Message-Id: <20220206115939.3091265-3-luca@lucaceresoli.net>
Date:   Sun,  6 Feb 2022 12:59:35 +0100
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     linux-media@...r.kernel.org, linux-i2c@...r.kernel.org
Cc:     Luca Ceresoli <luca@...aceresoli.net>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        Jacopo Mondi <jacopo@...ndi.org>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Peter Rosin <peda@...ntia.se>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        matti.vaittinen@...rohmeurope.com
Subject: [RFCv3 2/6] i2c: add I2C Address Translator (ATR) support

An ATR is a device that looks similar to an i2c-mux: it has an I2C
slave "upstream" port and N master "downstream" ports, and forwards
transactions from upstream to the appropriate downstream port. But is
is different in that the forwarded transaction has a different slave
address. The address used on the upstream bus is called the "alias"
and is (potentially) different from the physical slave address of the
downstream chip.

Add a helper file (just like i2c-mux.c for a mux or switch) to allow
implementing ATR features in a device driver. The helper takes care or
adapter creation/destruction and translates addresses at each transaction.

Signed-off-by: Luca Ceresoli <luca@...aceresoli.net>

---

Changes RFCv2 -> v3:

- fix const-related warnings

Changes RFCv1 -> RFCv2:

 RFCv1 was implemented inside i2c-mux.c and added yet more complexity
 there. RFCv2 creates a new file on its own, i2c-atr.c. Since many ATR
 features are not in a MUX and vice versa, the overlapping is low. This was
 almost a complete rewrite, but for the records here are the main
 differences from the old implementation:

 - change bus description
 - remove I2C_ATR_ARBITRATOR and I2C_ATR_GATE support
 - select() optional
 - rename i2c_atr_alloc -> i2c_atr_new, add i2c_atr_delete, move to bottom
 - lock the ATR, not the adapter or the muxes on the adapter
 - remove parent-locked code
 - remove I2C_MUX_LOCKED flag, now unused
 - remove I2C_ATR_ATR flag (always true)
 - translation in i2c_atr_smbus_xfer too
 - i2c_atr_map_msgs: don't ignore mapping errors
 - always require the "i2c-atr" DT node, no magic
 - remove ACPI support
 - one algo in the atrc, not one per adapter
 - remove unneeded i2c_atr_root_adapter
 - ditch force_nr
 - don't allocate private user memory, just provide a plain userdata pointer
 - consolidate all ops in a single struct, simplify naming
 - remove adapters individually, allocate in atrc->adapter[chan_id]
---
 MAINTAINERS             |   7 +
 drivers/i2c/Kconfig     |   9 +
 drivers/i2c/Makefile    |   1 +
 drivers/i2c/i2c-atr.c   | 557 ++++++++++++++++++++++++++++++++++++++++
 include/linux/i2c-atr.h |  82 ++++++
 5 files changed, 656 insertions(+)
 create mode 100644 drivers/i2c/i2c-atr.c
 create mode 100644 include/linux/i2c-atr.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 69a2935daf6c..7383aec87e4a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8961,6 +8961,13 @@ L:	linux-acpi@...r.kernel.org
 S:	Maintained
 F:	drivers/i2c/i2c-core-acpi.c
 
+I2C ADDRESS TRANSLATOR (ATR)
+M:	Luca Ceresoli <luca@...aceresoli.net>
+L:	linux-i2c@...r.kernel.org
+S:	Maintained
+F:	drivers/i2c/i2c-atr.c
+F:	include/linux/i2c-atr.h
+
 I2C CONTROLLER DRIVER FOR NVIDIA GPU
 M:	Ajay Gupta <ajayg@...dia.com>
 L:	linux-i2c@...r.kernel.org
diff --git a/drivers/i2c/Kconfig b/drivers/i2c/Kconfig
index 438905e2a1d0..c6d1a345ea6d 100644
--- a/drivers/i2c/Kconfig
+++ b/drivers/i2c/Kconfig
@@ -71,6 +71,15 @@ config I2C_MUX
 
 source "drivers/i2c/muxes/Kconfig"
 
