[v5,2/8] i2c: add I2C Address Translator (ATR) support

Message ID 20221208104006.316606-3-tomi.valkeinen@ideasonboard.com
State New
Headers
Series i2c-atr and FPDLink |

Commit Message

Tomi Valkeinen Dec. 8, 2022, 10:40 a.m. UTC
  From: Luca Ceresoli <luca@lucaceresoli.net>

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@lucaceresoli.net>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
 Documentation/i2c/index.rst         |   1 +
 Documentation/i2c/muxes/i2c-atr.rst |  78 +++++
 MAINTAINERS                         |   8 +
 drivers/i2c/Kconfig                 |   9 +
 drivers/i2c/Makefile                |   1 +
 drivers/i2c/i2c-atr.c               | 503 ++++++++++++++++++++++++++++
 include/linux/i2c-atr.h             |  82 +++++
 7 files changed, 682 insertions(+)
 create mode 100644 Documentation/i2c/muxes/i2c-atr.rst
 create mode 100644 drivers/i2c/i2c-atr.c
 create mode 100644 include/linux/i2c-atr.h
  

Comments

Andy Shevchenko Dec. 8, 2022, 12:53 p.m. UTC | #1
On Thu, Dec 08, 2022 at 12:40:00PM +0200, Tomi Valkeinen wrote:
> From: Luca Ceresoli <luca@lucaceresoli.net>
> 
> 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.

Besides comments given against diff between series versions, see below.

...

> +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;
> +	int ret;
> +
> +	c2a = kzalloc(sizeof(*c2a), GFP_KERNEL);
> +	if (!c2a)
> +		return -ENOMEM;
> +
> +	ret = atr->ops->attach_client(atr, chan->chan_id, info, client,
> +				      &alias_id);
> +	if (ret)
> +		goto err_free;

> +	if (alias_id == 0) {
> +		ret = -EINVAL;

I'm wondering why attach_client can't return this error and provide a guarantee
that if no error, the alias_id is never be 0?

> +		goto err_free;
> +	}
> +
> +	c2a->client = client;
> +	c2a->alias = alias_id;
> +	list_add(&c2a->node, &chan->alias_list);
> +
> +	return 0;
> +
> +err_free:
> +	kfree(c2a);
> +	return ret;
> +}

...

> +	if (bus_handle) {
> +		device_set_node(&chan->adap.dev, fwnode_handle_get(bus_handle));

I believe the correct way, while above still works, is

		device_set_node(&chan->adap.dev, bus_handle);
		fwnode_handle_get(dev_fwnode(&chan->adap.dev));

But I agree that this looks a bit verbose. And...

> +	} else {
> +		struct fwnode_handle *atr_node;
> +		struct fwnode_handle *child;
> +		u32 reg;
> +
> +		atr_node = device_get_named_child_node(dev, "i2c-atr");
> +
> +		fwnode_for_each_child_node(atr_node, child) {
> +			ret = fwnode_property_read_u32(child, "reg", &reg);
> +			if (ret)
> +				continue;
> +			if (chan_id == reg)
> +				break;
> +		}
> +
> +		device_set_node(&chan->adap.dev, child);

...OTOH, you set node with bumped reference here. So I leave all this to
the maintainers.

> +		fwnode_handle_put(atr_node);
> +	}

> +	ret = i2c_add_adapter(&chan->adap);
> +	if (ret) {
> +		dev_err(dev, "failed to add atr-adapter %u (error=%d)\n",
> +			chan_id, ret);
> +		goto err_mutex_destroy;
> +	}
> +
> +	snprintf(symlink_name, sizeof(symlink_name), "channel-%u",
> +		 chan->chan_id);
> +
> +	ret = sysfs_create_link(&chan->adap.dev.kobj, &dev->kobj, "atr_device");
> +	if (ret)
> +		dev_warn(dev, "can't create symlink to atr device\n");
> +	ret = sysfs_create_link(&dev->kobj, &chan->adap.dev.kobj, symlink_name);
> +	if (ret)
> +		dev_warn(dev, "can't create symlink for channel %u\n", chan_id);
> +
> +	dev_dbg(dev, "Added ATR child bus %d\n", i2c_adapter_id(&chan->adap));
> +
> +	atr->adapter[chan_id] = &chan->adap;
> +	return 0;
> +
> +err_mutex_destroy:

Now it's a bit misleading, wouldn't be better

err_put_fwnode:

?

