drivers/char/tpm/tpm_tis_i2c.c - linux/kernel/git/torvalds/linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (c) 2014-2021 Nuvoton Technology corporation
  * Copyright (C) 2019-2022 Infineon Technologies AG
  *
  * This device driver implements the TPM interface as defined in the TCG PC
  * Client Platform TPM Profile (PTP) Specification for TPM 2.0 v1.04
  * Revision 14.
  *
  * It is based on the tpm_tis_spi device driver.
  */

 #include <linux/i2c.h>
 #include <linux/crc-ccitt.h>
 #include "tpm_tis_core.h"

 /* TPM registers */
 #define TPM_I2C_LOC_SEL 0x00
 #define TPM_I2C_ACCESS 0x04
 #define TPM_I2C_INTERFACE_CAPABILITY 0x30
 #define TPM_I2C_DEVICE_ADDRESS 0x38
 #define TPM_I2C_DATA_CSUM_ENABLE 0x40
 #define TPM_DATA_CSUM 0x44
 #define TPM_I2C_DID_VID 0x48
 #define TPM_I2C_RID 0x4C

 /* TIS-compatible register address to avoid clash with TPM_ACCESS (0x00) */
 #define TPM_LOC_SEL 0x0FFF

 /* Mask to extract the I2C register from TIS register addresses */
 #define TPM_TIS_REGISTER_MASK 0x0FFF

 /* Default Guard Time of 250µs until interface capability register is read */
 #define GUARD_TIME_DEFAULT_MIN 250
 #define GUARD_TIME_DEFAULT_MAX 300

 /* Guard Time of 250µs after I2C slave NACK */
 #define GUARD_TIME_ERR_MIN 250
 #define GUARD_TIME_ERR_MAX 300

 /* Guard Time bit masks; SR is repeated start, RW is read then write, etc. */
 #define TPM_GUARD_TIME_SR_MASK 0x40000000
 #define TPM_GUARD_TIME_RR_MASK 0x00100000
 #define TPM_GUARD_TIME_RW_MASK 0x00080000
 #define TPM_GUARD_TIME_WR_MASK 0x00040000
 #define TPM_GUARD_TIME_WW_MASK 0x00020000
 #define TPM_GUARD_TIME_MIN_MASK 0x0001FE00
 #define TPM_GUARD_TIME_MIN_SHIFT 9

 /* Masks with bits that must be read zero */
 #define TPM_ACCESS_READ_ZERO 0x48
 #define TPM_INT_ENABLE_ZERO 0x7FFFFF6
 #define TPM_STS_READ_ZERO 0x23
 #define TPM_INTF_CAPABILITY_ZERO 0x0FFFF000
 #define TPM_I2C_INTERFACE_CAPABILITY_ZERO 0x80000000

 struct tpm_tis_i2c_phy {
 	struct tpm_tis_data priv;
 	struct i2c_client *i2c_client;
 	bool guard_time_read;
 	bool guard_time_write;
 	u16 guard_time_min;
 	u16 guard_time_max;
 	u8 *io_buf;
 };

 static inline struct tpm_tis_i2c_phy *
 to_tpm_tis_i2c_phy(struct tpm_tis_data *data)
 {
 	return container_of(data, struct tpm_tis_i2c_phy, priv);
 }

 /*
  * tpm_tis_core uses the register addresses as defined in Table 19 "Allocation
  * of Register Space for FIFO TPM Access" of the TCG PC Client PTP
  * Specification. In order for this code to work together with tpm_tis_core,
  * those addresses need to mapped to the registers defined for I2C TPMs in
  * Table 51 "I2C-TPM Register Overview".
  *
  * For most addresses this can be done by simply stripping off the locality
  * information from the address. A few addresses need to be mapped explicitly,
  * since the corresponding I2C registers have been moved around. TPM_LOC_SEL is
  * only defined for I2C TPMs and is also mapped explicitly here to distinguish
  * it from TPM_ACCESS(0).
  *
  * Locality information is ignored, since this driver assumes exclusive access
  * to the TPM and always uses locality 0.
  */
 static u8 tpm_tis_i2c_address_to_register(u32 addr)
 {
 	addr &= TPM_TIS_REGISTER_MASK;

