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

Tomi Valkeinen
2022-12-08T16:01:32Z

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