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

From	"Maciej W. Rozycki" <macro@imgtec.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] MIPS: Allow R6 compact branch policy to be left unspecified
Date	2016-04-22 18:00 +0200
Message-ID	<rqLET-4cJ-19@gated-at.bofh.it> (permalink)
References	<rqEWJ-7zC-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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

Thread

[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

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