linux-kernel - Re: [PATCH] tools lib api: respect CROSS

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Date:	Tue, 22 Mar 2016 11:22:53 -0700
From:	David Sharp <dhsharp@...gle.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Jiri Olsa <jolsa@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kernel@...gutronix.de, patchwork-lst@...gutronix.de,
	Wang Nan <wangnan0@...wei.com>
Subject: Re: [PATCH] tools lib api: respect CROSS_COMPILE for the linker

On Tue, Mar 22, 2016 at 7:42 AM, Arnaldo Carvalho de Melo
<acme@...nel.org> wrote:
> Em Tue, Mar 22, 2016 at 08:50:42AM -0400, Steven Rostedt escreveu:
>> On Tue, 22 Mar 2016 08:10:10 +0100
>> Jiri Olsa <jolsa@...hat.com> wrote:
>>
>> > On Mon, Mar 21, 2016 at 05:40:30PM -0300, Arnaldo Carvalho de Melo wrote:
>> > > Em Mon, Mar 21, 2016 at 09:08:52AM +0100, Jiri Olsa escreveu:
>> > > > On Fri, Mar 18, 2016 at 02:38:52PM -0300, Arnaldo Carvalho de Melo wrote:
>> > > > > Em Fri, Mar 18, 2016 at 12:16:23PM -0500, Josh Poimboeuf escreveu:
>> > > > > > So 'allow-override' would probably be a good option.
>> > >
>> > > > > Humm, my preference is to make tools/ look like the kernel, and the
>> > > > > kernel doesn't use that allow-override thing, right? So perhaps add what
>> > > > > is missing to make it look exactly like the kernel and then ditch this
>> > > > > allow-override thing?
>> > >
>> > > > Steven explained his reason for allow-override in the comment above it,
>> > > > please make sure the new solution follows that
>> > >
>> > > Sure, and I'm no make guru, but what puzzles me is why isn't this
>> > > required in:
>> > >
>> > > [acme@...et linux]$ grep -w ^CC Makefile
>> > > CC                = $(CROSS_COMPILE)gcc

This works, but doesn't allow setting CC from env since it is
unconditionally overwritten here.

>> > > [acme@...et linux]$
>> >
>> > Steve has special requirements I guess ;-) CC-ed
>> >
>>
>> I just copied what I had in trace-cmd. David Sharp is the one that
>> added that code.
>>
>>  Link: http://lkml.kernel.org/r/1299791491-1805-1-git-send-email-dhsharp@google.com
>
> David, so, what was the usecase for that? Something we can try to
> reproduce so that we can check if the kernel solution covers your
> specific case?
>
> - Arnaldo

It was a very long time ago, so I don't fully remember my specific
requirements at the time.

However, I'm sure I was wanting to set just "CROSS_COMPILE" from the
environment rather than all of CC, LD, AS, AR, etc, while still
allowing the flexibility to set CC, etc from the environment.

If you have just:

CC ?= $(CROSS_COMPILE)gcc

well, that line will never have any effect (not even the value of
"gcc"), because CC is already set by default by make itself (to "cc").

Demo (gmail won't let me paste tabs, so you'll have to fix the tabs in
the "all" rule):

"""
$ cat Makefile
define allow-override
  $(if $(or $(findstring environment,$(origin $(1))),\
            $(findstring command line,$(origin $(1)))),,\
    $(eval $(1) = $(2)))
endef

CC ?= $(CROSS_COMPILE)gcc
$(call allow-override,cross_CC,$(CROSS_COMPILE)gcc)

all:
        @echo "CC: $(CC)"
        @echo "cross_CC: $(cross_CC)"
$ make -f Makefile
CC: cc
cross_CC: gcc
$ CC=other-gcc cross_CC=other-gcc make -f Makefile
CC: other-gcc
cross_CC: other-gcc
$ CROSS_COMPILE=cross- make -f Makefile
CC: cc
cross_CC: cross-gcc
"""

So, note that without allow-override:
- The set default of "gcc" is not respected.
- Setting CC from the env does work.
- Setting CROSS_COMPILE from the env is not effective.

Hence the title of the patch and macro: "Makefiles suck". ;)