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[PATCH] MIPS: Allow R6 compact branch policy to be left unspecified

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  [PATCH] MIPS: Allow R6 compact branch policy to be left unspecified Paul Burton <paul.burton@imgtec.com> - 2016-04-22 10:50 +0200
    Re: [PATCH] MIPS: Allow R6 compact branch policy to be left  unspecified "Maciej W. Rozycki" <macro@imgtec.com> - 2016-04-22 18:00 +0200
      Re: [PATCH] MIPS: Allow R6 compact branch policy to be left  unspecified James Hogan <james.hogan@imgtec.com> - 2016-04-22 19:40 +0200

#1384793 — [PATCH] MIPS: Allow R6 compact branch policy to be left unspecified

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.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: c1a0e9bc885d ("MIPS: Allow compact branch policy to be changed")
Cc: stable <stable@vger.kernel.org> # v4.4+

---

 arch/mips/Kconfig.debug | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

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)"
-- 
2.8.0

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#1385326 — Re: [PATCH] MIPS: Allow R6 compact branch policy to be left unspecified

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
[...]
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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#1385398 — Re: [PATCH] MIPS: Allow R6 compact branch policy to be left unspecified

[Multipart message — attachments visible in raw view] — view raw

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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