$(CONFIG_BOSH_BNO055_SERIAL) += bno055_sl.o
diff --git a/drivers/iio/imu/bno055/bno055_sl.c b/drivers/iio/imu/bno055/bno055_sl.c
new file mode 100644
index 000000000000..1d1410fdaa7c
--- /dev/null
+++ b/drivers/iio/imu/bno055/bno055_sl.c
@@ -0,0 +1,568 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Serial line interface for Bosh BNO055 IMU (via serdev).
+ * This file implements serial communication up to the register read/write
+ * level.
+ *
+ * Copyright (C) 2021 Istituto Italiano di Tecnologia
+ * Electronic Design Laboratory
+ * Written by Andrea Merello <andrea.merello@iit.it>
+ *
+ * This driver is besed on
+ *	Plantower PMS7003 particulate matter sensor driver
+ *	Which is
+ *	Copyright (c) Tomasz Duszynski <tduszyns@gmail.com>
+ */
+
+#include <linux/completion.h>
+#include <linux/device.h>
+#include <linux/errno.h>
+#include <linux/jiffies.h>
+#include <linux/kernel.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/regmap.h>
+#include <linux/serdev.h>
+
+#include "bno055.h"
+
+/*
+ * Register writes cmd have the following format
+ * +------+------+-----+-----+----- ... ----+
+ * | 0xAA | 0xOO | REG | LEN | payload[LEN] |
+ * +------+------+-----+-----+----- ... ----+
+ *
+ * Register write responses have the following format
+ * +------+----------+
+ * | 0xEE | ERROCODE |
+ * +------+----------+
+ *
+ * Register read have the following format
+ * +------+------+-----+-----+
+ * | 0xAA | 0xO1 | REG | LEN |
+ * +------+------+-----+-----+
+ *
+ * Successful register read response have the following format
+ * +------+-----+----- ... ----+
+ * | 0xBB | LEN | payload[LEN] |
+ * +------+-----+----- ... ----+
+ *
+ * Failed register read response have the following format
+ * +------+--------+
+ * | 0xEE | ERRCODE|  (ERRCODE always > 1)
+ * +------+--------+
+ *
+ * Error codes are
+ * 01: OK
+ * 02: read/write FAIL
+ * 04: invalid address
+ * 05: write on RO
+ * 06: wrong start byte
+ * 07: bus overrun
+ * 08: len too high
+ * 09: len too low
+ * 10: bus RX byte timeout (timeout is 30mS)
+ *
+ *
+ * **WORKAROUND ALERT**
+ *
+ * Serial communication seems very fragile: the BNO055 buffer seems to overflow
+ * very easy; BNO055 seems able to sink few bytes, then it needs a brief pause.
+ * On the other hand, it is also picky on timeout: if there is a pause > 30mS in
+ * between two bytes then the transaction fails (IMU internal RX FSM resets).
+ *
+ * BMU055 has been seen also failing to process commands in case we send them
+ * too close each other (or if it is somehow busy?)
+ *
+ * One idea would be to split data in chunks, and then wait 1-2mS between
+ * chunks (we hope not to exceed 30mS delay for any reason - which should
+ * be pretty a lot of time for us), and eventually retry in case the BNO055
+ * gets upset for any reason. This seems to work in avoiding the overflow
+ * errors, but indeed it seems slower than just perform a retry when an overflow
+ * error occur.
+ * In particular I saw these scenarios:
+ * 1) If we send 2 bytes per time, then the IMU never(?) overflows.
+ * 2) If we send 4 bytes per time (i.e. the full header), then the IMU could
+ *    overflow, but it seem to sink all 4 bytes, then it returns error.
+ * 3) If we send more than 4 bytes, the IMU could overflow, and I saw it sending
+ *    error after 4 bytes are sent; we have troubles in synchronizing again,
+ *    because we are still sending data, and the IMU interprets it as the 1st
+ *    byte of a new command.
+ *
+ * So, we workaround all this in the following way:
+ * In case of read we don't split the header but we rely on retries; This seems
+ * convenient for data read (where we TX only the hdr).