 	switch (addr) {
 	case TPM_ACCESS(0):
 		return TPM_I2C_ACCESS;
 	case TPM_LOC_SEL:
 		return TPM_I2C_LOC_SEL;
 	case TPM_DID_VID(0):
 		return TPM_I2C_DID_VID;
 	case TPM_RID(0):
 		return TPM_I2C_RID;
 	default:
 		return addr;
 	}
 }

 static int tpm_tis_i2c_retry_transfer_until_ack(struct tpm_tis_data *data,
 						struct i2c_msg *msg)
 {
 	struct tpm_tis_i2c_phy *phy = to_tpm_tis_i2c_phy(data);
 	bool guard_time;
 	int i = 0;
 	int ret;

 	if (msg->flags & I2C_M_RD)
 		guard_time = phy->guard_time_read;
 	else
 		guard_time = phy->guard_time_write;

 	do {
 		ret = i2c_transfer(phy->i2c_client->adapter, msg, 1);
 		if (ret < 0)
 			usleep_range(GUARD_TIME_ERR_MIN, GUARD_TIME_ERR_MAX);
 		else if (guard_time)
 			usleep_range(phy->guard_time_min, phy->guard_time_max);
 		/* retry on TPM NACK */
 	} while (ret < 0 && i++ < TPM_RETRY);

 	return ret;
 }

 /* Check that bits which must be read zero are not set */
 static int tpm_tis_i2c_sanity_check_read(u8 reg, u16 len, u8 *buf)
 {
 	u32 zero_mask;
 	u32 value;

 	switch (len) {
 	case sizeof(u8):
 		value = buf[0];
 		break;
 	case sizeof(u16):
 		value = le16_to_cpup((__le16 *)buf);
 		break;
 	case sizeof(u32):
 		value = le32_to_cpup((__le32 *)buf);
 		break;
 	default:
 		/* unknown length, skip check */
 		return 0;
 	}

 	switch (reg) {
 	case TPM_I2C_ACCESS:
 		zero_mask = TPM_ACCESS_READ_ZERO;
 		break;
 	case TPM_INT_ENABLE(0) & TPM_TIS_REGISTER_MASK:
 		zero_mask = TPM_INT_ENABLE_ZERO;
 		break;
 	case TPM_STS(0) & TPM_TIS_REGISTER_MASK:
 		zero_mask = TPM_STS_READ_ZERO;
 		break;
 	case TPM_INTF_CAPS(0) & TPM_TIS_REGISTER_MASK:
 		zero_mask = TPM_INTF_CAPABILITY_ZERO;
 		break;
 	case TPM_I2C_INTERFACE_CAPABILITY:
 		zero_mask = TPM_I2C_INTERFACE_CAPABILITY_ZERO;
 		break;
 	default:
 		/* unknown register, skip check */
 		return 0;
 	}

 	if (unlikely((value & zero_mask) != 0x00)) {
 		pr_debug("TPM I2C read of register 0x%02x failed sanity check: 0x%x\n", reg, value);
 		return -EIO;
 	}

 	return 0;
 }

 static int tpm_tis_i2c_read_bytes(struct tpm_tis_data *data, u32 addr, u16 len,
 				  u8 *result, enum tpm_tis_io_mode io_mode)
 {
 	struct tpm_tis_i2c_phy *phy = to_tpm_tis_i2c_phy(data);
 	struct i2c_msg msg = { .addr = phy->i2c_client->addr };
 	u8 reg = tpm_tis_i2c_address_to_register(addr);
 	int i;
 	int ret;

 	for (i = 0; i < TPM_RETRY; i++) {
 		/* write register */
 		msg.len = sizeof(reg);
 		msg.buf = &reg;
 		msg.flags = 0;
 		ret = tpm_tis_i2c_retry_transfer_until_ack(data, &msg);
 		if (ret < 0)
 			return ret;

 		/* read data */
 		msg.buf = result;
 		msg.len = len;
 		msg.flags = I2C_M_RD;
 		ret = tpm_tis_i2c_retry_transfer_until_ack(data, &msg);
 		if (ret < 0)
 			return ret;

 		ret = tpm_tis_i2c_sanity_check_read(reg, len, result);
 		if (ret == 0)
 			return 0;

 		usleep_range(GUARD_TIME_ERR_MIN, GUARD_TIME_ERR_MAX);
 	}

 	return ret;
 }

 static int tpm_tis_i2c_write_bytes(struct tpm_tis_data *data, u32 addr, u16 len,
 				   const u8 *value,
 				   enum tpm_tis_io_mode io_mode)
 {
 	struct tpm_tis_i2c_phy *phy = to_tpm_tis_i2c_phy(data);
 	struct i2c_msg msg = { .addr = phy->i2c_client->addr };
 	u8 reg = tpm_tis_i2c_address_to_register(addr);
 	int ret;

 	if (len > TPM_BUFSIZE - 1)
 		return -EIO;