> +	fwnode_handle_put(dev_fwnode(&chan->adap.dev));
> +	mutex_destroy(&chan->orig_addrs_lock);
> +	kfree(chan);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(i2c_atr_add_adapter);

Wondering if we may put this into namespace from day 1.

...

> +/**
> + * 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[ATR_MAX_SYMLINK_LEN];
> +
> +	struct i2c_adapter *adap = atr->adapter[chan_id];
> +	struct i2c_atr_chan *chan = adap->algo_data;
> +	struct fwnode_handle *fwnode = dev_fwnode(&adap->dev);
> +	struct device *dev = atr->dev;

> +	if (!atr->adapter[chan_id]) {

Isn't it the same as

	if (!adap)

?

> +		dev_err(dev, "Adapter %d does not exist\n", chan_id);
> +		return;
> +	}
> +
> +	dev_dbg(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);
> +	fwnode_handle_put(fwnode);
> +	mutex_destroy(&chan->orig_addrs_lock);
> +	kfree(chan->orig_addrs);
> +	kfree(chan);
> +}

...

> +struct i2c_atr {
> +	/* private: internal use only */

What is private? The entire structure? Then why it's defined in
the include/linux/? Can't you make it opaque?

> +	struct i2c_adapter *parent;
> +	struct device *dev;
> +	const struct i2c_atr_ops *ops;
> +
> +	void *priv;
> +
> +	struct i2c_algorithm algo;
> +	/* lock for the I2C bus segment (see struct i2c_lock_operations) */
> +	struct mutex lock;
> +	int max_adapters;
> +
> +	struct i2c_adapter *adapter[];
> +};

...

> +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;
> +}

The function names are misleading, because I would think this is about driver
data that has been set.

I would rather use name like

	i2c_atr_get_priv()
	i2c_atr_set_priv()
  
Tomi Valkeinen Dec. 8, 2022, 4:01 p.m. UTC | #2
Hi Andy,

On 08/12/2022 14:53, Andy Shevchenko wrote:
> On Thu, Dec 08, 2022 at 12:40:00PM +0200, Tomi Valkeinen wrote:
>> From: Luca Ceresoli <luca@lucaceresoli.net>
>>
>> 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.
> 
> Besides comments given against diff between series versions, see below.
> 
> ...
> 
>> +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;
>> +	int ret;
>> +
>> +	c2a = kzalloc(sizeof(*c2a), GFP_KERNEL);
>> +	if (!c2a)
>> +		return -ENOMEM;
>> +
>> +	ret = atr->ops->attach_client(atr, chan->chan_id, info, client,
>> +				      &alias_id);
>> +	if (ret)
>> +		goto err_free;
> 
>> +	if (alias_id == 0) {
>> +		ret = -EINVAL;
> 
> I'm wondering why attach_client can't return this error and provide a guarantee
> that if no error, the alias_id is never be 0?

I think that's a valid point. I see no reason to check for alias_id == 0 
here.

>> +		goto err_free;
>> +	}
>> +
>> +	c2a->client = client;
>> +	c2a->alias = alias_id;
>> +	list_add(&c2a->node, &chan->alias_list);
>> +
>> +	return 0;
>> +
>> +err_free:
>> +	kfree(c2a);
>> +	return ret;
>> +}
> 
> ...
> 
>> +	if (bus_handle) {
>> +		device_set_node(&chan->adap.dev, fwnode_handle_get(bus_handle));
> 
> I believe the correct way, while above still works, is
> 
> 		device_set_node(&chan->adap.dev, bus_handle);
> 		fwnode_handle_get(dev_fwnode(&chan->adap.dev));

Hmm, why is that correct? Shouldn't you give device_set_node() an fwnode 
that has been referenced?

