Add support for generic i2c-mux base-nr property

[pelwell]

It can be desirable to have a fixed, known base address for the channels of a mux. However, there is no easy way to do that from a DT overlay (aliases ought to work, but the overlay application logic can't perform the required string pasting to create "i2c", and creating one alias per channel is tedious). To that end, create a base-nr DT property and a base parameter that allows the bus number for channel 0 to be specified, with the remaining channels allocated sequentially after it.

[pelwell]

This has been seen to work: https://forums.raspberrypi.com/viewtopic.php?p=2203123#p2203123

[popcornmix]

You have a merge commit, but otherwise I'm okay with it.

[6by9]

Note comments on a7c1456 over unintended consequences of this for i2c0mux failing.
(I think it's actually due to 841bb8e at fault)

Changing force_nr should be conditional on of_property_read_u32 returning a value, otherwise with no force_nr we get the channels initialised with 0, 1, 2, etc, not all with 0.

[pelwell]

I've been staring at that line, and not noticing that I had an unsaved change in my editor:

 		of_property_read_u32(dev_node, "base-nr", &base_nr);
-		if (!force_nr)
+		if (!force_nr && base_nr)
 			force_nr = base_nr + chan_id;

Now it makes sense.

[6by9]

:-)

I was about to go for

@@ -358,8 +358,7 @@ int i2c_mux_add_adapter(struct i2c_mux_core *muxc,
                u32 base_nr = 0;
                u32 reg;
 
-               of_property_read_u32(dev_node, "base-nr", &base_nr);
-               if (!force_nr)
+               if (!force_nr && !of_property_read_u32(dev_node, "base-nr", &base_nr))
                        force_nr = base_nr + chan_id;
 
                if (muxc->arbitrator)

Technically you could have base-nr defined as 0 if the mux was the only thing in your system, and the parent had been assigned somewhere that it wasn't going to clash. The mux child buses would then be 0, 1, 2, 3, etc, with the parent as eg 10.

[pelwell]

However, other parts of the bus numbering system reject 0 as a forced value, so it's academic.