 	/* write register and data in one go */
 	phy->io_buf[0] = reg;
 	memcpy(phy->io_buf + sizeof(reg), value, len);

 	msg.len = sizeof(reg) + len;
 	msg.buf = phy->io_buf;
 	ret = tpm_tis_i2c_retry_transfer_until_ack(data, &msg);
 	if (ret < 0)
 		return ret;

 	return 0;
 }

 static int tpm_tis_i2c_verify_crc(struct tpm_tis_data *data, size_t len,
 				  const u8 *value)
 {
 	u16 crc_tpm, crc_host;
 	int rc;

 	rc = tpm_tis_read16(data, TPM_DATA_CSUM, &crc_tpm);
 	if (rc < 0)
 		return rc;

 	/* reflect crc result, regardless of host endianness */
 	crc_host = swab16(crc_ccitt(0, value, len));
 	if (crc_tpm != crc_host)
 		return -EIO;

 	return 0;
 }

 /*
  * Guard Time:
  * After each I2C operation, the TPM might require the master to wait.
  * The time period is vendor-specific and must be read from the
  * TPM_I2C_INTERFACE_CAPABILITY register.
  *
  * Before the Guard Time is read (or after the TPM failed to send an I2C NACK),
  * a Guard Time of 250µs applies.
  *
  * Various flags in the same register indicate if a guard time is needed:
  *  - SR: <I2C read with repeated start> <guard time> <I2C read>
  *  - RR: <I2C read> <guard time> <I2C read>
  *  - RW: <I2C read> <guard time> <I2C write>
  *  - WR: <I2C write> <guard time> <I2C read>
  *  - WW: <I2C write> <guard time> <I2C write>
  *
  * See TCG PC Client PTP Specification v1.04, 8.1.10 GUARD_TIME
  */
 static int tpm_tis_i2c_init_guard_time(struct tpm_tis_i2c_phy *phy)
 {
 	u32 i2c_caps;
 	int ret;

 	phy->guard_time_read = true;
 	phy->guard_time_write = true;
 	phy->guard_time_min = GUARD_TIME_DEFAULT_MIN;
 	phy->guard_time_max = GUARD_TIME_DEFAULT_MAX;

 	ret = tpm_tis_i2c_read_bytes(&phy->priv, TPM_I2C_INTERFACE_CAPABILITY,
 				     sizeof(i2c_caps), (u8 *)&i2c_caps,
 				     TPM_TIS_PHYS_32);
 	if (ret)
 		return ret;

 	phy->guard_time_read = (i2c_caps & TPM_GUARD_TIME_RR_MASK) ||
 			       (i2c_caps & TPM_GUARD_TIME_RW_MASK);
 	phy->guard_time_write = (i2c_caps & TPM_GUARD_TIME_WR_MASK) ||
 				(i2c_caps & TPM_GUARD_TIME_WW_MASK);
 	phy->guard_time_min = (i2c_caps & TPM_GUARD_TIME_MIN_MASK) >>
 			      TPM_GUARD_TIME_MIN_SHIFT;
 	/* guard_time_max = guard_time_min * 1.2 */
 	phy->guard_time_max = phy->guard_time_min + phy->guard_time_min / 5;

 	return 0;
 }

 static SIMPLE_DEV_PM_OPS(tpm_tis_pm, tpm_pm_suspend, tpm_tis_resume);

 static const struct tpm_tis_phy_ops tpm_i2c_phy_ops = {
 	.read_bytes = tpm_tis_i2c_read_bytes,
 	.write_bytes = tpm_tis_i2c_write_bytes,
 	.verify_crc = tpm_tis_i2c_verify_crc,
 };

 static int tpm_tis_i2c_probe(struct i2c_client *dev,
 			     const struct i2c_device_id *id)
 {
 	struct tpm_tis_i2c_phy *phy;
 	const u8 crc_enable = 1;
 	const u8 locality = 0;
 	int ret;

 	phy = devm_kzalloc(&dev->dev, sizeof(struct tpm_tis_i2c_phy),
 			   GFP_KERNEL);
 	if (!phy)
 		return -ENOMEM;

 	phy->io_buf = devm_kzalloc(&dev->dev, TPM_BUFSIZE, GFP_KERNEL);
 	if (!phy->io_buf)
 		return -ENOMEM;

 	phy->i2c_client = dev;