> But I agree that this looks a bit verbose. And...
> 
>> +	} else {
>> +		struct fwnode_handle *atr_node;
>> +		struct fwnode_handle *child;
>> +		u32 reg;
>> +
>> +		atr_node = device_get_named_child_node(dev, "i2c-atr");
>> +
>> +		fwnode_for_each_child_node(atr_node, child) {
>> +			ret = fwnode_property_read_u32(child, "reg", &reg);
>> +			if (ret)
>> +				continue;
>> +			if (chan_id == reg)
>> +				break;
>> +		}
>> +
>> +		device_set_node(&chan->adap.dev, child);
> 
> ...OTOH, you set node with bumped reference here. So I leave all this to
> the maintainers.
> 
>> +		fwnode_handle_put(atr_node);
>> +	}
> 
>> +	ret = i2c_add_adapter(&chan->adap);
>> +	if (ret) {
>> +		dev_err(dev, "failed to add atr-adapter %u (error=%d)\n",
>> +			chan_id, ret);
>> +		goto err_mutex_destroy;
>> +	}
>> +
>> +	snprintf(symlink_name, sizeof(symlink_name), "channel-%u",
>> +		 chan->chan_id);
>> +
>> +	ret = sysfs_create_link(&chan->adap.dev.kobj, &dev->kobj, "atr_device");
>> +	if (ret)
>> +		dev_warn(dev, "can't create symlink to atr device\n");
>> +	ret = sysfs_create_link(&dev->kobj, &chan->adap.dev.kobj, symlink_name);
>> +	if (ret)
>> +		dev_warn(dev, "can't create symlink for channel %u\n", chan_id);
>> +
>> +	dev_dbg(dev, "Added ATR child bus %d\n", i2c_adapter_id(&chan->adap));
>> +
>> +	atr->adapter[chan_id] = &chan->adap;
>> +	return 0;
>> +
>> +err_mutex_destroy:
> 
> Now it's a bit misleading, wouldn't be better
> 
> err_put_fwnode:
> 
> ?

Yes.

>> +	fwnode_handle_put(dev_fwnode(&chan->adap.dev));
>> +	mutex_destroy(&chan->orig_addrs_lock);
>> +	kfree(chan);
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(i2c_atr_add_adapter);
> 
> Wondering if we may put this into namespace from day 1.

Right, that's something I didn't look at all for v5. I have not heard 
anyone else commenting about the NS, though. I could have a look at it 
anyway, just to learn about NSes.

> ...
> 
>> +/**
>> + * 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[ATR_MAX_SYMLINK_LEN];
>> +
>> +	struct i2c_adapter *adap = atr->adapter[chan_id];
>> +	struct i2c_atr_chan *chan = adap->algo_data;
>> +	struct fwnode_handle *fwnode = dev_fwnode(&adap->dev);
>> +	struct device *dev = atr->dev;
> 
>> +	if (!atr->adapter[chan_id]) {
> 
> Isn't it the same as
> 
> 	if (!adap)
> 
> ?

Yes.

> 
>> +		dev_err(dev, "Adapter %d does not exist\n", chan_id);
>> +		return;
>> +	}
>> +
>> +	dev_dbg(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);
>> +	fwnode_handle_put(fwnode);
>> +	mutex_destroy(&chan->orig_addrs_lock);
>> +	kfree(chan->orig_addrs);
>> +	kfree(chan);
>> +}
> 
> ...
> 
>> +struct i2c_atr {
>> +	/* private: internal use only */
> 
> What is private? The entire structure? Then why it's defined in
> the include/linux/? Can't you make it opaque?

Good point, I see no reason to keep this in the public header. 
i2c_atr_set/get_clientdata used it, but I can move their implementations 
into the .c file.