+ * For TX we split the transmission in 2-bytes chunks so that, we should not
+ * only avoid case 2 (which is still manageable), but we also hopefully avoid
+ * case 3, that would be by far worse.
+ */
+
+/*
+ * Read operation overhead:
+ *  4 bytes req + 2byte resp hdr.
+ *  6 bytes = 60 bit (considering 1start + 1stop bits).
+ *  60/115200 = ~520uS.
+ *
+ * In 520uS we could read back about 34 bytes that means 3 samples, this means
+ * that in case of scattered read in which the gap is 3 samples or less it is
+ * still convenient to go for a burst.
+ * We have to take into account also IMU response time - IMU seems to be often
+ * reasonably quick to respond, but sometimes it seems to be in some "critical
+ * section" in which it delays handling of serial protocol.
+ * By experiment, it seems convenient to burst up to about 5/6-samples-long gap.
+ */
+
+#define BNO055_SL_XFER_BURST_BREAK_THRESHOLD 6
+
+struct bno055_sl_priv {
+	struct serdev_device *serdev;
+	struct completion cmd_complete;
+	enum {
+		CMD_NONE,
+		CMD_READ,
+		CMD_WRITE,
+	} expect_response;
+	int expected_data_len;
+	u8 *response_buf;
+
+	/**
+	 * enum cmd_status - represent the status of a command sent to the HW.
+	 * @STATUS_OK:   The command executed successfully.
+	 * @STATUS_FAIL: The command failed: HW responded with an error.
+	 * @STATUS_CRIT: The command failed: the serial communication failed.
+	 */
+	enum {
+		STATUS_OK = 0,
+		STATUS_FAIL = 1,
+		STATUS_CRIT = -1
+	} cmd_status;
+	struct mutex lock;
+
+	/* Only accessed in behalf of RX callback context. No lock needed. */
+	struct {
+		enum {
+			RX_IDLE,
+			RX_START,
+			RX_DATA
+		} state;
+		int databuf_count;
+		int expected_len;
+		int type;
+	} rx;
+
+	/* Never accessed in behalf of RX callback context. No lock needed */
+	bool cmd_stale;
+};
+
+static int bno055_sl_send_chunk(struct bno055_sl_priv *priv, u8 *data, int len)
+{
+	int ret;
+
+	dev_dbg(&priv->serdev->dev, "send (len: %d): %*ph", len, len, data);
+	ret = serdev_device_write(priv->serdev, data, len,
+				  msecs_to_jiffies(25));
+	if (ret < 0)
+		return ret;
+
+	if (ret < len)
+		return -EIO;
+
+	return 0;
+}
+
+/*
+ * Sends a read or write command.
+ * 'data' can be NULL (used in read case). 'len' parameter is always valid; in
+ * case 'data' is non-NULL then it must match 'data' size.
+ */
+static int bno055_sl_do_send_cmd(struct bno055_sl_priv *priv,
+				 int read, int addr, int len, u8 *data)
+{
+	int ret;
+	int chunk_len;
+	u8 hdr[] = {0xAA, !!read, addr, len};
+
+	if (read) {
+		ret = bno055_sl_send_chunk(priv, hdr, 4);
+	} else {
+		ret = bno055_sl_send_chunk(priv, hdr, 2);
+		if (ret)
+			goto fail;
+
+		usleep_range(2000, 3000);
+		ret = bno055_sl_send_chunk(priv, hdr + 2, 2);
+	}
+	if (ret)
+		goto fail;
+
+	if (data) {
+		while (len) {
+			chunk_len = min(len, 2);
+			usleep_range(2000, 3000);
+			ret = bno055_sl_send_chunk(priv, data, chunk_len);
+			if (ret)
+				goto fail;
+			data += chunk_len;
+			len -= chunk_len;
+		}
+	}
+
+	return 0;
+fail:
+	/* waiting more than 30mS should clear the BNO055 internal state */
+	usleep_range(40000, 50000);
+	return ret;
+}
+
+static int bno_sl_send_cmd(struct bno055_sl_priv *priv,
+			   int read, int addr, int len, u8 *data)
+{
+	const int retry_max = 5;
+	int retry = retry_max;
+	int ret = 0;
+
+	/*
+	 * In case previous command was interrupted we still neet to wait it to
+	 * complete before we can issue new commands
+	 */
+	if (priv->cmd_stale) {
+		ret = wait_for_completion_interruptible_timeout(&priv->cmd_complete,
+								msecs_to_jiffies(100));
+		if (ret == -ERESTARTSYS)
+			return -ERESTARTSYS;
+
+		priv->cmd_stale = false;
+		/* if serial protocol broke, bail out */
+		if (priv->cmd_status == STATUS_CRIT)
+			goto exit;
+	}
+
+	/*
+	 * Try to convince the IMU to cooperate.. as explained in the comments
+	 * at the top of this file, the IMU could also refuse the command (i.e.