 	/* must precede all communication with the tpm */
 	ret = tpm_tis_i2c_init_guard_time(phy);
 	if (ret)
 		return ret;

 	ret = tpm_tis_i2c_write_bytes(&phy->priv, TPM_LOC_SEL, sizeof(locality),
 				      &locality, TPM_TIS_PHYS_8);
 	if (ret)
 		return ret;

 	ret = tpm_tis_i2c_write_bytes(&phy->priv, TPM_I2C_DATA_CSUM_ENABLE,
 				      sizeof(crc_enable), &crc_enable,
 				      TPM_TIS_PHYS_8);
 	if (ret)
 		return ret;

 	return tpm_tis_core_init(&dev->dev, &phy->priv, -1, &tpm_i2c_phy_ops,
 				 NULL);
 }

 static int tpm_tis_i2c_remove(struct i2c_client *client)
 {
 	struct tpm_chip *chip = i2c_get_clientdata(client);

 	tpm_chip_unregister(chip);
 	tpm_tis_remove(chip);
 	return 0;
 }

 static const struct i2c_device_id tpm_tis_i2c_id[] = {
 	{ "tpm_tis_i2c", 0 },
 	{}
 };
 MODULE_DEVICE_TABLE(i2c, tpm_tis_i2c_id);

 #ifdef CONFIG_OF
 static const struct of_device_id of_tis_i2c_match[] = {
 	{ .compatible = "infineon,slb9673", },
 	{}
 };
 MODULE_DEVICE_TABLE(of, of_tis_i2c_match);
 #endif

 static struct i2c_driver tpm_tis_i2c_driver = {
 	.driver = {
 		.name = "tpm_tis_i2c",
 		.pm = &tpm_tis_pm,
 		.of_match_table = of_match_ptr(of_tis_i2c_match),
 	},
 	.probe = tpm_tis_i2c_probe,
 	.remove = tpm_tis_i2c_remove,
 	.id_table = tpm_tis_i2c_id,
 };
 module_i2c_driver(tpm_tis_i2c_driver);

 MODULE_DESCRIPTION("TPM Driver for native I2C access");
 MODULE_LICENSE("GPL");
	// SPDX-License-Identifier: GPL-2.0
	/*
	* Copyright (c) 2014-2021 Nuvoton Technology corporation
	* Copyright (C) 2019-2022 Infineon Technologies AG
	*
	* This device driver implements the TPM interface as defined in the TCG PC
	* Client Platform TPM Profile (PTP) Specification for TPM 2.0 v1.04
	* Revision 14.
	*
	* It is based on the tpm_tis_spi device driver.
	*/

	#include <linux/i2c.h>
	#include <linux/crc-ccitt.h>
	#include "tpm_tis_core.h"

	/* TPM registers */
	#define TPM_I2C_LOC_SEL 0x00
	#define TPM_I2C_ACCESS 0x04
	#define TPM_I2C_INTERFACE_CAPABILITY 0x30
	#define TPM_I2C_DEVICE_ADDRESS 0x38
	#define TPM_I2C_DATA_CSUM_ENABLE 0x40
	#define TPM_DATA_CSUM 0x44
	#define TPM_I2C_DID_VID 0x48
	#define TPM_I2C_RID 0x4C

	/* TIS-compatible register address to avoid clash with TPM_ACCESS (0x00) */
	#define TPM_LOC_SEL 0x0FFF

	/* Mask to extract the I2C register from TIS register addresses */
	#define TPM_TIS_REGISTER_MASK 0x0FFF

	/* Default Guard Time of 250µs until interface capability register is read */
	#define GUARD_TIME_DEFAULT_MIN 250
	#define GUARD_TIME_DEFAULT_MAX 300

	/* Guard Time of 250µs after I2C slave NACK */
	#define GUARD_TIME_ERR_MIN 250
	#define GUARD_TIME_ERR_MAX 300