+config I2C_ATR
+	tristate "I2C Address Translator (ATR) support"
+	help
+	  Enable support for I2C Address Translator (ATR) chips.
+
+	  An ATR allows accessing multiple I2C busses from a single
+	  physical bus via address translation instead of bus selection as
+	  i2c-muxes do.
+
 config I2C_HELPER_AUTO
 	bool "Autoselect pertinent helper modules"
 	default y
diff --git a/drivers/i2c/Makefile b/drivers/i2c/Makefile
index c1d493dc9bac..3f71ce4711e3 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -13,6 +13,7 @@ i2c-core-$(CONFIG_OF) 		+= i2c-core-of.o
 obj-$(CONFIG_I2C_SMBUS)		+= i2c-smbus.o
 obj-$(CONFIG_I2C_CHARDEV)	+= i2c-dev.o
 obj-$(CONFIG_I2C_MUX)		+= i2c-mux.o
+obj-$(CONFIG_I2C_ATR)		+= i2c-atr.o
 obj-y				+= algos/ busses/ muxes/
 obj-$(CONFIG_I2C_STUB)		+= i2c-stub.o
 obj-$(CONFIG_I2C_SLAVE_EEPROM)	+= i2c-slave-eeprom.o
diff --git a/drivers/i2c/i2c-atr.c b/drivers/i2c/i2c-atr.c
new file mode 100644
index 000000000000..f44a5696f7d8
--- /dev/null
+++ b/drivers/i2c/i2c-atr.c
@@ -0,0 +1,557 @@
+// SPDX-License-Identifier: GPL-2.0
+/**
+ * I2C Address Translator
+ *
+ * Copyright (c) 2019 Luca Ceresoli <luca@...aceresoli.net>
+ *
+ * An I2C Address Translator (ATR) is a device with an I2C slave parent
+ * ("upstream") port and N I2C master child ("downstream") ports, and
+ * forwards transactions from upstream to the appropriate downstream port
+ * with a modified slave address. The address used on the parent bus is
+ * called the "alias" and is (potentially) different from the physical
+ * slave address of the child bus. Address translation is done by the
+ * hardware.
+ *
+ * An ATR looks similar to an i2c-mux except:
+ * - the address on the parent and child busses can be different
+ * - there is normally no need to select the child port; the alias used on
+ *   the parent bus implies it
+ *
+ * The ATR functionality can be provided by a chip with many other
+ * features. This file provides a helper to implement an ATR within your
+ * driver.
+ *
+ * The ATR creates a new I2C "child" adapter on each child bus. Adding
+ * devices on the child bus ends up in invoking the driver code to select
+ * an available alias. Maintaining an appropriate pool of available aliases
+ * and picking one for each new device is up to the driver implementer. The
+ * ATR maintains an table of currently assigned alias and uses it to modify
+ * all I2C transactions directed to devices on the child buses.
+ *
+ * A typical example follows.
+ *
+ * Topology:
+ *
+ *                       Slave X @ 0x10
+ *               .-----.   |
+ *   .-----.     |     |---+---- B
+ *   | CPU |--A--| ATR |
+ *   `-----'     |     |---+---- C
+ *               `-----'   |
+ *                       Slave Y @ 0x10
+ *
+ * Alias table:
+ *
+ *   Client  Alias
+ *   -------------
+ *      X    0x20
+ *      Y    0x30
+ *
+ * Transaction:
+ *
+ *  - Slave X driver sends a transaction (on adapter B), slave address 0x10
+ *  - ATR driver rewrites messages with address 0x20, forwards to adapter A
+ *  - Physical I2C transaction on bus A, slave address 0x20
+ *  - ATR chip propagates transaction on bus B with address translated to 0x10
+ *  - Slave X chip replies on bus B
