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

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Message-ID: <alpine.DEB.2.00.1604221648580.21846@tp.orcam.me.uk>
Date:	Fri, 22 Apr 2016 16:56:02 +0100
From:	"Maciej W. Rozycki" <macro@...tec.com>
To:	Paul Burton <paul.burton@...tec.com>
CC:	<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, 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