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

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Date:	Tue, 14 Jul 2015 14:00:05 +0000
From:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:	"acme@...nel.org" <acme@...nel.org>
CC:	"aaro.koskinen@...ia.com" <aaro.koskinen@...ia.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
	"jolsa@...nel.org" <jolsa@...nel.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Revert "perf tools: Allow to specify custom linker
 command"

Hi Arnaldo,

On Tue, 2015-07-14 at 10:46 -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jul 13, 2015 at 11:24:06PM +0300, Aaro Koskinen escreveu:
> > 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.
> 
> So, I think this is an "Acked-by: Aaro Koskinen <aaro.koskinen@....fi>"
> for the revert, right?
>  
> > 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?
> 
> When you guys get to some conclusion, please submit a new patch, for now
> I am taking the revert with the (implied) Acked-by Aaro, ok?

Please find my patch that both reverts initial Aaro's patch and implements
proper handling of CC, AR and LD substitution here - https://lkml.org/lkml/2015/7/14/149

-Alexey--
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