diff --git a/Documentation/i2c/slave-testunit-backend.rst b/Documentation/i2c/slave-testunit-backend.rst
new file mode 100644
index 0000000000000000000000000000000000000000..f537c62a8a83933eec22012eb34c7324818d0eeb
--- /dev/null
+++ b/Documentation/i2c/slave-testunit-backend.rst
@@ -0,0 +1,66 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+Linux I2C slave testunit backend
+================================
+
+by Wolfram Sang <wsa@sang-engineering.com> in 2020
+
+This backend can be used to trigger test cases for I2C bus masters which
+require a remote device with certain capabilities (and which are usually not so
+easy to obtain). Examples include multi-master testing, and SMBus Host Notify
+testing. For some tests, the I2C slave controller must be able to switch
+between master and slave mode because it needs to send data, too.
+
+Note that this is a device for testing and debugging. It should not be enabled
+in a production build. And while there is some versioning and we try hard to
+keep backward compatibility, there is no stable ABI guaranteed!
+
+Instantiating the device is regular. Example for bus 0, address 0x30:
+
+# echo "slave-testunit 0x1030" > /sys/bus/i2c/devices/i2c-0/new_device
+
+After that, you will have a write-only device listening. Reads will return an
+8-bit version number. The device consists of 4 8-bit registers and all must be
+written to start a testcase, i.e. you must always write 4 bytes to the device.
+The registers are:
+
+0x00 CMD - which test to trigger
+0x01 DATAL - configuration byte 1 for the test
+0x02 DATAH - configuration byte 2 for the test
+0x03 DELAY - delay in n * 10ms until test is started
+
+Using 'i2cset' from the i2c-tools package, the generic command looks like:
+
+# i2cset -y <bus_num> <testunit_address> <CMD> <DATAL> <DATAH> <DELAY> i
+
+DELAY is a generic parameter which will delay the execution of the test in CMD.
+The commands are described in the following section. An invalid command will
+result in the transfer not being acknowledged.
+
+Commands
+--------
+
+0x00 NOOP (reserved for future use)
+
+0x01 READ_BYTES (also needs master mode)
+ DATAL - address to read data from
+ DATAH - number of bytes to read
+
+This is useful to test if your bus master driver is handling multi-master
+correctly. You can trigger the testunit to read bytes from another device on
+the bus. If the bus master under test also wants to access the bus at the same
+time, the bus will be busy. Example to read 128 bytes from device 0x50 after
+50ms of delay:
+
+# i2cset -y 0 0x30 0x01 0x50 0x80 0x05 i
+
+0x02 SMBUS_HOST_NOTIFY (also needs master mode)
+ DATAL - low byte of the status word to send
+ DATAH - high byte of the status word to send
+
+This test will send an SMBUS_HOST_NOTIFY message to the host. Note that the
+status word is currently ignored in the Linux Kernel. Example to send a
+notification after 10ms:
+
+# i2cset -y 0 0x30 0x02 0x42 0x64 0x01 i
diff --git a/drivers/i2c/Kconfig b/drivers/i2c/Kconfig
index 5449729cdb878e603bfee69b34cfb733b7d1f09a..438905e2a1d0b4c4921a3eb00c9a1cccfd0333e3 100644
--- a/drivers/i2c/Kconfig
+++ b/drivers/i2c/Kconfig
@@ -125,6 +125,14 @@ config I2C_SLAVE_EEPROM
This backend makes Linux behave like an I2C EEPROM. Please read
Documentation/i2c/slave-eeprom-backend.rst for further details.
+config I2C_SLAVE_TESTUNIT
+ tristate "I2C eeprom testunit driver"
+ help
+ This backend can be used to trigger test cases for I2C bus masters
+ which require a remote device with certain capabilities, e.g.
+ multi-master, SMBus Host Notify, etc. Please read
+ Documentation/i2c/slave-testunit-backend.rst for further details.
+
endif
config I2C_DEBUG_CORE
diff --git a/drivers/i2c/Makefile b/drivers/i2c/Makefile
index bed6ba63c98329cb73d6c7cbb45a72962245c793..c1d493dc9bac325dacf10d2a7ec8faadedf5ef32 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -16,5 +16,6 @@ obj-$(CONFIG_I2C_MUX) += i2c-mux.o
obj-y += algos/ busses/ muxes/
obj-$(CONFIG_I2C_STUB) += i2c-stub.o
obj-$(CONFIG_I2C_SLAVE_EEPROM) += i2c-slave-eeprom.o
+obj-$(CONFIG_I2C_SLAVE_TESTUNIT) += i2c-slave-testunit.o
ccflags-$(CONFIG_I2C_DEBUG_CORE) := -DDEBUG
diff --git a/drivers/i2c/i2c-slave-testunit.c b/drivers/i2c/i2c-slave-testunit.c