+ *  - ATR chip forwards reply on bus A
+ *  - ATR driver rewrites messages with address 0x10
+ *  - Slave X driver gets back the msgs[], with reply and address 0x10
+ *
+ * Usage:
+ *
+ *  1. In your driver (typically in the probe function) add an ATR by
+ *     calling i2c_atr_new() passing your attach/detach callbacks
+ *  2. When the attach callback is called pick an appropriate alias,
+ *     configure it in your chip and return the chosen alias in the
+ *     alias_id parameter
+ *  3. When the detach callback is called, deconfigure the alias from
+ *     your chip and put it back in the pool for later usage
+ *
+ * Originally based on i2c-mux.c
+ */
+
+#include <linux/i2c.h>
+#include <linux/i2c-atr.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/of.h>
+#include <linux/slab.h>
+
+/**
+ * struct i2c_atr_cli2alias_pair - Hold the alias assigned to a client.
+ * @node:   List node
+ * @client: Pointer to the client on the child bus
+ * @alias:  I2C alias address assigned by the driver.
+ *          This is the address that will be used to issue I2C transactions
+ *          on the parent (physical) bus.
+ */
+struct i2c_atr_cli2alias_pair {
+	struct list_head node;
+	const struct i2c_client *client;
+	u16 alias;
+};
+
+/*
+ * Data for each channel (child bus)
+ */
+struct i2c_atr_chan {
+	struct i2c_adapter adap;
+	struct i2c_atr *atr;
+	u32 chan_id;
+
+	struct list_head alias_list;
+
+	u16 *orig_addrs;
+	unsigned int orig_addrs_size;
+	struct mutex orig_addrs_lock; /* Lock orig_addrs during xfer */
+};
+
+static struct i2c_atr_cli2alias_pair *
+i2c_atr_find_mapping_by_client(const struct list_head *list,
+			       const struct i2c_client *client)
+{
+	struct i2c_atr_cli2alias_pair *c2a;
+
+	list_for_each_entry(c2a, list, node) {
+		if (c2a->client == client)
+			return c2a;
+	}
+
+	return NULL;
+}
+
+static struct i2c_atr_cli2alias_pair *
+i2c_atr_find_mapping_by_addr(const struct list_head *list,
+			     u16 phys_addr)
+{
+	struct i2c_atr_cli2alias_pair *c2a;
+
+	list_for_each_entry(c2a, list, node) {
+		if (c2a->client->addr == phys_addr)
+			return c2a;
+	}
+
+	return NULL;
+}
+
+/*
+ * Replace all message addresses with their aliases, saving the original
+ * addresses.
+ *
+ * This function is internal for use in i2c_atr_master_xfer(). It must be
+ * followed by i2c_atr_unmap_msgs() to restore the original addresses.
+ */
+static int i2c_atr_map_msgs(struct i2c_atr_chan *chan,
+			    struct i2c_msg msgs[], int num)
+
+{
+	struct i2c_atr *atr = chan->atr;
+	static struct i2c_atr_cli2alias_pair *c2a;
+	int i;
+
+	/* Ensure we have enough room to save the original addresses */
+	if (unlikely(chan->orig_addrs_size < num)) {
+		void *new_buf = kmalloc(num * sizeof(chan->orig_addrs[0]),
+					GFP_KERNEL);
+		if (new_buf == NULL)
+			return -ENOMEM;
+
+		kfree(chan->orig_addrs);
+		chan->orig_addrs = new_buf;
+		chan->orig_addrs_size = num;
+	}
+
+	for (i = 0; i < num; i++) {
+		chan->orig_addrs[i] = msgs[i].addr;
+
+		c2a = i2c_atr_find_mapping_by_addr(&chan->alias_list,
+						   msgs[i].addr);
+		if (c2a) {
+			msgs[i].addr = c2a->alias;
+		} else {
+			dev_err(atr->dev, "client 0x%02x not mapped!\n",
+				msgs[i].addr);
+			return -ENXIO;
+		}
+	}
+