>> +	struct i2c_adapter *parent;
>> +	struct device *dev;
>> +	const struct i2c_atr_ops *ops;
>> +
>> +	void *priv;
>> +
>> +	struct i2c_algorithm algo;
>> +	/* lock for the I2C bus segment (see struct i2c_lock_operations) */
>> +	struct mutex lock;
>> +	int max_adapters;
>> +
>> +	struct i2c_adapter *adapter[];
>> +};
> 
> ...
> 
>> +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;
>> +}
> 
> The function names are misleading, because I would think this is about driver
> data that has been set.
> 
> I would rather use name like
> 
> 	i2c_atr_get_priv()
> 	i2c_atr_set_priv()

Indeed, set_clientdata is probably wrong. But i2c_atr_set_priv() sounds 
like it's private to the i2c-atr itself. Maybe i2c_atr_set_driver_data?

  Tomi
  
Andy Shevchenko Dec. 8, 2022, 6:09 p.m. UTC | #3
On Thu, Dec 08, 2022 at 06:01:19PM +0200, Tomi Valkeinen wrote:
> On 08/12/2022 14:53, Andy Shevchenko wrote:
> > On Thu, Dec 08, 2022 at 12:40:00PM +0200, Tomi Valkeinen wrote:

...

> > > +	if (bus_handle) {
> > > +		device_set_node(&chan->adap.dev, fwnode_handle_get(bus_handle));
> > 
> > I believe the correct way, while above still works, is
> > 
> > 		device_set_node(&chan->adap.dev, bus_handle);
> > 		fwnode_handle_get(dev_fwnode(&chan->adap.dev));
> 
> Hmm, why is that correct? Shouldn't you give device_set_node() an fwnode
> that has been referenced?

You take a reference on the adap->dev and not on input. It's just a logical,
But as I said your variant still works.

> > But I agree that this looks a bit verbose. And...
> > 
> > > +	} else {
> > > +		struct fwnode_handle *atr_node;
> > > +		struct fwnode_handle *child;
> > > +		u32 reg;
> > > +
> > > +		atr_node = device_get_named_child_node(dev, "i2c-atr");
> > > +
> > > +		fwnode_for_each_child_node(atr_node, child) {
> > > +			ret = fwnode_property_read_u32(child, "reg", &reg);
> > > +			if (ret)
> > > +				continue;
> > > +			if (chan_id == reg)
> > > +				break;
> > > +		}
> > > +
> > > +		device_set_node(&chan->adap.dev, child);
> > 
> > ...OTOH, you set node with bumped reference here. So I leave all this to
> > the maintainers.
> > 
> > > +		fwnode_handle_put(atr_node);
> > > +	}

...

> > > +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;
> > > +}
> > 
> > The function names are misleading, because I would think this is about driver
> > data that has been set.
> > 
> > I would rather use name like
> > 
> > 	i2c_atr_get_priv()
> > 	i2c_atr_set_priv()
> 
> Indeed, set_clientdata is probably wrong. But i2c_atr_set_priv() sounds like
> it's private to the i2c-atr itself. Maybe i2c_atr_set_driver_data?

Works for me.
  

Patch

diff --git a/Documentation/i2c/index.rst b/Documentation/i2c/index.rst
index 6270f1fd7d4e..aaf33d1315f4 100644
--- a/Documentation/i2c/index.rst
+++ b/Documentation/i2c/index.rst
@@ -16,6 +16,7 @@  Introduction
    instantiating-devices
    busses/index
    i2c-topology
+   muxes/i2c-atr
    muxes/i2c-mux-gpio
    i2c-sysfs
 
diff --git a/Documentation/i2c/muxes/i2c-atr.rst b/Documentation/i2c/muxes/i2c-atr.rst
new file mode 100644
index 000000000000..14597c9ec19b
--- /dev/null
+++ b/Documentation/i2c/muxes/i2c-atr.rst
@@ -0,0 +1,78 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Kernel driver i2c-atr
+=====================
+
+Author: Luca Ceresoli <luca@lucaceresoli.net>
+
+Description
+-----------
+
+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:
+
+.. 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
diff --git a/MAINTAINERS b/MAINTAINERS
index 379945f82a64..4c375e30f951 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9568,6 +9568,14 @@  L:	linux-acpi@vger.kernel.org
 S:	Maintained
 F:	drivers/i2c/i2c-core-acpi.c
 