+	 * it is not ready yet); retry in this case.
+	 */
+	while (retry--) {
+		mutex_lock(&priv->lock);
+		priv->expect_response = read ? CMD_READ : CMD_WRITE;
+		reinit_completion(&priv->cmd_complete);
+		mutex_unlock(&priv->lock);
+
+		if (retry != (retry_max - 1))
+			dev_dbg(&priv->serdev->dev, "cmd retry: %d",
+				retry_max - retry);
+		ret = bno055_sl_do_send_cmd(priv, read, addr, len, data);
+		if (ret)
+			continue;
+
+		ret = wait_for_completion_interruptible_timeout(&priv->cmd_complete,
+								msecs_to_jiffies(100));
+		if (ret == -ERESTARTSYS) {
+			priv->cmd_stale = true;
+			return -ERESTARTSYS;
+		} else if (!ret) {
+			ret = -ETIMEDOUT;
+			break;
+		}
+		ret = 0;
+
+		/*
+		 * Poll if the IMU returns error (i.e busy), break if the IMU
+		 * returns OK or if the serial communication broke
+		 */
+		if (priv->cmd_status <= 0)
+			break;
+	}
+
+exit:
+	if (ret)
+		return ret;
+	if (priv->cmd_status == STATUS_CRIT)
+		return -EIO;
+	if (priv->cmd_status == STATUS_FAIL)
+		return -EINVAL;
+	return 0;
+}
+
+static int bno055_sl_write_reg(void *context, const void *data, size_t count)
+{
+	int ret;
+	int reg;
+	u8 *write_data = (u8 *)data + 1;
+	struct bno055_sl_priv *priv = context;
+
+	if (count < 2) {
+		dev_err(&priv->serdev->dev, "Invalid write count %zu", count);
+		return -EINVAL;
+	}
+
+	reg = ((u8 *)data)[0];
+	dev_dbg(&priv->serdev->dev, "wr reg 0x%x = 0x%x", reg, ((u8 *)data)[1]);
+	ret = bno_sl_send_cmd(priv, 0, reg, count - 1, write_data);
+
+	return ret;
+}
+
+static int bno055_sl_read_reg(void *context,
+			      const void *reg, size_t reg_size,
+			      void *val, size_t val_size)
+{
+	int ret;
+	int reg_addr;
+	struct bno055_sl_priv *priv = context;
+
+	if (val_size > 128) {
+		dev_err(&priv->serdev->dev, "Invalid read valsize %d",
+			val_size);
+		return -EINVAL;
+	}
+
+	reg_addr = ((u8 *)reg)[0];
+	dev_dbg(&priv->serdev->dev, "rd reg 0x%x (len %d)", reg_addr, val_size);
+	mutex_lock(&priv->lock);
+	priv->expected_data_len = val_size;
+	priv->response_buf = val;
+	mutex_unlock(&priv->lock);
+
+	ret = bno_sl_send_cmd(priv, 1, reg_addr, val_size, NULL);
+
+	mutex_lock(&priv->lock);
+	priv->response_buf = NULL;
+	mutex_unlock(&priv->lock);
+
+	return ret;
+}
+
+/*
+ * Handler for received data; this is called from the reicever callback whenever
+ * it got some packet from the serial bus. The status tell us whether the
+ * packet is valid (i.e. header ok && received payload len consistent wrt the
+ * header). It's now our responsability to check whether this is what we
+ * expected, of whether we got some unexpected, yet valid, packet.