	/* Guard Time bit masks; SR is repeated start, RW is read then write, etc. */
	#define TPM_GUARD_TIME_SR_MASK 0x40000000
	#define TPM_GUARD_TIME_RR_MASK 0x00100000
	#define TPM_GUARD_TIME_RW_MASK 0x00080000
	#define TPM_GUARD_TIME_WR_MASK 0x00040000
	#define TPM_GUARD_TIME_WW_MASK 0x00020000
	#define TPM_GUARD_TIME_MIN_MASK 0x0001FE00
	#define TPM_GUARD_TIME_MIN_SHIFT 9

	/* Masks with bits that must be read zero */
	#define TPM_ACCESS_READ_ZERO 0x48
	#define TPM_INT_ENABLE_ZERO 0x7FFFFF6
	#define TPM_STS_READ_ZERO 0x23
	#define TPM_INTF_CAPABILITY_ZERO 0x0FFFF000
	#define TPM_I2C_INTERFACE_CAPABILITY_ZERO 0x80000000

	struct tpm_tis_i2c_phy {
	struct tpm_tis_data priv;
	struct i2c_client *i2c_client;
	bool guard_time_read;
	bool guard_time_write;
	u16 guard_time_min;
	u16 guard_time_max;
	u8 *io_buf;
	};

	static inline struct tpm_tis_i2c_phy *
	to_tpm_tis_i2c_phy(struct tpm_tis_data *data)
	{
	return container_of(data, struct tpm_tis_i2c_phy, priv);
	}

	/*
	* tpm_tis_core uses the register addresses as defined in Table 19 "Allocation
	* of Register Space for FIFO TPM Access" of the TCG PC Client PTP
	* Specification. In order for this code to work together with tpm_tis_core,
	* those addresses need to mapped to the registers defined for I2C TPMs in
	* Table 51 "I2C-TPM Register Overview".
	*
	* For most addresses this can be done by simply stripping off the locality
	* information from the address. A few addresses need to be mapped explicitly,
	* since the corresponding I2C registers have been moved around. TPM_LOC_SEL is
	* only defined for I2C TPMs and is also mapped explicitly here to distinguish
	* it from TPM_ACCESS(0).
	*
	* Locality information is ignored, since this driver assumes exclusive access
	* to the TPM and always uses locality 0.
	*/
	static u8 tpm_tis_i2c_address_to_register(u32 addr)
	{
	addr &= TPM_TIS_REGISTER_MASK;

	switch (addr) {
	case TPM_ACCESS(0):
	return TPM_I2C_ACCESS;
	case TPM_LOC_SEL:
	return TPM_I2C_LOC_SEL;
	case TPM_DID_VID(0):
	return TPM_I2C_DID_VID;
	case TPM_RID(0):
	return TPM_I2C_RID;
	default:
	return addr;
	}
	}

	static int tpm_tis_i2c_retry_transfer_until_ack(struct tpm_tis_data *data,
	struct i2c_msg *msg)
	{
	struct tpm_tis_i2c_phy *phy = to_tpm_tis_i2c_phy(data);
	bool guard_time;
	int i = 0;
	int ret;

	if (msg->flags & I2C_M_RD)
	guard_time = phy->guard_time_read;
	else
	guard_time = phy->guard_time_write;

	do {
	ret = i2c_transfer(phy->i2c_client->adapter, msg, 1);
	if (ret < 0)
	usleep_range(GUARD_TIME_ERR_MIN, GUARD_TIME_ERR_MAX);
	else if (guard_time)
	usleep_range(phy->guard_time_min, phy->guard_time_max);
	/* retry on TPM NACK */
	} while (ret < 0 && i++ < TPM_RETRY);

	return ret;
	}

	/* Check that bits which must be read zero are not set */
	static int tpm_tis_i2c_sanity_check_read(u8 reg, u16 len, u8 *buf)
	{
	u32 zero_mask;
	u32 value;

	switch (len) {
	case sizeof(u8):
	value = buf[0];
	break;
	case sizeof(u16):
	value = le16_to_cpup((__le16 *)buf);
	break;
	case sizeof(u32):
	value = le32_to_cpup((__le32 *)buf);
	break;
	default:
	/* unknown length, skip check */
	return 0;
	}

