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

Re: [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.

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

[Jason Gunthorpe]

On Mon, Jul 29, 2013 at 01:23:39PM -0400, Jason Cooper wrote:

> 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.

That seems like a really smart idea. dtc could store the startup-order
and dependencies in the dtb someplace and the kernel can just run
through that. Using a combination of declaration order and
topo-phandle sorting would probably cover all cases???

Jason

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

[Mark Brown]

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On Mon, Jul 29, 2013 at 01:23:39PM -0400, Jason Cooper wrote:

> 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.

FWIW this is broadly speaking what deferred probing is supposed to do -
it seemed too complex and fragile to make the graph for board files so
Grant came up with the idea of deferred probing which aims to sidestep
the problem and work it out dynamically, also providing a fallback in
case something that has actually optimised the ordering misses some case
(cleaning up after the effects of parallel probes for example).

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

[David Gibson]

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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.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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

[Jason Cooper]

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.

thx,

Jason.

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

[David Gibson]

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On Mon, Jul 29, 2013 at 06:48:40PM -0400, Jason Cooper wrote:
> 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.

I never count on weird and wonderful arrangements _not_ appearing in
embedded.

But, in regards to falling back do deferred probing.  If you're
thinking of the fdt ordering as purely an optimization, rather than
_required_ to get device init correct, then that's a very different
matter.  I have no problem with optimizing the ordering, as long as
its expected that the kernel will still be correct with arbitrary
ordering.

> 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.

Well.  It depends what you mean here.  To do this fully would require
dtc to interpret properties much more than it currently does - in
nearly all cases it treats them as opaque blobs, whereas many
different kinds of properties can potentially include phandles, and
you have to know how to parse them to discover them.

On the other hand, if what you're referring to is dtc's &-syntax for
phandle references, then I guess we could topsort on that.  As long as
we bear in mind that that can miss cases, if people hand craft their
phandles, instead of using references.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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

[Jason Cooper]

On Tue, Jul 30, 2013 at 09:45:32AM +1000, David Gibson wrote:
> On Mon, Jul 29, 2013 at 06:48:40PM -0400, Jason Cooper wrote:
> > 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.
> 
> I never count on weird and wonderful arrangements _not_ appearing in
> embedded.

:-)

> But, in regards to falling back do deferred probing.  If you're
> thinking of the fdt ordering as purely an optimization, rather than
> _required_ to get device init correct, then that's a very different
> matter.  I have no problem with optimizing the ordering, as long as
> its expected that the kernel will still be correct with arbitrary
> ordering.

Yes, after sleeping on it a night, I agree.

> > 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.
> 
> Well.  It depends what you mean here.  To do this fully would require
> dtc to interpret properties much more than it currently does - in
> nearly all cases it treats them as opaque blobs, whereas many
> different kinds of properties can potentially include phandles, and
> you have to know how to parse them to discover them.
> 
> On the other hand, if what you're referring to is dtc's &-syntax for
> phandle references, then I guess we could topsort on that.  As long as
> we bear in mind that that can miss cases, if people hand craft their
> phandles, instead of using references.

Just this would eliminate a majority of probe deferrals.

For example, on the Seagate Wireless Plus I was recently trying to put a
mainline kernel on, all of the probe deferrals were pinctrl related.
Just topsorting that node would've fixed the problem.

thx,

Jason.

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

[David Lang]

On Mon, 29 Jul 2013, Jason Cooper wrote:

> 
>>
>> 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.

if you make the initializaiton order 'magicly' correct by following the order of 
the flat representation, how do you reflect the case where initialization can be 
overlapped for different devices?

you are just trading one side of the problem for the other.

David Lang

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

[jonsmirl]

On Mon, Jul 29, 2013 at 8:41 PM, David Lang <david@lang.hm> wrote:
> On Mon, 29 Jul 2013, Jason Cooper wrote:
>
>>
>>>
>>> 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.
>
>
> if you make the initializaiton order 'magicly' correct by following the
> order of the flat representation, how do you reflect the case where
> initialization can be overlapped for different devices?

I agree with David, using DT to try and eliminate deferred probes
isn't a good solution.  Overlapped probes and doing probes on multiple
CPUs introduces a temporal angle to the problem. Best to just let the
deferred probing code dynamically solve the problem. From what I can
see the deferred probing solution is working out nicely.

Plus there isn't that much code being run in deferred probing. I
suspect potential savings (if there even is any) are under a
millisecond.

>
> you are just trading one side of the problem for the other.
>
> David Lang



-- 
Jon Smirl
jonsmirl@gmail.com

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

[David Gibson]

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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'm not actually sure what you mean by this.  dtc already preserves
order between input and output.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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

[Jason Cooper]

On Tue, Jul 30, 2013 at 11:50:31AM +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'm not actually sure what you mean by this.  dtc already preserves
> order between input and output.

This is an old comment (~ 1d, wow).  My position has evolved to seeing
if we can allow dtc to topsort nodes it can easily tell are needed first
as an optimization.  *Not* a requirement.  Deferred probing would still
be a fall back.

thx,

Jason.

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

[Thomas Petazzoni]

Dear Tomasz Figa,

On Wed, 31 Jul 2013 14:21:49 +0200, Tomasz Figa wrote:

> u-boot still does not have generic driver model or at least did not have when 
> I last checked it not so long ago, so it seems like there is still long way to 
> go for using FDT as source of information about hardware in u-boot.
> 
> barebox [1], aka u-boot-v2, might be closer to this approach (it might be 
> already using it, but this needs to be checked) as it already has generic 
> driver model and support for DT.

There are already some platforms in Barebox that are using the Device
Tree. For example, the basic Marvell platforms support in Barebox now
uses the Device Tree.

Best regards,

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com