linux-kernel - Re: [PATCH] ARM: OMAP: drop "select MACH_NOKIA

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Date:	Sat, 09 Mar 2013 20:48:30 +0100
From:	Paul Bolle <pebolle@...cali.nl>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Tony Lindgren <tony@...mide.com>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	linux-arm-kernel@...ts.infradead.org, linux-omap@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: OMAP: drop "select MACH_NOKIA_RM696"

On Sat, 2013-03-09 at 00:01 +0000, Russell King - ARM Linux wrote:
> It's actually quite clever.  There's two levels to it.
> 
> The first is that CONFIG_MACH_xxx result in their machine_is_xxx() macros
> being defined to constant zero if the CONFIG option is not enabled.  That
> allows the compiler to throw away code for disabled platforms because
> the expression is always false.
> 
> Otherwise, they end up as (machine_arch_type == MACH_TYPE_xxx).
> 
> The second is the magic which happens when two CONFIG_MACH_xxx are
> selected.  If only one is selected, then machine_arch_type is defined
> to the appropriate MACH_TYPE_xxx.  This means that the above expression
> becomes constant-true, and the conditional is eliminated.
> 
> If more than one is selected, then machine_arch_type is defined to a
> variable which is appropriately set to one of the MACH_TYPE_xxx values.

At boot?

> So, the result is that:
> - de-selected platforms have their if (machine_is_xxx()) { } optimised
>   out of the kernel.
> - for a kernel built targetting one platform, all the
>   if (machine_is_xxx()) tests are optimised away, leaving only the
>   relevant code behind.
> - otherwise, we get the _appropriate_ conditional code for the
>   configuration generated.

Thanks for clarifying this. Quite clever indeed. 

> However, going back to that MACH_NOKIA_RM696.  If there exists only a
> select of this symbol and no "config MACH_NOKIA_RM696" entry, then the
> symbol will never be generated in the output .config file.
> 
>[...]
> 
> My conclusion is... it's a mess.

That mess can only be fully cleaned up if the code for the RM-696 that
now is maintained in some unknown to me repository gets merged into
mainline, can't it?

In the meantime, how do you prefer I solve the (trivial) issue of an
useless select for MACH_NOKIA_RM696? Drop that select or add an (equally
useless) config entry for MACH_NOKIA_RM696? Or should I try to ignore it
for the time being?


Paul Bolle

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