+	return 0;
+}
+
+/*
+ * Restore all message address aliases with the original addresses.
+ *
+ * This function is internal for use in i2c_atr_master_xfer().
+ *
+ * @see i2c_atr_map_msgs()
+ */
+static void i2c_atr_unmap_msgs(struct i2c_atr_chan *chan,
+			       struct i2c_msg msgs[], int num)
+{
+	int i;
+
+	for (i = 0; i < num; i++)
+		msgs[i].addr = chan->orig_addrs[i];
+}
+
+static int i2c_atr_master_xfer(struct i2c_adapter *adap,
+			       struct i2c_msg msgs[], int num)
+{
+	struct i2c_atr_chan *chan = adap->algo_data;
+	struct i2c_atr *atr = chan->atr;
+	struct i2c_adapter *parent = atr->parent;
+	int ret = 0;
+
+	/* Switch to the right atr port */
+	if (atr->ops->select) {
+		ret = atr->ops->select(atr, chan->chan_id);
+		if (ret < 0)
+			goto out;
+	}
+
+	/* Translate addresses */
+	mutex_lock(&chan->orig_addrs_lock);
+	ret = i2c_atr_map_msgs(chan, msgs, num);
+	if (ret < 0) {
+		mutex_unlock(&chan->orig_addrs_lock);
+		goto out;
+	}
+
+	/* Perform the transfer */
+	ret = i2c_transfer(parent, msgs, num);
+
+	/* Restore addresses */
+	i2c_atr_unmap_msgs(chan, msgs, num);
+	mutex_unlock(&chan->orig_addrs_lock);
+
+out:
+	if (atr->ops->deselect)
+		atr->ops->deselect(atr, chan->chan_id);
+
+	return ret;
+}
+
+static int i2c_atr_smbus_xfer(struct i2c_adapter *adap,
+			      u16 addr, unsigned short flags,
+			      char read_write, u8 command,
+			      int size, union i2c_smbus_data *data)
+{
+	struct i2c_atr_chan *chan = adap->algo_data;
+	struct i2c_atr *atr = chan->atr;
+	struct i2c_adapter *parent = atr->parent;
+	struct i2c_atr_cli2alias_pair *c2a;
+	int err = 0;
+
+	c2a = i2c_atr_find_mapping_by_addr(&chan->alias_list, addr);
+	if (!c2a) {
+		dev_err(atr->dev, "client 0x%02x not mapped!\n", addr);
+		return -ENXIO;
+	}
+
+	if (atr->ops->select)
+		err = atr->ops->select(atr, chan->chan_id);
+	if (!err)
+		err = i2c_smbus_xfer(parent, c2a->alias, flags,
+				     read_write, command, size, data);
+	if (atr->ops->deselect)
+		atr->ops->deselect(atr, chan->chan_id);
+
+	return err;
+}
+
+static u32 i2c_atr_functionality(struct i2c_adapter *adap)
+{
+	struct i2c_atr_chan *chan = adap->algo_data;
+	struct i2c_adapter *parent = chan->atr->parent;
+
+	return parent->algo->functionality(parent);
+}
+
+static void i2c_atr_lock_bus(struct i2c_adapter *adapter, unsigned int flags)
+{
+	struct i2c_atr_chan *chan = adapter->algo_data;
+	struct i2c_atr *atr = chan->atr;
+
+	mutex_lock(&atr->lock);
+}
+
+static int i2c_atr_trylock_bus(struct i2c_adapter *adapter, unsigned int flags)
+{
+	struct i2c_atr_chan *chan = adapter->algo_data;
+	struct i2c_atr *atr = chan->atr;
+
+	return mutex_trylock(&atr->lock);
+}
+
+static void i2c_atr_unlock_bus(struct i2c_adapter *adapter, unsigned int flags)
+{
+	struct i2c_atr_chan *chan = adapter->algo_data;
+	struct i2c_atr *atr = chan->atr;
+
+	mutex_unlock(&atr->lock);
+}
+
+static const struct i2c_lock_operations i2c_atr_lock_ops = {
+	.lock_bus =    i2c_atr_lock_bus,
+	.trylock_bus = i2c_atr_trylock_bus,
+	.unlock_bus =  i2c_atr_unlock_bus,
+};
+
+static int i2c_atr_attach_client(struct i2c_adapter *adapter,
+				 const struct i2c_board_info *info,
+				 const struct i2c_client *client)
+{
+	struct i2c_atr_chan *chan = adapter->algo_data;