+I2C ADDRESS TRANSLATOR (ATR)
+M:	Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
+R:	Luca Ceresoli <luca.ceresoli@bootlin.com>
+L:	linux-i2c@vger.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@nvidia.com>
 L:	linux-i2c@vger.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..1d3b25a6550f
--- /dev/null
+++ b/drivers/i2c/i2c-atr.c
@@ -0,0 +1,503 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * I2C Address Translator
+ *
+ * Copyright (c) 2019,2022 Luca Ceresoli <luca@lucaceresoli.net>
+ *
+ * Originally based on i2c-mux.c
+ */
+
+#include <linux/fwnode.h>
+#include <linux/i2c-atr.h>
+#include <linux/i2c.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/slab.h>
+
+#define ATR_MAX_ADAPTERS 99	/* Just a sanity limit */
+#define ATR_MAX_SYMLINK_LEN 16	/* Longest name is 10 chars: "channel-99" */
+
+/**
+ * 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)) {
+		u16 *new_buf;
+
+		new_buf = kmalloc_array(num, sizeof(chan->orig_addrs[0]),
+					GFP_KERNEL);
+		if (!new_buf)
+			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) {
+			dev_err(atr->dev, "client 0x%02x not mapped!\n",
+				msgs[i].addr);
+			return -ENXIO;
+		}
+
+		msgs[i].addr = c2a->alias;
+	}
+
+	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;
+
+	/* Switch to the right atr port */
+	if (atr->ops->select) {
+		ret = atr->ops->select(atr, chan->chan_id);
+		if (ret < 0)
+			goto out_deselect;
+	}
+
+	/* Translate addresses */
+	mutex_lock(&chan->orig_addrs_lock);
+	ret = i2c_atr_map_msgs(chan, msgs, num);
+	if (ret < 0)
+		goto out_unlock_deselect;
+
+	/* Perform the transfer */
+	ret = i2c_transfer(parent, msgs, num);
+
+	/* Restore addresses */
+	i2c_atr_unmap_msgs(chan, msgs, num);
+
+out_unlock_deselect:
+	mutex_unlock(&chan->orig_addrs_lock);
+
+out_deselect:
+	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 ret = 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)
+		ret = atr->ops->select(atr, chan->chan_id);
+	if (!ret)
+		ret = i2c_smbus_xfer(parent, c2a->alias, flags, read_write,
+				     command, size, data);
+	if (atr->ops->deselect)
+		atr->ops->deselect(atr, chan->chan_id);
+
+	return ret;
+}
+
+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;
+	int ret;
+
+	c2a = kzalloc(sizeof(*c2a), GFP_KERNEL);
+	if (!c2a)
+		return -ENOMEM;
+
+	ret = atr->ops->attach_client(atr, chan->chan_id, info, client,
+				      &alias_id);
+	if (ret)
+		goto err_free;
+	if (alias_id == 0) {
+		ret = -EINVAL;
+		goto err_free;
+	}
+
+	c2a->client = client;
+	c2a->alias = alias_id;
+	list_add(&c2a->node, &chan->alias_list);
+
+	return 0;
+
+err_free:
+	kfree(c2a);
+	return ret;
+}
+
+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) {
+		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`.
+ * @bus_handle: The fwnode handle that points to the adapter's i2c
+ *              peripherals, or NULL.
+ *
+ * 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.
+ *
+ * The adapter's fwnode is set to 'bus_handle', or if 'bus_handle' is NULL the
+ * function looks for a child node whose 'reg' property matches the chan_id
+ * under the i2c-atr device's 'i2c-atr' node.
+
+ * 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 fwnode_handle *bus_handle)
+{
+	struct i2c_adapter *parent = atr->parent;
+	struct device *dev = atr->dev;
+	struct i2c_atr_chan *chan;
+	char symlink_name[ATR_MAX_SYMLINK_LEN];
+	int ret;
+
+	if (chan_id >= atr->max_adapters) {
