Re: [Ksummit-2013-discuss] Defining schemas for Device Tree
From: Jason Cooper
Date: Mon Jul 29 2013 - 18:49:25 EST
On Tue, Jul 30, 2013 at 08:29:20AM +1000, David Gibson wrote:
> On Mon, Jul 29, 2013 at 01:23:39PM -0400, Jason Cooper wrote:
> > 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.
> I've always considered the DT to be unordered, although the flattened
> representation obviously has to have some order. It is much safer to
> explicitly represent any required orderings with properties, rather
> than to rely on the flattened tree order. I really don't think trying
> to have dtc magically understand device initialization ordering in
> this way is a good idea.
> Fwiw, dtc generally preserves order between input and output, with the
> exception of the -s option, which sorts the subnodes of each node by
> name (useful for dtdiff).
> > > 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.
> It's not necessarily possible to encode device initialization order in
> flattened tree order. Suppose you have bus A with devices A1 and A2,
> and bus B with devices B1 and B2. A1 must be initialized before B1,
> but B2 must be initialized before A2. There are no loops there, it's
> a valid set of initialization order constraints, but you can't get
> both of them right in the flat tree ordering.
True, but is there a real scenario where this is the case? In any
event, this could still fall back to deferred probing.
As I think about it more, working with only what dtc can definitely see,
eg busses and phandles, some ordering optimization could be done to
reduce the number of probe deferrals.
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