Re: [PATCH v10 1/8] i2c: add I2C Address Translator (ATR) support

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

On 20/03/2023 10:28, Luca Ceresoli wrote:
> Hello Matthias,
> 
> thanks for the in-depth review!
> 
> On Mon, 20 Mar 2023 07:34:34 +0100
> zzam@gentoo.org wrote:
> 
>> Some inline comments below.
>>
>> Regards
>> Matthias
>>
>> Am 22.02.23 um 14:29 schrieb Tomi Valkeinen:
>>> 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 it
>>> 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.
>>>
>>> Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>> ---
>>>    Documentation/i2c/index.rst         |   1 +
>>>    Documentation/i2c/muxes/i2c-atr.rst |  97 +++++
>>>    MAINTAINERS                         |   8 +
>>>    drivers/i2c/Kconfig                 |   9 +
>>>    drivers/i2c/Makefile                |   1 +
>>>    drivers/i2c/i2c-atr.c               | 548 ++++++++++++++++++++++++++++
>>>    include/linux/i2c-atr.h             | 116 ++++++
>>>    7 files changed, 780 insertions(+)
>>>    create mode 100644 Documentation/i2c/muxes/i2c-atr.rst
>>>    create mode 100644 drivers/i2c/i2c-atr.c
>>>    create mode 100644 include/linux/i2c-atr.h
>>>    
>> [...]
>>> diff --git a/drivers/i2c/i2c-atr.c b/drivers/i2c/i2c-atr.c
>>> new file mode 100644
>>> index 000000000000..5ab890b83670
>>> --- /dev/null
>>> +++ b/drivers/i2c/i2c-atr.c
>>> @@ -0,0 +1,548 @@
>> [...]
>>> +
>>> +/*
>>> + * Replace all message addresses with their aliases, saving the original
>>> + * addresses.
>>> + *
>>> + * This function is internal for use in i2c_atr_master_xfer(). It must be
>>> + * followed by i2c_atr_unmap_msgs() to restore the original addresses.
>>> + */
>>> +static int i2c_atr_map_msgs(struct i2c_atr_chan *chan, struct i2c_msg *msgs,
>>> +			    int num)
>>> +{
>>> +	struct i2c_atr *atr = chan->atr;
>>> +	static struct i2c_atr_cli2alias_pair *c2a;
>>> +	int i;
>>> +
>>> +	/* Ensure we have enough room to save the original addresses */
>>> +	if (unlikely(chan->orig_addrs_size < num)) {
>>> +		u16 *new_buf;
>>> +
>>> +		/* We don't care about old data, hence no realloc() */
>>> +		new_buf = kmalloc_array(num, sizeof(*new_buf), GFP_KERNEL);
>>> +		if (!new_buf)
>>> +			return -ENOMEM;
>>> +
>>> +		kfree(chan->orig_addrs);
>>> +		chan->orig_addrs = new_buf;
>>> +		chan->orig_addrs_size = num;
>>> +	}
>>> +
>>> +	for (i = 0; i < num; i++) {
>>> +		chan->orig_addrs[i] = msgs[i].addr;
>>> +
>>> +		c2a = i2c_atr_find_mapping_by_addr(&chan->alias_list,
>>> +						   msgs[i].addr);
>>> +		if (!c2a) {
>>> +			dev_err(atr->dev, "client 0x%02x not mapped!\n",
>>> +				msgs[i].addr);
>>> +			return -ENXIO;
>> I miss the roll-back of previously modified msgs[].addr values.
> 
> Indeed you have a point. There is a subtle error in case all of the
> following happen in a single i2c_atr_master_xfer() call:
> 
>   * there are 2+ messages, having different addresses
>   * msg[0] is mapped correctly
>   * msg[n] (n > 0) fails mapping
> 
> It's very unlikely, but in this case we'd get back to the caller with
> an error and modified addresses for the first n messages. Which in turn
> is unlikely to create any problems, but it could.
> 
> Tomi, do you agree?
> 
> This looks like a simple solution:
> 
>     if (!c2a) {
> +    i2c_atr_unmap_msgs(chan, msgs, i);
>       ...
>     }

Wouldn't that possibly restore the address from orig_addrs[x] also for 
messages we haven't handled yet?

I think a simple

while (i--)
	msgs[i].addr = chan->orig_addrs[i];

should do here. It is also, perhaps, a bit more clear this way, as you 
can see the assignments to msgs[i].addr nearby, and the rollback here 
with the above code. Instead of seeing a call to an unmap function, 
having to go and see what exactly it will do.

> While there, maybe switching to dev_err_probe would make code cleaner.

The while loop above has to be done after the print, if we use the same 
i variable in both. dev_err_probe could still be used, but... I don't 
know if it's worth trying to push it in.

>>> +/*
>>> + * Restore all message address aliases with the original addresses. This
>>> + * function is internal for use in i2c_atr_master_xfer().
>>> + *
>>> + * @see i2c_atr_map_msgs()
>>> + */
>>> +static void i2c_atr_unmap_msgs(struct i2c_atr_chan *chan, struct i2c_msg *msgs,
>>> +			       int num)
>>> +{
>>> +	int i;
>>> +
>>> +	for (i = 0; i < num; i++)
>>> +		msgs[i].addr = chan->orig_addrs[i];
>> Does this code needs null and size checks for orig_addrs/orig_addrs_size
>> to protect from oopses?
>> This cannot happen now as i2c_atr_master_xfer returns early when
>> i2c_atr_map_msgs fails.
> 
> The map/unmap functions are really a part of i2c_atr_master_xfer() that
> has been extracted for code readability, as the comments say, and I
> can't think of a different use for them. So I think this code is OK as
> is.
> 
> However a small comment might help future readers, especially in case
> code will change and these functions gain new use cases.
> E.g.
> 
>     This function is internal for use in i2c_atr_master_xfer()
> +  and for this reason it needs no null and size checks on orig_addr.
>     It must be followed by i2c_atr_unmap_msgs() to restore the original addresses.

I can add a comment. as Luca said, it's an internal helper function, I 
don't think we need to check the parameters there for cases which can't 
happen.

  Tomi