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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