+ */
+static void bno055_sl_handle_rx(struct bno055_sl_priv *priv, int status)
+{
+	mutex_lock(&priv->lock);
+	switch (priv->expect_response) {
+	case CMD_NONE:
+		dev_warn(&priv->serdev->dev, "received unexpected, yet valid, data from sensor");
+		mutex_unlock(&priv->lock);
+		return;
+
+	case CMD_READ:
+		priv->cmd_status = status;
+		if (status == STATUS_OK &&
+		    priv->rx.databuf_count != priv->expected_data_len) {
+			/*
+			 * If we got here, then the lower layer serial protocol
+			 * seems consistent with itself; if we got an unexpected
+			 * amount of data then signal it as a non critical error
+			 */
+			priv->cmd_status = STATUS_FAIL;
+			dev_warn(&priv->serdev->dev, "received an unexpected amount of, yet valid, data from sensor");
+		}
+		break;
+
+	case CMD_WRITE:
+		priv->cmd_status = status;
+		break;
+	}
+
+	priv->expect_response = CMD_NONE;
+	complete(&priv->cmd_complete);
+	mutex_unlock(&priv->lock);
+}
+
+/*
+ * Serdev receiver FSM. This tracks the serial communication and parse the
+ * header. It pushes packets to bno055_sl_handle_rx(), eventually communicating
+ * failures (i.e. malformed packets).
+ * Ideally it doesn't know anything about upper layer (i.e. if this is the
+ * packet we were really expecting), but since we copies the payload into the
+ * receiver buffer (that is not valid when i.e. we don't expect data), we
+ * snoop a bit in the upper layer..
+ * Also, we assume to RX one pkt per time (i.e. the HW doesn't send anything
+ * unless we require to AND we don't queue more than one request per time).
+ */
+static int bno055_sl_receive_buf(struct serdev_device *serdev,
+				 const unsigned char *buf, size_t size)
+{
+	int status;
+	struct bno055_sl_priv *priv = serdev_device_get_drvdata(serdev);
+	int _size = size;
+
+	if (size == 0)
+		return 0;
+
+	dev_dbg(&priv->serdev->dev, "recv (len %zu): %*ph ", size, size, buf);
+	switch (priv->rx.state) {
+	case RX_IDLE:
+		/*
+		 * New packet.
+		 * Check for its 1st byte, that identifies the pkt type.
+		 */
+		if (buf[0] != 0xEE && buf[0] != 0xBB) {
+			dev_err(&priv->serdev->dev,
+				"Invalid packet start %x", buf[0]);
+			bno055_sl_handle_rx(priv, STATUS_CRIT);
+			break;
+		}
+		priv->rx.type = buf[0];
+		priv->rx.state = RX_START;
+		size--;
+		buf++;
+		priv->rx.databuf_count = 0;
+		fallthrough;
+
+	case RX_START:
+		/*
+		 * Packet RX in progress, we expect either 1-byte len or 1-byte
+		 * status depending by the packet type.
+		 */
+		if (size == 0)
+			break;
+
+		if (priv->rx.type == 0xEE) {
+			if (size > 1) {
+				dev_err(&priv->serdev->dev, "EE pkt. Extra data received");
+				status = STATUS_CRIT;
+
+			} else {
+				status = (buf[0] == 1) ? STATUS_OK : STATUS_FAIL;
+			}
+			bno055_sl_handle_rx(priv, status);
+			priv->rx.state = RX_IDLE;
+			break;
+
+		} else {
+			/*priv->rx.type == 0xBB */
+			priv->rx.state = RX_DATA;
+			priv->rx.expected_len = buf[0];
+			size--;
+			buf++;
+		}
+		fallthrough;
+
+	case RX_DATA:
+		/* Header parsed; now receiving packet data payload */
+		if (size == 0)
+			break;
+
+		if (priv->rx.databuf_count + size > priv->rx.expected_len) {
+			/*
+			 * This is a inconsistency in serial protocol, we lost
+			 * sync and we don't know how to handle further data
+			 */
+			dev_err(&priv->serdev->dev, "BB pkt. Extra data received");
+			bno055_sl_handle_rx(priv, STATUS_CRIT);
+			priv->rx.state = RX_IDLE;
+			break;
+		}
+
+		mutex_lock(&priv->lock);
+		/*
+		 * NULL e.g. when read cmd is stale or when no read cmd is
+		 * actually pending.
