How complete should the DTS be?

[Josh Boyer]

On Thu, 10 Jan 2008 17:50:49 +1100
Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:

> 
> On Thu, 2008-01-10 at 17:02 +1100, David Gibson wrote:
> > On Thu, Jan 10, 2008 at 12:53:57AM -0500, Sean MacLennan wrote:
> > > David Gibson wrote:
> > > > Hrm... I'd say this is not something which has a firm convention yet.
> > > > It's going to become more of an issue once we get a macros system for
> > > > dtc, so the "440EP" macro would have all the devices, even if some are
> > > > not connected on a given board.
> > > >
> > > > I'm contemplating suggesting that we adopt the "status" property from
> > > > IEEE1275 to cover this.
> > > >
> > > >   
> > > When I am laying out the dts, leaving out what isn't used makes the dts 
> > > file cleaner, at least in my view. It doesn't hurt to have the second 
> > > i2c bus there, but it also doesn't help and leaving it out points out 
> > > that it is not used.
> > > 
> > > When we get a macro system I assume the second i2c bus will be there but 
> > > hidden by a macro. It will still be clean and shouldn't cause grief.
> > 
> > Right, but if it is there we'll want to mark it as unused in some way
> > so that the kernel doesn't waste resources attempting to drive it.
> 
> Sure but I don't want to make it mandatory for people to put unused
> devices in. If the macro system brings them in, then yes, it's good to
> have a way to properly mark them unused. But people hand crafting DTS
> shouldn't have to bloat them.
> 
> There is -one- case where you may want to put unused devices, is if you
> do some kind of resource management on that specific bus (like need to
> be able to dynamically allocate space on the bus). In this case, you
> want to know everything that's there and potentially decodes addresses
> to avoid collisions.

There are other uses too.  E.g. pin sharing between devices based on
DIP switch settings.  You'd want to enumerate all the devices, and then
add 'status = "failed-not-connected"' to the ones that don't have pins.

josh