+	struct i2c_atr *atr = chan->atr;
+	struct i2c_atr_cli2alias_pair *c2a;
+	u16 alias_id = 0;
+	int err = 0;
+
+	c2a = kzalloc(sizeof(struct i2c_atr_cli2alias_pair), GFP_KERNEL);
+	if (!c2a) {
+		err = -ENOMEM;
+		goto err_alloc;
+	}
+
+	err = atr->ops->attach_client(atr, chan->chan_id, info, client,
+				      &alias_id);
+	if (err)
+		goto err_attach;
+	if (alias_id == 0) {
+		err = -EINVAL;
+		goto err_attach;
+	}
+
+	c2a->client = client;
+	c2a->alias = alias_id;
+	list_add(&c2a->node, &chan->alias_list);
+
+	return 0;
+
+err_attach:
+	kfree(c2a);
+err_alloc:
+	return err;
+}
+
+static void i2c_atr_detach_client(struct i2c_adapter *adapter,
+				  const struct i2c_client *client)
+{
+	struct i2c_atr_chan *chan = adapter->algo_data;
+	struct i2c_atr *atr = chan->atr;
+	struct i2c_atr_cli2alias_pair *c2a;
+
+	atr->ops->detach_client(atr, chan->chan_id, client);
+
+	c2a = i2c_atr_find_mapping_by_client(&chan->alias_list, client);
+	if (c2a != NULL) {
+		list_del(&c2a->node);
+		kfree(c2a);
+	}
+}
+
+static const struct i2c_attach_operations i2c_atr_attach_ops = {
+	.attach_client = i2c_atr_attach_client,
+	.detach_client = i2c_atr_detach_client,
+};
+
+/**
+ * i2c_atr_add_adapter - Create a child ("downstream") I2C bus.
+ * @atr:     The I2C ATR
+ * @chan_id: Index of the new adapter (0 .. max_adapters-1).  This value is
+ *           passed to the callbacks in `struct i2c_atr_ops`.
+ *
+ * After calling this function a new i2c bus will appear. Adding and
+ * removing devices on the downstream bus will result in calls to the
+ * `attach_client` and `detach_client` callbacks for the driver to assign
+ * an alias to the device.
+ *
+ * If there is a device tree node under "i2c-atr" whose "reg" property
+ * equals chan_id, the new adapter will receive that node and perhaps start
+ * adding devices under it. The callbacks for those additions will be made
+ * before i2c_atr_add_adapter() returns.
+ *
+ * Call i2c_atr_del_adapter() to remove the adapter.
+ *
+ * Return: 0 on success, a negative error code otherwise.
+ */
+int i2c_atr_add_adapter(struct i2c_atr *atr, u32 chan_id)
+{
+	struct i2c_adapter *parent = atr->parent;
+	struct device *dev = atr->dev;
+	struct i2c_atr_chan *chan;
+	char symlink_name[20];
+	int err;
+
+	if (chan_id >= atr->max_adapters)
+		return -EINVAL;
+
+	if (atr->adapter[chan_id]) {
+		dev_err(dev, "Adapter %d already present\n", chan_id);
+		return -EEXIST;
+	}
+
+	chan = kzalloc(sizeof(*chan), GFP_KERNEL);
+	if (!chan)
+		return -ENOMEM;
+
+	chan->atr = atr;
+	chan->chan_id = chan_id;
+	INIT_LIST_HEAD(&chan->alias_list);
+	mutex_init(&chan->orig_addrs_lock);
+
+	snprintf(chan->adap.name, sizeof(chan->adap.name),
+		 "i2c-%d-atr-%d", i2c_adapter_id(parent), chan_id);
+	chan->adap.owner = THIS_MODULE;
+	chan->adap.algo = &atr->algo;
+	chan->adap.algo_data = chan;
+	chan->adap.dev.parent = dev;
+	chan->adap.retries = parent->retries;
+	chan->adap.timeout = parent->timeout;
+	chan->adap.quirks = parent->quirks;
+	chan->adap.lock_ops = &i2c_atr_lock_ops;
+	chan->adap.attach_ops = &i2c_atr_attach_ops;
+
+	if (dev->of_node) {
+		struct device_node *atr_node;
+		struct device_node *child;
+		u32 reg;
+
