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

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Message-Id: <2FE8B176-8A95-457C-A9B9-D089FC525E6A@gmail.com>
Date:	Mon, 18 May 2015 17:13:21 +0300
From:	Pantelis Antoniou <pantelis.antoniou@...il.com>
To:	frowand.list@...il.com
Cc:	Geert Uytterhoeven <geert+renesas@...der.be>,
	linux-kbuild@...r.kernel.org,
	devicetree <devicetree@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] kbuild: Create a rule for building device tree overlay objects

Hi Frank,

> On May 16, 2015, at 05:16 , Frank Rowand <frowand.list@...il.com> wrote:
> 
> 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…
> 

Many people asked for overlays with hot-plugging probably busses like PCI and USB.

Think about an FPGA on a PCI board that needs to be instantiated after insertion.

Expect more activity on that particular front; probably this week.

>> 
>> 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

Regards

— Pantelis

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