new file mode 100644
index 0000000000000000000000000000000000000000..c288102de324f189b53c3a9f73b9d0ad7bceca47
--- /dev/null
+++ b/drivers/i2c/i2c-slave-testunit.c
@@ -0,0 +1,175 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * I2C slave mode testunit
+ *
+ * Copyright (C) 2020 by Wolfram Sang, Sang Engineering <wsa@sang-engineering.com>
+ * Copyright (C) 2020 by Renesas Electronics Corporation
+ */
+
+#include <linux/bitops.h>
+#include <linux/i2c.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/of.h>
+#include <linux/slab.h>
+#include <linux/workqueue.h> /* FIXME: is system_long_wq the best choice? */
+
+#define TU_CUR_VERSION 0x01
+
+enum testunit_cmds {
+ TU_CMD_READ_BYTES = 1, /* save 0 for ABORT, RESET or similar */
+ TU_CMD_HOST_NOTIFY,
+ TU_NUM_CMDS
+};
+
+enum testunit_regs {
+ TU_REG_CMD,
+ TU_REG_DATAL,
+ TU_REG_DATAH,
+ TU_REG_DELAY,
+ TU_NUM_REGS
+};
+
+enum testunit_flags {
+ TU_FLAG_IN_PROCESS,
+};
+
+struct testunit_data {
+ unsigned long flags;
+ u8 regs[TU_NUM_REGS];
+ u8 reg_idx;
+ struct i2c_client *client;
+ struct delayed_work worker;
+};
+
+static void i2c_slave_testunit_work(struct work_struct *work)
+{
+ struct testunit_data *tu = container_of(work, struct testunit_data, worker.work);
+ struct i2c_msg msg;
+ u8 msgbuf[256];
+ int ret = 0;
+
+ msg.addr = I2C_CLIENT_END;
+ msg.buf = msgbuf;
+
+ switch (tu->regs[TU_REG_CMD]) {
+ case TU_CMD_READ_BYTES:
+ msg.addr = tu->regs[TU_REG_DATAL];
+ msg.flags = I2C_M_RD;
+ msg.len = tu->regs[TU_REG_DATAH];
+ break;
+
+ case TU_CMD_HOST_NOTIFY:
+ msg.addr = 0x08;
+ msg.flags = 0;
+ msg.len = 3;
+ msgbuf[0] = tu->client->addr;
+ msgbuf[1] = tu->regs[TU_REG_DATAL];
+ msgbuf[2] = tu->regs[TU_REG_DATAH];
+ break;
+
+ default:
+ break;
+ }
+
+ if (msg.addr != I2C_CLIENT_END) {
+ ret = i2c_transfer(tu->client->adapter, &msg, 1);
+ /* convert '0 msgs transferred' to errno */
+ ret = (ret == 0) ? -EIO : ret;
+ }
+
+ if (ret < 0)
+ dev_err(&tu->client->dev, "CMD%02X failed (%d)\n", tu->regs[TU_REG_CMD], ret);
+
+ clear_bit(TU_FLAG_IN_PROCESS, &tu->flags);
+}
+
+static int i2c_slave_testunit_slave_cb(struct i2c_client *client,
+ enum i2c_slave_event event, u8 *val)
+{
+ struct testunit_data *tu = i2c_get_clientdata(client);
+ int ret = 0;
+
+ switch (event) {
+ case I2C_SLAVE_WRITE_RECEIVED:
+ if (test_bit(TU_FLAG_IN_PROCESS, &tu->flags))
+ return -EBUSY;
+
+ if (tu->reg_idx < TU_NUM_REGS)
+ tu->regs[tu->reg_idx] = *val;
+ else
+ ret = -EMSGSIZE;
+
+ if (tu->reg_idx <= TU_NUM_REGS)
+ tu->reg_idx++;
+
+ /* TU_REG_CMD always written at this point */
+ if (tu->regs[TU_REG_CMD] >= TU_NUM_CMDS)
+ ret = -EINVAL;
+
+ break;
+
+ case I2C_SLAVE_STOP:
+ if (tu->reg_idx == TU_NUM_REGS) {
+ set_bit(TU_FLAG_IN_PROCESS, &tu->flags);
+ queue_delayed_work(system_long_wq, &tu->worker,
+ msecs_to_jiffies(10 * tu->regs[TU_REG_DELAY]));
+ }
+ fallthrough;
+
+ case I2C_SLAVE_WRITE_REQUESTED:
+ tu->reg_idx = 0;
+ break;
+
+ case I2C_SLAVE_READ_REQUESTED:
+ case I2C_SLAVE_READ_PROCESSED:
+ *val = TU_CUR_VERSION;
+ break;
+ }
+
+ return ret;
+}
+
+static int i2c_slave_testunit_probe(struct i2c_client *client)
+{
+ struct testunit_data *tu;
+
+ tu = devm_kzalloc(&client->dev, sizeof(struct testunit_data), GFP_KERNEL);
+ if (!tu)
+ return -ENOMEM;
+
+ tu->client = client;
+ i2c_set_clientdata(client, tu);
+ INIT_DELAYED_WORK(&tu->worker, i2c_slave_testunit_work);
+
+ return i2c_slave_register(client, i2c_slave_testunit_slave_cb);
+};
+
+static int i2c_slave_testunit_remove(struct i2c_client *client)
+{
+ struct testunit_data *tu = i2c_get_clientdata(client);
+
+ cancel_delayed_work_sync(&tu->worker);
+ i2c_slave_unregister(client);
+ return 0;
+}
+
+static const struct i2c_device_id i2c_slave_testunit_id[] = {
+ { "slave-testunit", 0 },
+ { }
+};
+MODULE_DEVICE_TABLE(i2c, i2c_slave_testunit_id);
+
+static struct i2c_driver i2c_slave_testunit_driver = {
+ .driver = {
+ .name = "i2c-slave-testunit",
+ },
+ .probe_new = i2c_slave_testunit_probe,
+ .remove = i2c_slave_testunit_remove,
+ .id_table = i2c_slave_testunit_id,
+};
+module_i2c_driver(i2c_slave_testunit_driver);
+
+MODULE_AUTHOR("Wolfram Sang <wsa@sang-engineering.com>");
+MODULE_DESCRIPTION("I2C slave mode test unit");
+MODULE_LICENSE("GPL v2");