linux-kernel - Re: [PATCH] Revert "perf tools: Allow to specify custom linker command"

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Date:	Tue, 14 Jul 2015 08:31:52 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Aaro Koskinen <aaro.koskinen@....fi>
Cc:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	linux-arch@...r.kernel.org,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Aaro Koskinen <aaro.koskinen@...ia.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "perf tools: Allow to specify custom linker
 command"

On Mon, Jul 13, 2015 at 11:24:06PM +0300, Aaro Koskinen wrote:
> Hi,
> 
> On Mon, Jul 13, 2015 at 02:10:53PM +0300, Alexey Brodkin wrote:
> > This reverts commit 5ef7bbb09f7b
> > ("perf tools: Allow to specify custom linker command").
> > 
> > LD is a pre-defined variable in GNU Make. I.e. it is always defined.
> > Which means there's no point to check "LD ?= ..." because it will never
> > succeed. And so LD will be either that explicitly passed to make like
> > this:
> >  ------->8-------
> >  make LD=path_to_my_ld ...
> >  ------->8-------
> >  or default value, which is host's "ld".
> > 
> > Latter leads to failure of cross-linkage because instead of cross linker
> > "$(CROSS_COMPILE)ld" host's "ld" is used.
> > 
> > As for commit which is reverted here:
> >  [1] Usually for selection of non-default flavour of CPU core/options
> >      linker flags are used like "-mtune=xxx" or "-mMyCPUType" etc.
> > 
> >  [2] Still to implement ability to use "ld" that differs from
> >     "$(CROSS_COMPILE)ld" one will need to add new makefile variable like
> >     TARGET_LD and then check if $(TARGET_LD) is not specified on make
> >     invocation then use "$(CROSS_COMPILE)ld".
> > 
> > But for now to fix cross-building of perf this revert is enough.
> 
> Hmm. You are probably right, my build system always exports LD with
> correct linker for cross builds so perhaps that's why I this "worked"
> for me when testing. Sorry.
> 
> I guess the correct fix would be [1], i.e. there should be some new
> variable to pass flags to ld command.
> 
> Or maybe [2], could we use make "origin" function? If LD is "default",
> then use "$(CROSS_COMPILE)ld", otherwise use what the user passed?

thats what Steven did in traceevent/Makefile:

---
# Makefiles suck: This macro sets a default value of $(2) for the
# variable named by $(1), unless the variable has been set by
# environment or command line. This is necessary for CC and AR
# because make sets default values, so the simpler ?= approach
# won't work as expected.
define allow-override
  $(if $(or $(findstring environment,$(origin $(1))),\
            $(findstring command line,$(origin $(1)))),,\
    $(eval $(1) = $(2)))
endef

# Allow setting CC and AR, or setting CROSS_COMPILE as a prefix.
$(call allow-override,CC,$(CROSS_COMPILE)gcc)
$(call allow-override,AR,$(CROSS_COMPILE)ar)
$(call allow-override,NM,$(CROSS_COMPILE)nm)
---

jirka
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