+		atr_node = of_get_child_by_name(dev->of_node, "i2c-atr");
+
+		for_each_child_of_node(atr_node, child) {
+			err = of_property_read_u32(child, "reg", &reg);
+			if (err)
+				continue;
+			if (chan_id == reg)
+				break;
+		}
+
+		chan->adap.dev.of_node = child;
+		of_node_put(atr_node);
+	}
+
+	err = i2c_add_adapter(&chan->adap);
+	if (err) {
+		dev_err(dev, "failed to add atr-adapter %u (error=%d)\n",
+			chan_id, err);
+		goto err_add_adapter;
+	}
+
+	WARN(sysfs_create_link(&chan->adap.dev.kobj, &dev->kobj, "atr_device"),
+	     "can't create symlink to atr device\n");
+	snprintf(symlink_name, sizeof(symlink_name), "channel-%u", chan_id);
+	WARN(sysfs_create_link(&dev->kobj, &chan->adap.dev.kobj, symlink_name),
+	     "can't create symlink for channel %u\n", chan_id);
+
+	dev_info(dev, "Added ATR child bus %d\n", i2c_adapter_id(&chan->adap));
+
+	atr->adapter[chan_id] = &chan->adap;
+	return 0;
+
+err_add_adapter:
+	mutex_destroy(&chan->orig_addrs_lock);
+	kfree(chan);
+	return err;
+}
+EXPORT_SYMBOL_GPL(i2c_atr_add_adapter);
+
+/**
+ * i2c_atr_del_adapter - Remove a child ("downstream") I2C bus added by
+ * i2c_atr_del_adapter().
+ * @atr:     The I2C ATR
+ * @chan_id: Index of the `adapter to be removed (0 .. max_adapters-1)
+ */
+void i2c_atr_del_adapter(struct i2c_atr *atr, u32 chan_id)
+{
+	char symlink_name[20];
+
+	struct i2c_adapter *adap = atr->adapter[chan_id];
+	struct i2c_atr_chan *chan = adap->algo_data;
+	struct device_node *np = adap->dev.of_node;
+	struct device *dev = atr->dev;
+
+	if (atr->adapter[chan_id] == NULL) {
+		dev_err(dev, "Adapter %d does not exist\n", chan_id);
+		return;
+	}
+
+	dev_info(dev, "Removing ATR child bus %d\n", i2c_adapter_id(adap));
+
+	atr->adapter[chan_id] = NULL;
+
+	snprintf(symlink_name, sizeof(symlink_name),
+		 "channel-%u", chan->chan_id);
+	sysfs_remove_link(&dev->kobj, symlink_name);
+	sysfs_remove_link(&chan->adap.dev.kobj, "atr_device");
+
+	i2c_del_adapter(adap);
+	of_node_put(np);
+	mutex_destroy(&chan->orig_addrs_lock);
+	kfree(chan);
+}
+EXPORT_SYMBOL_GPL(i2c_atr_del_adapter);
+
+/**
+ * i2c_atr_new() - Allocate and initialize an I2C ATR helper.
+ * @parent:       The parent (upstream) adapter
+ * @dev:          The device acting as an ATR
+ * @ops:          Driver-specific callbacks
+ * @max_adapters: Maximum number of child adapters
+ *
+ * The new ATR helper is connected to the parent adapter but has no child
+ * adapters. Call i2c_atr_add_adapter() to add some.
+ *
+ * Call i2c_atr_delete() to remove.
+ *
+ * Return: pointer to the new ATR helper object, or ERR_PTR
+ */
+struct i2c_atr *i2c_atr_new(struct i2c_adapter *parent, struct device *dev,
+			    const struct i2c_atr_ops *ops, int max_adapters)
+{
+	struct i2c_atr *atr;
+
+	if (!ops || !ops->attach_client || !ops->detach_client)
+		return ERR_PTR(-EINVAL);
+
+	atr = devm_kzalloc(dev, sizeof(*atr)
+			    + max_adapters * sizeof(atr->adapter[0]),
+			    GFP_KERNEL);
+	if (!atr)
+		return ERR_PTR(-ENOMEM);
+
+	mutex_init(&atr->lock);
+
+	atr->parent = parent;
+	atr->dev = dev;
+	atr->ops = ops;
+	atr->max_adapters = max_adapters;
+
+	if (parent->algo->master_xfer)
+		atr->algo.master_xfer = i2c_atr_master_xfer;
+	if (parent->algo->smbus_xfer)
