linux-kernel - Re: [PATCH] MIPS: Allow R6 compact branch policy to be left unspecified

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Message-ID: <20160422173245.GC2467@jhogan-linux.le.imgtec.org>
Date:	Fri, 22 Apr 2016 18:32:45 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	"Maciej W. Rozycki" <macro@...tec.com>
CC:	Paul Burton <paul.burton@...tec.com>, <linux-mips@...ux-mips.org>,
	"Ralf Baechle" <ralf@...ux-mips.org>, <fengguang.wu@...el.com>,
	"stable # v4 . 4+" <stable@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: Allow R6 compact branch policy to be left
 unspecified

On Fri, Apr 22, 2016 at 04:56:02PM +0100, Maciej W. Rozycki wrote:
> On Fri, 22 Apr 2016, Paul Burton wrote:
> 
> > It turns out that some toolchains which support MIPS R6 don't support
> > the -mcompact-branches flag to specify compact branch behaviour. Default
> > to not providing the -mcompact-branch option to the compiler such that
> > we can build with such toolchains.
> 
>  Good idea overall, one further suggestion below.
> 
> > diff --git a/arch/mips/Kconfig.debug b/arch/mips/Kconfig.debug
> > index f0e314c..e91b3d1 100644
> > --- a/arch/mips/Kconfig.debug
> > +++ b/arch/mips/Kconfig.debug
> > @@ -117,7 +117,15 @@ if CPU_MIPSR6
> >  
> >  choice
> >  	prompt "Compact branch policy"
> > -	default MIPS_COMPACT_BRANCHES_OPTIMAL
> > +	default MIPS_COMPACT_BRANCHES_DEFAULT
> > +
> > +config MIPS_COMPACT_BRANCHES_DEFAULT
> > +	bool "Toolchain Default (don't specify)"
> > +	help
> > +	  Don't pass the -mcompact-branches flag to the compiler, allowing it
> > +	  to use its default (generally "optimal"). This is particularly
> > +	  useful for early R6-supporting toolchains which don't support the
> > +	  -mcompact-branches flag.
> >  
> >  config MIPS_COMPACT_BRANCHES_NEVER
> >  	bool "Never (force delay slot branches)"
> 
>  How about making the option depend on DEBUG_KERNEL and maybe making it an 
> umbrella setting to hide details from users who do not want to be 
> bothered, i.e. something like:
> 
> config MIPS_COMPACT_BRANCHES_OVERRIDE
>      bool "Override the toolchain default for compact branch policy"
>      depends on DEBUG_KERNEL
>      default n

Although note that "default n" is redundant.

Cheers
James

> [...]
> if MIPS_COMPACT_BRANCHES_OVERRIDE
> choice
>     prompt "Compact branch policy"
>     default MIPS_COMPACT_BRANCHES_OPTIMAL
> [...]
> endif # MIPS_COMPACT_BRANCHES_OVERRIDE
> 
> ?
> 
>   Maciej
> 

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