Re: [PATCH v7 1/7] i2c: add I2C Address Translator (ATR) support

[Andy Shevchenko]

On Wed, Jan 18, 2023 at 02:40:25PM +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

is is ?

> 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.

...

> +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

I'm not sure I got the real / virtual status of the adapters. Are the B and C
virtual ones, while A is the real?

...

> +#define ATR_MAX_ADAPTERS 99	/* Just a sanity limit */

Hmm... It's not clear why this is not 100, for example, and how 99 below is
related to that, assuming channel numbering is started from 0.

> +#define ATR_MAX_SYMLINK_LEN 16	/* Longest name is 10 chars: "channel-99" */

...

> +	/* 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(*new_buf), GFP_KERNEL);

I remember that I asked why we don't use krealloc_array() here... Perhaps
that we don't need to copy the old mapping table? Can we put a short comment
to clarify this in the code?

> +		if (!new_buf)
> +			return -ENOMEM;
> +
> +		kfree(chan->orig_addrs);
> +		chan->orig_addrs = new_buf;
> +		chan->orig_addrs_size = num;
> +	}

...

> +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;
> +	int ret;
> +
> +	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);

How do you know (or why do we limit) that the scope of this function will be
only in ->probe()? Even though, I would replace devm_ by non-devm_ since there
is the tear-down function has to be called by the user anyway.

> +	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;
> +
> +	atr->i2c_nb.notifier_call = i2c_atr_bus_notifier_call;
> +	ret = bus_register_notifier(&i2c_bus_type, &atr->i2c_nb);
> +	if (ret) {
> +		mutex_destroy(&atr->lock);
> +		return ERR_PTR(ret);
> +	}
> +
> +	return atr;
> +}

...

> +void i2c_atr_del_adapter(struct i2c_atr *atr, u32 chan_id)
> +{
> +	char symlink_name[ATR_MAX_SYMLINK_LEN];

> +

Redundant blank line.

> +	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 (!adap)
> +		return;

Redundant check (it will be optimized out by compiler) or wrong assignments
above.

> +	dev_dbg(dev, "Removing ATR child bus %d\n", i2c_adapter_id(adap));
> +
> +	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);
> +
> +	atr->adapter[chan_id] = NULL;
> +
> +	fwnode_handle_put(fwnode);
> +	mutex_destroy(&chan->orig_addrs_lock);
> +	kfree(chan->orig_addrs);
> +	kfree(chan);
> +}

...

> +void i2c_atr_set_driver_data(struct i2c_atr *atr, void *data)
> +{
> +	atr->priv = data;
> +}
> +EXPORT_SYMBOL_NS_GPL(i2c_atr_set_driver_data, I2C_ATR);
> +
> +void *i2c_atr_get_driver_data(struct i2c_atr *atr)
> +{
> +	return atr->priv;
> +}
> +EXPORT_SYMBOL_NS_GPL(i2c_atr_get_driver_data, I2C_ATR);

Just to be sure: Is it really _driver_ data and not _device instance_ data?

-- 
With Best Regards,
Andy Shevchenko