Re: [PATCH v2 5/7] dt-bindings: i3c: Document core bindings

[Boris Brezillon]

Hi Rob,

On Tue, 26 Dec 2017 12:29:34 -0600
Rob Herring <robh@xxxxxxxxxx> wrote:

> >> > > > +Optional properties
> >> > > > +-------------------
> >> > > > +- reg: static address. Only valid is the device has a static address.
> >> > > > +- i3c-dynamic-address: dynamic address to be assigned to this device. This
> >> > > > +                      property depends on the reg property.  
> >> > >
> >> > > Perhaps "assigned-address" property would be appropriate. I'm not all
> >> > > that familiar with it though.  
> >> >
> >> > Again, the spec use the term "dynamic address" everywhere, and I'd like
> >> > to stay as close as possible to the spec.  
> >>
> >> I looked at assigned-addresses a bit more and that won't really fit
> >> because it should be the same format as reg. So I think reg should
> >> always be the PID as that is fixed and always present. Then the DAA
> >> address is separate and can be the i3c-dynamic-address property.
> >>
> >> However, there's still part I don't understand...
> >>  
> >> > > > +               /* I3C device with a static address. */
> >> > > > +               thermal_sensor: sensor@68 {
> >> > > > +                       reg = <0x68>;
> >> > > > +                       i3c-dynamic-address = <0xa>;  
> >>
> >> I'm confused as to how/why you have both reg and dynamic address?  
> >
> > Some I3C devices have an I2C address (also called static or legacy
> > address in a few places). The static/I2C/legacy address is used until
> > the I3C device is assigned a dynamic address by the master. The whole
> > point of specifying both an I2C address (through the reg property) and
> > a dynamic address (through the i3c-dynamic-address) is to tell the
> > controller that a specific dynamic address should be assigned to this
> > device using the SETSADA (Set Dynamic Address from Static Address)
> > command before a DAA (Dynamic Address Assignment) procedure is started.
> > This way, the device will not participate to the DAA (because it
> > already has a valid DA) and the dynamic address can't be assigned to
> > a different device (which is one of the problem with the automatic DAA
> > procedure).  
> 
> Okay, think I got it now.
> 
> I think we should extend "reg" to have either I2C address, I3C PID, or
> both (in a defined order). I'm assuming you can always distinguish a
> static I2C address and an I3C PID just by upper bits all being 0s for
> I2C addresses. Maybe both is not needed? This means we'd have to allow
> 64-bit I2C addresses (#address-cells=2), but that should be easily
> fixed if that causes problems in the kernel.
> 
> So i3c-pid would go away and i3c-dynamic-address stays.

Hm, actually I'm not sure this is a good idea. Sounds like we're
abusing the purpose of reg here. For busses, reg is supposed to encode
the id of the device on the bus that is used to communicate with this
device (CS line for SPI, I2C address for I2C devs, ...). With I3C, the
PID is just a way to uniquely identify a device, but is not used during
communications (we either use the static/I2C address or the dynamic
address assigned by the master).

If your concern is just about I3C dev naming convention, maybe we
could have something like:

	i3c-master@xxxx {
		...
		i2cdev@xx {
			reg = <xx>;
			i3c-lvr = <yy>;
			...
		};
		...

		i3cdev-<i3c-pid>[@zz] {
			i3c-pid = <pppppp>;
			/*
			 * reg only defined if the device has a static
			 * address.
			 */
			[reg = <zz>;]
			/*
			 * i3c-dynamic-address only defined if a
			 * specific dynamic address is requested.
			 */
			[i3c-dynamic-address = <dd>;]
		};
	}; 

With this approach we have a way to quickly identify i3c devices by
their pid when looking at their names (with the -<i3c-pid> suffix), and
we keep reg for static/i2c addresses only.

Regards,

Boris