+		dev_err(dev, "No room for more i2c-atr adapters\n");
+		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 (bus_handle) {
+		device_set_node(&chan->adap.dev, fwnode_handle_get(bus_handle));
+	} else {
+		struct fwnode_handle *atr_node;
+		struct fwnode_handle *child;
+		u32 reg;
+
+		atr_node = device_get_named_child_node(dev, "i2c-atr");
+
+		fwnode_for_each_child_node(atr_node, child) {
+			ret = fwnode_property_read_u32(child, "reg", &reg);
+			if (ret)
+				continue;
+			if (chan_id == reg)
+				break;
+		}
+
+		device_set_node(&chan->adap.dev, child);
+		fwnode_handle_put(atr_node);
+	}
+
+	ret = i2c_add_adapter(&chan->adap);
+	if (ret) {
+		dev_err(dev, "failed to add atr-adapter %u (error=%d)\n",
+			chan_id, ret);
+		goto err_mutex_destroy;
+	}
+
+	snprintf(symlink_name, sizeof(symlink_name), "channel-%u",
+		 chan->chan_id);
+
+	ret = sysfs_create_link(&chan->adap.dev.kobj, &dev->kobj, "atr_device");
+	if (ret)
+		dev_warn(dev, "can't create symlink to atr device\n");
+	ret = sysfs_create_link(&dev->kobj, &chan->adap.dev.kobj, symlink_name);
+	if (ret)
+		dev_warn(dev, "can't create symlink for channel %u\n", chan_id);
+
+	dev_dbg(dev, "Added ATR child bus %d\n", i2c_adapter_id(&chan->adap));
+
+	atr->adapter[chan_id] = &chan->adap;
+	return 0;
+
+err_mutex_destroy:
+	fwnode_handle_put(dev_fwnode(&chan->adap.dev));
+	mutex_destroy(&chan->orig_addrs_lock);
+	kfree(chan);
+	return ret;
+}
+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[ATR_MAX_SYMLINK_LEN];
+
+	struct i2c_adapter *adap = atr->adapter[chan_id];
+	struct i2c_atr_chan *chan = adap->algo_data;
+	struct fwnode_handle *fwnode = dev_fwnode(&adap->dev);
+	struct device *dev = atr->dev;
+
+	if (!atr->adapter[chan_id]) {
+		dev_err(dev, "Adapter %d does not exist\n", chan_id);
+		return;
+	}
+
+	dev_dbg(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);
+	fwnode_handle_put(fwnode);
+	mutex_destroy(&chan->orig_addrs_lock);
+	kfree(chan->orig_addrs);
+	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 (max_adapters > ATR_MAX_ADAPTERS)
+		return ERR_PTR(-EINVAL);
+
+	if (!ops || !ops->attach_client || !ops->detach_client)
+		return ERR_PTR(-EINVAL);
+
+	atr = devm_kzalloc(dev, struct_size(atr, adapter, max_adapters),
+			   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@lucaceresoli.net>");
+MODULE_DESCRIPTION("I2C Address Translator");
+MODULE_LICENSE("GPL");
diff --git a/include/linux/i2c-atr.h b/include/linux/i2c-atr.h
new file mode 100644
index 000000000000..044c87c5b336
--- /dev/null
+++ b/include/linux/i2c-atr.h
@@ -0,0 +1,82 @@ 
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * I2C Address Translator
+ *
+ * Copyright (c) 2019,2022 Luca Ceresoli <luca@lucaceresoli.net>
+ *
+ * Based on i2c-mux.h
+ */
+
+#ifndef _LINUX_I2C_ATR_H
+#define _LINUX_I2C_ATR_H
+
+#include <linux/i2c.h>
+#include <linux/mutex.h>
+#include <linux/types.h>
+
+struct device;
+struct i2c_atr;
+struct fwnode_handle;
+
+/**
+ * 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);
+};
+
+/**
+ * struct i2c_atr - Represents the I2C ATR instance
+ */
+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;
+	/* lock for the I2C bus segment (see struct i2c_lock_operations) */
+	struct mutex lock;
+	int max_adapters;
+
+	struct i2c_adapter *adapter[];
+};
+
+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,
+			struct fwnode_handle *bus_np);
+void i2c_atr_del_adapter(struct i2c_atr *atr, u32 chan_id);
+
+#endif /* _LINUX_I2C_ATR_H */