	switch (reg) {
	case TPM_I2C_ACCESS:
	zero_mask = TPM_ACCESS_READ_ZERO;
	break;
	case TPM_INT_ENABLE(0) & TPM_TIS_REGISTER_MASK:
	zero_mask = TPM_INT_ENABLE_ZERO;
	break;
	case TPM_STS(0) & TPM_TIS_REGISTER_MASK:
	zero_mask = TPM_STS_READ_ZERO;
	break;
	case TPM_INTF_CAPS(0) & TPM_TIS_REGISTER_MASK:
	zero_mask = TPM_INTF_CAPABILITY_ZERO;
	break;
	case TPM_I2C_INTERFACE_CAPABILITY:
	zero_mask = TPM_I2C_INTERFACE_CAPABILITY_ZERO;
	break;
	default:
	/* unknown register, skip check */
	return 0;
	}

	if (unlikely((value & zero_mask) != 0x00)) {
	pr_debug("TPM I2C read of register 0x%02x failed sanity check: 0x%x\n", reg, value);
	return -EIO;
	}

	return 0;
	}

	static int tpm_tis_i2c_read_bytes(struct tpm_tis_data *data, u32 addr, u16 len,
	u8 *result, enum tpm_tis_io_mode io_mode)
	{
	struct tpm_tis_i2c_phy *phy = to_tpm_tis_i2c_phy(data);
	struct i2c_msg msg = { .addr = phy->i2c_client->addr };
	u8 reg = tpm_tis_i2c_address_to_register(addr);
	int i;
	int ret;

	for (i = 0; i < TPM_RETRY; i++) {
	/* write register */
	msg.len = sizeof(reg);
	msg.buf = ®
	msg.flags = 0;
	ret = tpm_tis_i2c_retry_transfer_until_ack(data, &msg);
	if (ret < 0)
	return ret;

	/* read data */
	msg.buf = result;
	msg.len = len;
	msg.flags = I2C_M_RD;
	ret = tpm_tis_i2c_retry_transfer_until_ack(data, &msg);
	if (ret < 0)
	return ret;

	ret = tpm_tis_i2c_sanity_check_read(reg, len, result);
	if (ret == 0)
	return 0;

	usleep_range(GUARD_TIME_ERR_MIN, GUARD_TIME_ERR_MAX);
	}

	return ret;
	}

	static int tpm_tis_i2c_write_bytes(struct tpm_tis_data *data, u32 addr, u16 len,
	const u8 *value,
	enum tpm_tis_io_mode io_mode)
	{
	struct tpm_tis_i2c_phy *phy = to_tpm_tis_i2c_phy(data);
	struct i2c_msg msg = { .addr = phy->i2c_client->addr };
	u8 reg = tpm_tis_i2c_address_to_register(addr);
	int ret;

	if (len > TPM_BUFSIZE - 1)
	return -EIO;

	/* write register and data in one go */
	phy->io_buf[0] = reg;
	memcpy(phy->io_buf + sizeof(reg), value, len);

	msg.len = sizeof(reg) + len;
	msg.buf = phy->io_buf;
	ret = tpm_tis_i2c_retry_transfer_until_ack(data, &msg);
	if (ret < 0)
	return ret;

	return 0;
	}

	static int tpm_tis_i2c_verify_crc(struct tpm_tis_data *data, size_t len,
	const u8 *value)
	{
	u16 crc_tpm, crc_host;
	int rc;

	rc = tpm_tis_read16(data, TPM_DATA_CSUM, &crc_tpm);
	if (rc < 0)
	return rc;

	/* reflect crc result, regardless of host endianness */
	crc_host = swab16(crc_ccitt(0, value, len));
	if (crc_tpm != crc_host)
	return -EIO;

	return 0;
	}

	/*
	* Guard Time:
	* After each I2C operation, the TPM might require the master to wait.
	* The time period is vendor-specific and must be read from the
	* TPM_I2C_INTERFACE_CAPABILITY register.
	*
	* Before the Guard Time is read (or after the TPM failed to send an I2C NACK),
	* a Guard Time of 250µs applies.
	*
	* Various flags in the same register indicate if a guard time is needed:
	* - SR: <I2C read with repeated start> <guard time> <I2C read>
	* - RR: <I2C read> <guard time> <I2C read>
	* - RW: <I2C read> <guard time> <I2C write>
	* - WR: <I2C write> <guard time> <I2C read>
	* - WW: <I2C write> <guard time> <I2C write>
	*
	* See TCG PC Client PTP Specification v1.04, 8.1.10 GUARD_TIME
	*/
	static int tpm_tis_i2c_init_guard_time(struct tpm_tis_i2c_phy *phy)
	{
	u32 i2c_caps;
	int ret;

