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

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

Hi Tomi,

On Mon, 20 Mar 2023 14:12:32 +0200
Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> wrote:

> 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?

No, because there is  'i' as the 3rd argument, not 'num'. But...

> 
> 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.

...sure, this would work. If I had connected my brain at the
appropriate time I would have realized it's two lines only. And
definitely less spaghetti-coded that what I had suggested.

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