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

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Date:	Fri, 15 May 2015 17:47:38 -0700
From:	Frank Rowand <frowand.list@...il.com>
To:	Pantelis Antoniou <pantelis.antoniou@...il.com>
CC:	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 5/12/2015 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?

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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