linux-kernel - Re: [PATCH/RFC] kbuild: Create a rule for building device tree overlay objects

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Message-ID: <5556A893.9060204@gmail.com>
Date:	Fri, 15 May 2015 19:16:51 -0700
From:	Frank Rowand <frowand.list@...il.com>
To:	frowand.list@...il.com
CC:	Pantelis Antoniou <pantelis.antoniou@...il.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	linux-kbuild@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] kbuild: Create a rule for building device tree overlay
 objects

On 5/15/2015 5:47 PM, Frank Rowand wrote:
> On 5/12/2015 7:33 AM, Pantelis Antoniou wrote:
>> Hi Geert,
>>
>>> On May 12, 2015, at 14:56 , Geert Uytterhoeven <geert+renesas@...der.be> wrote:
>>>
>>> This allows to handle device tree overlays like plain device trees.
>>>
>>> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>> ---
>>> Questions:
>>>  - Do we want dtso files under arch/<arch>/boot/dts/, too?
>>>  - Do we want to move the dts files outside the kernel repository
>>>    first?
>>>
>>
>> Oh that’s a nice hornet’s nest you’ve kicked here.
>>
>> arch/<arch>/boot/dts should not be the place, cause overlays are not related with boot per se.
>> As they are right now are board (family) specific.
> 
> Aren't overlays meant to describe child boards (capes, shields, whatever) that may
> vary from system to system, but are not expected to be hot-plugged while the OS
> is up?  Or is hot-plug a design goal?

To reply to myself, there is a current discussion about whether to use overlays to
help with a powerpc pci desire to add and delete subtrees:

   http://www.spinics.net/lists/linux-pci/msg40740.html

That sound like hot-plug to me...

> 
> If no hot-plug, then to me an overlay is just as related to boot as the base dts.
> It is a mere implementation detail that overlays are "loaded" from userspace
> instead of by the booting kernel (I don't really know the details of using
> overlays, so please correct me if I am wrong about how the kernel becomes aware
> of an overlay).
> 
>>
>> I think we should try to keep an external kernel repo with them for now until we
>> figure out where to put them.
>>
>>> scripts/Makefile.lib | 3 +++
>>> 1 file changed, 3 insertions(+)
>>>
>>> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
>>> index 79e86613712f2230..4b14eef1d4b2ce8f 100644
>>> --- a/scripts/Makefile.lib
>>> +++ b/scripts/Makefile.lib
>>> @@ -292,6 +292,9 @@ cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \
>>> $(obj)/%.dtb: $(src)/%.dts FORCE
>>> 	$(call if_changed_dep,dtc)
>>>
>>> +$(obj)/%.dtbo: $(src)/%.dtso FORCE
>>> +	$(call if_changed_dep,dtc)
>>> +
>>> dtc-tmp = $(subst $(comma),_,$(dot-target).dts.tmp)
>>>
>>> # Bzip2
>>> -- 
>>> 1.9.1
>>
>> Regards
>>
>> — Pantelis
>>
>> --
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>>
> 
> 

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