[Ksummit-2013-discuss] Defining schemas for Device Tree

[Jason Cooper]

On Mon, Jul 29, 2013 at 05:49:05PM +0100, Dave Martin wrote:
> On Mon, Jul 29, 2013 at 11:01:24AM -0400, Jason Cooper wrote:
> > On Mon, Jul 29, 2013 at 02:21:52AM +0200, Tomasz Figa wrote:

> > > b) What information should be specified in schemas? What level of 
> > >    granularity is required?
> > 
> > One item I don't see in this list is node ordering.  There's been some
> > discussion lately on deferred probing (re boot times).  If we were to
> > intentionally declare that DT are parsed in the order written, then a
> > lot of deferred probes could be avoided by moving eg the pinctrl node to
> > near the top of the tree.
> > 
> > This doesn't impact buses as much, since the nodes needing the bus are
> > already children.  However, anything accessed via phandles: pins,
> > clocks, regulators, etc could benefit from declaring and enforcing this.
> > Eg having the dtc warn when a phandle is used before it's corresponding
> > node is declared.
> > 
> > Not critical though, just a thought.
> 
> I don't think that siblings have any defined order in DT.  If reading a
> device tree, there's no guarantee you get nodes or properties out in the
> same order as the original .dts file.

That's why I raised the point.  If people think encoding initialization
order in the DT is a good idea, then we should change the dtc so it
compiles/decompiles in the same order.

> Provided child/parent relationships are maintained and the set of nodes
> and values is the same, I think completely rearranging a .dts file does
> not change its meaning.
> 
> "depends-on" relationships mostly have to come from the semantics of
> the bindings themselves: for example, if a device is connected to some
> clocks and regulators, the kernel may need to probe those first.

true, the answer to this problem may be to create a depgraph of the
nodes based on phandles and child status, then init.  However, if the
goal is to accelerate boot times, then that should not be calculated
during each boot, especially since it doesn't likely change from boot to
boot.

Which means it would either go in the dtc (dts node ordering is
irrelevant), or in the dts.  I'm inclined to say dtc should do it, but I
like the aesthetics of things being in the proper order in something I
can read.  After all, C requires functions to be declared before use,
even though the compiler could figure it out.

thx,

Jason.