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

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

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);
     ...
   }

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

> > +/*
> > + * 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.

Regards,
Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com