	phy->guard_time_read = true;
	phy->guard_time_write = true;
	phy->guard_time_min = GUARD_TIME_DEFAULT_MIN;
	phy->guard_time_max = GUARD_TIME_DEFAULT_MAX;

	ret = tpm_tis_i2c_read_bytes(&phy->priv, TPM_I2C_INTERFACE_CAPABILITY,
	sizeof(i2c_caps), (u8 *)&i2c_caps,
	TPM_TIS_PHYS_32);
	if (ret)
	return ret;

	phy->guard_time_read = (i2c_caps & TPM_GUARD_TIME_RR_MASK) \|\|
	(i2c_caps & TPM_GUARD_TIME_RW_MASK);
	phy->guard_time_write = (i2c_caps & TPM_GUARD_TIME_WR_MASK) \|\|
	(i2c_caps & TPM_GUARD_TIME_WW_MASK);
	phy->guard_time_min = (i2c_caps & TPM_GUARD_TIME_MIN_MASK) >>
	TPM_GUARD_TIME_MIN_SHIFT;
	/* guard_time_max = guard_time_min * 1.2 */
	phy->guard_time_max = phy->guard_time_min + phy->guard_time_min / 5;

	return 0;
	}

	static SIMPLE_DEV_PM_OPS(tpm_tis_pm, tpm_pm_suspend, tpm_tis_resume);

	static const struct tpm_tis_phy_ops tpm_i2c_phy_ops = {
	.read_bytes = tpm_tis_i2c_read_bytes,
	.write_bytes = tpm_tis_i2c_write_bytes,
	.verify_crc = tpm_tis_i2c_verify_crc,
	};

	static int tpm_tis_i2c_probe(struct i2c_client *dev,
	const struct i2c_device_id *id)
	{
	struct tpm_tis_i2c_phy *phy;
	const u8 crc_enable = 1;
	const u8 locality = 0;
	int ret;

	phy = devm_kzalloc(&dev->dev, sizeof(struct tpm_tis_i2c_phy),
	GFP_KERNEL);
	if (!phy)
	return -ENOMEM;

	phy->io_buf = devm_kzalloc(&dev->dev, TPM_BUFSIZE, GFP_KERNEL);
	if (!phy->io_buf)
	return -ENOMEM;

	phy->i2c_client = dev;

	/* must precede all communication with the tpm */
	ret = tpm_tis_i2c_init_guard_time(phy);
	if (ret)
	return ret;

	ret = tpm_tis_i2c_write_bytes(&phy->priv, TPM_LOC_SEL, sizeof(locality),
	&locality, TPM_TIS_PHYS_8);
	if (ret)
	return ret;

	ret = tpm_tis_i2c_write_bytes(&phy->priv, TPM_I2C_DATA_CSUM_ENABLE,
	sizeof(crc_enable), &crc_enable,
	TPM_TIS_PHYS_8);
	if (ret)
	return ret;

	return tpm_tis_core_init(&dev->dev, &phy->priv, -1, &tpm_i2c_phy_ops,
	NULL);
	}

	static int tpm_tis_i2c_remove(struct i2c_client *client)
	{
	struct tpm_chip *chip = i2c_get_clientdata(client);

	tpm_chip_unregister(chip);
	tpm_tis_remove(chip);
	return 0;
	}

	static const struct i2c_device_id tpm_tis_i2c_id[] = {
	{ "tpm_tis_i2c", 0 },
	{}
	};
	MODULE_DEVICE_TABLE(i2c, tpm_tis_i2c_id);

	#ifdef CONFIG_OF
	static const struct of_device_id of_tis_i2c_match[] = {
	{ .compatible = "infineon,slb9673", },
	{}
	};
	MODULE_DEVICE_TABLE(of, of_tis_i2c_match);
	#endif

	static struct i2c_driver tpm_tis_i2c_driver = {
	.driver = {
	.name = "tpm_tis_i2c",
	.pm = &tpm_tis_pm,
	.of_match_table = of_match_ptr(of_tis_i2c_match),
	},
	.probe = tpm_tis_i2c_probe,
	.remove = tpm_tis_i2c_remove,
	.id_table = tpm_tis_i2c_id,
	};
	module_i2c_driver(tpm_tis_i2c_driver);

	MODULE_DESCRIPTION("TPM Driver for native I2C access");
	MODULE_LICENSE("GPL");