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

Andy Shevchenko
2022-12-08T12:53:42Z

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()

--
With Best Regards,
Andy Shevchenko