linux-kernel - Re: [PATCH] Kbuild: Avoid DTB rebuilds if source files are untouched

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Date:	Thu, 4 Apr 2013 11:00:35 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Michal Marek <mmarek@...e.cz>,
	Stephen Warren <swarren@...dia.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	<linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH] Kbuild: Avoid DTB rebuilds if source files are untouched

On 04/03/2013 09:48 PM, Stephen Warren wrote:
> On 04/03/2013 01:14 AM, Vineet Gupta wrote:
>> forgot to CC linux-arch
>>
>> On 04/03/2013 12:42 PM, Vineet Gupta wrote:
>>> Currently, for every ARC kernel build I see the following:
>>>
>>> --------------->8-----------------
>>>   DTB    arch/arc/boot/dts/angel4.dtb.S
>>>   AS      arch/arc/boot/dts/angel4.dtb.o
>>>   LD      arch/arc/boot/dts/built-in.o
>>> rm arch/arc/boot/dts/angel4.dtb.S        <-- forces rebuild next iter
>>>   CHK     kernel/config_data.h
>>> --------------->8-----------------
> I assume that's because the file is an intermediate file, and only built
> due to a chain of build rules, and hence make clean it up itself after
> the build?

Indeed - I should have made that explicit in the Changelog.

>>> +.PRECIOUS: $(obj)/%.dtb.S
>>> +
>>>  $(obj)/%.dtb.S: $(obj)/%.dtb
>>>  	$(call cmd,dt_S_dtb)
> I'm not sure if .PRECIOUS is correct here. That prevents make from
> deleting the file if make is CTRL-C'd in the middle of generating it.
> Couldn't that leave a stale/corrupt file around that'd break the build.
> Judging by:
>
> http://www.gnu.org/software/make/manual/html_node/Special-Targets.html
>
> I think .SECONDARY might be a better choice? Does that solve the problem
> you're seeing?

Technically .SECONDARY is better - however it doesn't seem to work.
Running make with various debug toggles doesn't seem to be helping with why
.PRECIOUS works but not this.
That is also likely reason for a bunch of other .PRECIOUS entries in the same file
but no .SECONDARY.

I presume this is something not specific to ARC kernel or my host.
Can you try the .SECONDARY locally to see if it works for you.

Thx,
-Vineet
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