+		atr->algo.smbus_xfer = i2c_atr_smbus_xfer;
+	atr->algo.functionality = i2c_atr_functionality;
+
+	return atr;
+}
+EXPORT_SYMBOL_GPL(i2c_atr_new);
+
+/**
+ * i2c_atr_delete - Delete an I2C ATR helper.
+ * @atr: I2C ATR helper to be deleted.
+ *
+ * Precondition: all the adapters added with i2c_atr_add_adapter() mumst be
+ * removed by calling i2c_atr_del_adapter().
+ */
+void i2c_atr_delete(struct i2c_atr *atr)
+{
+	mutex_destroy(&atr->lock);
+}
+EXPORT_SYMBOL_GPL(i2c_atr_delete);
+
+MODULE_AUTHOR("Luca Ceresoli <luca@...aceresoli.net>");
+MODULE_DESCRIPTION("I2C Address Translator");
+MODULE_LICENSE("GPL v2");
diff --git a/include/linux/i2c-atr.h b/include/linux/i2c-atr.h
new file mode 100644
index 000000000000..019816e5a50c
--- /dev/null
+++ b/include/linux/i2c-atr.h
@@ -0,0 +1,82 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/**
+ * drivers/i2c/i2c-atr.h -- I2C Address Translator
+ *
+ * Copyright (c) 2019 Luca Ceresoli <luca@...aceresoli.net>
+ *
+ * Based on i2c-mux.h
+ */
+
+#ifndef _LINUX_I2C_ATR_H
+#define _LINUX_I2C_ATR_H
+
+#ifdef __KERNEL__
+
+#include <linux/i2c.h>
+#include <linux/mutex.h>
+
+struct i2c_atr;
+
+/**
+ * struct i2c_atr_ops - Callbacks from ATR to the device driver.
+ * @select:        Ask the driver to select a child bus (optional)
+ * @deselect:      Ask the driver to deselect a child bus (optional)
+ * @attach_client: Notify the driver of a new device connected on a child
+ *                 bus. The driver must choose an I2C alias, configure the
+ *                 hardware to use it and return it in `alias_id`.
+ * @detach_client: Notify the driver of a device getting disconnected. The
+ *                 driver must configure the hardware to stop using the
+ *                 alias.
+ *
+ * All these functions return 0 on success, a negative error code otherwise.
+ */
+struct i2c_atr_ops {
+	int  (*select)(struct i2c_atr *atr, u32 chan_id);
+	int  (*deselect)(struct i2c_atr *atr, u32 chan_id);
+	int  (*attach_client)(struct i2c_atr *atr, u32 chan_id,
+			      const struct i2c_board_info *info,
+			      const struct i2c_client *client,
+			      u16 *alias_id);
+	void (*detach_client)(struct i2c_atr *atr, u32 chan_id,
+			      const struct i2c_client *client);
+};
+
+/**
+ * Helper to add I2C ATR features to a device driver.
+ */
+struct i2c_atr {
+	/* private: internal use only */
+
+	struct i2c_adapter *parent;
+	struct device *dev;
+	const struct i2c_atr_ops *ops;
+
+	void *priv;
+
+	struct i2c_algorithm algo;
+	struct mutex lock;
+	int max_adapters;
+
+	struct i2c_adapter *adapter[0];
+};
+
+struct i2c_atr *i2c_atr_new(struct i2c_adapter *parent, struct device *dev,
+			    const struct i2c_atr_ops *ops, int max_adapters);
+void i2c_atr_delete(struct i2c_atr *atr);
+
+static inline void i2c_atr_set_clientdata(struct i2c_atr *atr, void *data)
+{
+	atr->priv = data;
+}
+
+static inline void *i2c_atr_get_clientdata(struct i2c_atr *atr)
+{
+	return atr->priv;
+}
+
+int i2c_atr_add_adapter(struct i2c_atr *atr, u32 chan_id);
+void i2c_atr_del_adapter(struct i2c_atr *atr, u32 chan_id);
+
+#endif /* __KERNEL__ */
+
+#endif /* _LINUX_I2C_ATR_H */
-- 
2.25.1

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