+		 */
+		if (priv->response_buf &&
+		    /*
+		     * Snoop on the upper layer protocol stuff to make sure not
+		     * to write to an invalid memory. Apart for this, let's the
+		     * upper layer manage any inconsistency wrt expected data
+		     * len (as long as the serial protocol is consistent wrt
+		     * itself (i.e. response header is consistent with received
+		     * response len.
+		     */
+		    (priv->rx.databuf_count + size <= priv->expected_data_len))
+			memcpy(priv->response_buf + priv->rx.databuf_count,
+			       buf, size);
+		mutex_unlock(&priv->lock);
+
+		priv->rx.databuf_count += size;
+
+		/*
+		 * Reached expected len advertised by the IMU for the current
+		 * packet. Pass it to the upper layer (for us it is just valid).
+		 */
+		if (priv->rx.databuf_count == priv->rx.expected_len) {
+			bno055_sl_handle_rx(priv, STATUS_OK);
+			priv->rx.state = RX_IDLE;
+		}
+		break;
+	}
+
+	return _size;
+}
+
+static const struct serdev_device_ops bno055_sl_serdev_ops = {
+	.receive_buf = bno055_sl_receive_buf,
+	.write_wakeup = serdev_device_write_wakeup,
+};
+
+static struct regmap_bus bno055_sl_regmap_bus = {
+	.write = bno055_sl_write_reg,
+	.read = bno055_sl_read_reg,
+};
+
+static int bno055_sl_probe(struct serdev_device *serdev)
+{
+	struct bno055_sl_priv *priv;
+	struct regmap *regmap;
+	int ret;
+
+	priv = devm_kzalloc(&serdev->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	serdev_device_set_drvdata(serdev, priv);
+	priv->serdev = serdev;
+	mutex_init(&priv->lock);
+	init_completion(&priv->cmd_complete);
+
+	serdev_device_set_client_ops(serdev, &bno055_sl_serdev_ops);
+	ret = devm_serdev_device_open(&serdev->dev, serdev);
+	if (ret)
+		return ret;
+
+	if (serdev_device_set_baudrate(serdev, 115200) != 115200) {
+		dev_err(&serdev->dev, "Cannot set required baud rate");
+		return -EIO;
+	}
+
+	ret = serdev_device_set_parity(serdev, SERDEV_PARITY_NONE);
+	if (ret) {
+		dev_err(&serdev->dev, "Cannot set required parity setting");
+		return ret;
+	}
+	serdev_device_set_flow_control(serdev, false);
+
+	regmap = devm_regmap_init(&serdev->dev, &bno055_sl_regmap_bus,
+				  priv, &bno055_regmap_config);
+	if (IS_ERR(regmap)) {
+		dev_err(&serdev->dev, "Unable to init register map");
+		return PTR_ERR(regmap);
+	}
+
+	return bno055_probe(&serdev->dev, regmap,
+			    BNO055_SL_XFER_BURST_BREAK_THRESHOLD);
+}
+
+static const struct of_device_id bno055_sl_of_match[] = {
+	{ .compatible = "bosch,bno055" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, bno055_sl_of_match);
+
+static struct serdev_device_driver bno055_sl_driver = {
+	.driver = {
+		.name = "bno055-sl",
+		.of_match_table = bno055_sl_of_match,
+	},
+	.probe = bno055_sl_probe,
+};
+module_serdev_device_driver(bno055_sl_driver);
+
+MODULE_AUTHOR("Andrea Merello <andrea.merello@iit.it>");
+MODULE_DESCRIPTION("Bosch BNO055 serdev interface");
+MODULE_LICENSE("GPL v2");
-- 
2.17.1