Re: [PATCH v4 2/8] i2c: add I2C Address Translator (ATR) support

Tomi Valkeinen
2022-11-04T11:59:16Z

Hi Andy,

On 01/11/2022 16:30, Andy Shevchenko wrote:
> On Tue, Nov 01, 2022 at 03:20:26PM +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.
>
> ...
>
>> i2c-topology
>> muxes/i2c-mux-gpio
>> i2c-sysfs
>> + muxes/i2c-atr
>
> Doesn't make sense to group muxes/*, that they are following each other?

Ok.

> ...
>
>> +I2C ADDRESS TRANSLATOR (ATR)
>> +M: Luca Ceresoli <luca@lucaceresoli.net>
>
> Hmm... Are you going to maintain this? Or Review? Why not?

We haven't discussed with Luca if he wants to maintain this (this is
mostly his code). But, indeed, I should add my name there.

>> +L: linux-i2c@vger.kernel.org
>> +S: Maintained
>> +F: drivers/i2c/i2c-atr.c
>> +F: include/linux/i2c-atr.h
>
> ...
>
>> + void *new_buf = kmalloc_array(num, sizeof(chan->orig_addrs[0]),
>> + GFP_KERNEL);
>> + if (new_buf == NULL)
>> + return -ENOMEM;
>
> Isn't it better to write this as
>
> void *new_buf;
>
> new_buf = kmalloc_array(num, sizeof(chan->orig_addrs[0]), GFP_KERNEL);
> if (!new_buf)
> return -ENOMEM;

Ok.

> Remarks:
> - note the style of the conditional
> - why is it void?

No idea. I'll change it.

>
> Also, does it make sense to use krealloc_array() or is it complete replacement
> of the data?

The whole array will be rewritten, so we don't need to preserve the
current data.

The buffer allocated here (i.e. orig_addrs) is only used for the
duration of the i2c_atr_master_xfer(). So, we could allocate a new
buffer for every xfer call, but to avoid that, we retain the old buffer.
Any old data in the buffer can be discarded.

>> + kfree(chan->orig_addrs);
>> + chan->orig_addrs = new_buf;
>> + chan->orig_addrs_size = num;
>
> ...
>
>> +static void i2c_atr_unmap_msgs(struct i2c_atr_chan *chan, struct i2c_msg msgs[],
>> + int num)
>
> [] in the function parameter is longer than * and actually doesn't make
> difference in C. Ditto for the rest of similar cases.

Ok. I missed a few, it seems.

> ...
>
>> +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)
>
> Can flags be fixed size (yes I understand that in our case short would probably
> never be different to u16, but for the sake of clearness)?

The parameters and their types come from the ops in struct i2c_algorithm.

> ...
>
>> +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 = 0;
>
> Can we split assignment from the definition and locate it closer to the first
> use?

Actually, I don't think we need to initialize it at all. If
attach_client() fails, we don't care about alias_id. If attach_client()
succeeds, it _must_ return alias_id.

>> + int ret = 0;
>
> Useless assignment.

Yep.

>> +
>> + c2a = kzalloc(sizeof(*c2a), GFP_KERNEL);
>> + if (!c2a)
>> + return -ENOMEM;
>> +
>> + ret = atr->ops->attach_client(atr, chan->chan_id, info, client,
>> + &alias_id);
>
> On one line looks better.

I agree, but it doesn't fit into 80 characters. I personally think
that's a too narrow a limit, but some maintainers absolutely require max
80 chars, so I try to limit the lines to 80 unless it looks really ugly.

>> + 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;
>> +}
>
> ...
>
>> +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;
>> + 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);
>> + }
>
> It seems you have OF independent code, but by some reason you included of.h
> instead of property.h. Am I right?

Just an leftover from the conversion from of to fwnode.

>> + 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_add_adapter;
>> + }
>> +
>> + symlink_name = kasprintf(GFP_KERNEL, "channel-%u", chan_id);
>
> No NULL check?

Right, missed that.

>> + WARN(sysfs_create_link(&chan->adap.dev.kobj, &dev->kobj, "atr_device"),
>> + "can't create symlink to atr device\n");
>> + WARN(sysfs_create_link(&dev->kobj, &chan->adap.dev.kobj, symlink_name),
>> + "can't create symlink for channel %u\n", chan_id);
>
> Why WARNs? sysfs has already some in their implementation.

True, and I can drop these if required. But afaics, sysfs_create_link
only warns if there's a duplicate entry, not for other errors.

>> +
>> + kfree(symlink_name);
>> +
>> + dev_dbg(dev, "Added ATR child bus %d\n", i2c_adapter_id(&chan->adap));
>> +
>> + atr->adapter[chan_id] = &chan->adap;
>> + return 0;
>> +
>> +err_add_adapter:
>> + mutex_destroy(&chan->orig_addrs_lock);
>> + kfree(chan);
>> + return ret;
>> +}
>
> ...
>
>> + struct fwnode_handle *fwnode = adap->dev.fwnode;
>
> Please don't dereference fwnode like this, we have dev_fwnode() for that.

Ok.

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

Yep.

>> + dev_err(dev, "Adapter %d does not exist\n", chan_id);
>> + return;
>> + }
>
> ...
>
>> + snprintf(symlink_name, sizeof(symlink_name),
>> + "channel-%u", chan->chan_id);
>
> Once line?

80 char limit here too. But I see that this is (kind of) broken. In the
i2c_atr_add_adapter() I used dynamic alloc for the symlink_name, but
here we still have the fixed size buffer.

>
> ...
>
>> + atr_size = struct_size(atr, adapter, max_adapters);
>
>> + if (atr_size == SIZE_MAX)
>> + return ERR_PTR(-EOVERFLOW);
>
> Dunno if you really need this to be separated from devm_kzalloc(), either way
> you will get an error, but in embedded case it will be -ENOMEM.

Yep. Well... I kind of like it to be explicit. Calling alloc(SIZE_MAX)
doesn't feel nice.

>> + atr = devm_kzalloc(dev, atr_size, GFP_KERNEL);
>> + if (!atr)
>> + return ERR_PTR(-ENOMEM);
>
> ...
>
>> +EXPORT_SYMBOL_GPL(i2c_atr_delete);
>
> I would put these to their own namespace from day 1.

What would be the namespace? Isn't this something that should be
subsystem-wide decision? I have to admit I have never used symbol
namespaces, and don't know much about them.

>
> ...
>
>> +/**
>> + * Helper to add I2C ATR features to a device driver.
>> + */
>
> ??? Copy'n'paste typo?

No idea where that is from... I'll fix it.

>> +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;
>> + struct mutex lock;
>> + int max_adapters;
>> +
>> + struct i2c_adapter *adapter[0];
>
> No VLAs.

Ok.

I'm not arguing against any of the comments you've made, I think they
are all valid, but I want to point out that many of them are in a code
copied from i2c-mux.

Whether there's any value in keeping i2c-mux and i2c-atr similar in
design/style... Maybe not.

>> +};
>
> ...
>
>> +int i2c_atr_add_adapter(struct i2c_atr *atr, u32 chan_id,
>> + struct fwnode_handle *bus_np);
>
> Missing
>
> struct fwnode_handle;
>
> at the top of the file?

Ok.

Tomi