[fedora-arm] precedence of built-in vs. platform trees?

[fedora-arm] precedence of built-in vs. platform trees?

[Jon Masters]

Hey guys,

I apologize if I should have RTFM. If a platform provides a device tree
at boot time, and the kernel also has a tree appended, what behavior is
supposed to happen? i.e. what is the standard that is anticipated here?

Jon.
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Re: [fedora-arm] precedence of built-in vs. platform trees?

[Leif Lindholm]

On Thu, Nov 29, 2012 at 01:39:56AM -0500, Jon Masters wrote:
> Hey guys,
> 
> I apologize if I should have RTFM. If a platform provides a device tree
> at boot time, and the kernel also has a tree appended, what behavior is
> supposed to happen? i.e. what is the standard that is anticipated here?

From the kernel config help on CONFIG_ARM_APPENDED_DTB:
<<<
This is meant as a backward compatibility convenience for those
systems with a bootloader that can't be upgraded to accommodate
the documented boot protocol using a device tree.
>>>
and
<<<
Do not leave this option active in a production kernel
if you don't intend to always append a DTB.
>>>

Meaning that if the loader supports passing DTB, the kernel shouldn't
have an appended one.

Apart from that, if you use an appended DTB, this completely overrides
any DTB passed by the loader, so (unless you also enable the unholy
CONFIG_ARM_ATAG_DTB_COMPAT) you also lose the ability to pass kernel
command line options from the loader.

/
	Leif
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Re: precedence of built-in vs. platform trees?

[Grant Likely]

On Thu, Nov 29, 2012 at 6:39 AM, Jon Masters <jonathan-Zp4isUonpHBD60Wz+7aTrA@public.gmane.org> wrote:
> Hey guys,
>
> I apologize if I should have RTFM. If a platform provides a device tree
> at boot time, and the kernel also has a tree appended, what behavior is
> supposed to happen? i.e. what is the standard that is anticipated here?

Hmmm, nobody has asked that before. I don't think it is really defined :-)

I presume that the built-in DT will end up getting used from what I
remember about the code.

g.

Re: precedence of built-in vs. platform trees?

[Mitch Bradley]

On 11/29/2012 1:46 AM, Grant Likely wrote:
> On Thu, Nov 29, 2012 at 6:39 AM, Jon Masters <jonathan-Zp4isUonpHBD60Wz+7aTrA@public.gmane.org> wrote:
>> Hey guys,
>>
>> I apologize if I should have RTFM. If a platform provides a device tree
>> at boot time, and the kernel also has a tree appended, what behavior is
>> supposed to happen? i.e. what is the standard that is anticipated here?
> 
> Hmmm, nobody has asked that before. I don't think it is really defined :-)
> 
> I presume that the built-in DT will end up getting used from what I
> remember about the code.

That sounds like the right default, but it would be nice if one could
override it with a cmdline option.  It's *usually* easier to change the
kernel than the platform firmware or bootloader, but there are
exceptions.  In our (OLPC's) case, DT mods can be made trivially by
editing a script (and the firmware includes an editor).  We use this
feature all the time in development, and sometimes even for customers,
for testing bug fixes.

> 
> g.
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