linux-kernel - Re: Rewriting kernel config after generation

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Message-ID: <4EE36C43.2040508@suse.cz>
Date:	Sat, 10 Dec 2011 15:27:15 +0100
From:	Michal Marek <mmarek@...e.cz>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	sam@...nborg.org, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Rewriting kernel config after generation

On 6.12.2011 12:45, Alexey Dobriyan wrote:
> On Fri, Dec 2, 2011 at 7:33 AM, Alexey Dobriyan <adobriyan@...il.com> wrote:
>> I'm trying to add compiling with -march=native to the kernel.
>> The problem is that then some config options become discoverable at
>> the beginning of the compilation. For example,
>> CONFIG_X86_L1_CACHE_SHIFT=6 can be derived from "--param l1-cache-line-size=64".
>> I suspect this is no-go, because all the dependency logic must be
>> duplicated somewhere else (in the march=native script) for the above
>> to work.
>>
>> Is there any cleaner way I'm missing?
>>
>> In theory, such script could even turn off CONFIG_CPU_SUP_AMD
>> if it detects Intel CPU.
> 
> Hrph, this is harder than it looks because "silentoldconfig" is done at
> the beginning rewriting whatever changes done to .config by hand. :-(

X86_L1_CACHE_SHIFT has no prompt, so it cannot be modified by the user.
You would need to add a prompt if you want to modify it, e.g.

config X86_L1_CACHE_SHIFT
        int
	prompt "..." if X86_MARCH_NATIVE

But I wonder why is this needed. To build for the native cpu, you only
need to detect what cpu you are running and select the matching
processor family. The x86 Kconfig/Makefile will then take care of
setting the right kconfig options and the right -march= parameter for gcc.

Michal
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