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

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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@xxxxxxxxxxxxxxxx>

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




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