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



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