Hi Ralf,

2014-08-26 3:41 GMT+08:00 Ralf Baechle [off-list ref]:

On Mon, Aug 25, 2014 at 08:35:17PM +0100, Maciej W. Rozycki wrote:

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On Mon, 25 Aug 2014, Ralf Baechle wrote:

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+cflags-$(CONFIG_CPU_LOONGSON3)  += -D_MIPS_ARCH_LOONGSON3A

The _MIPS_ARCH_* namespace belongs to GCC.  While it seems current GCC
does not define this symbol _MIPS_ARCH_LOONGSON3A runs into the danger
of causing a conflict when GCC eventually will define the symbol.

When this symbol will be defined? With option '-march=loongson3a'?

Well, not currently (at least not in my gcc 4.9.0) - but it might.  In
fact, I'm wondering why it doesn't.  Maciej?

 No idea, a _MIPS_ARCH_foo macro gets defined automagically by GCC
whenever `-march=foo' is in effect (be it implicitly or with the use of a
command-line option), so there should be one.

 Has support for "loongson3a" been present in 4.9.x (it is in trunk)?  If
so, then what _MIPS_ARCH_* macro gets defined for `-march=loongson3a'?

Hmm - I must have fatfingered something.  Now I'm getting:

$ mips-linux-gcc < /dev/null -E -C -dM -march=loongson3a - | grep _MIPS_ARCH
#define _MIPS_ARCH_LOONGSON3A 1
#define _MIPS_ARCH "loongson3a"
$

So that would conflict with a manual definition, thus the patch would not
be acceptable as it because:

$ cat > c.c << EOF
foo(){}
EOF
$ mips-linux-gcc -D_MIPS_ARCH_LOONGSON3A -march=loongson3a -Wall -c c.c
c.c:1:1: warning: return type defaults to ‘int’ [-Wreturn-type]
 foo(){}
 ^
c.c: In function ‘foo’:
c.c:1:1: warning: control reaches end of non-void function [-Wreturn-type]
 foo(){}
 ^
$

Thanks for the explanation. We will enable "-march=loongson3a" and do
some test internally to make sure doing so wouldn't cause any
problems.

To James:

Any reason not to just refer directly to CONFIG_CPU_LOONGSON3 from the
source rather than adding an intermediate definition?

Cheers
James

I guess it's because "arch/mips/include/uapi/asm/swab.h" is in "uapi/"
directory(http://lwn.net/Articles/507794/), that means it contains
"user-space API-related definitions", hence using CONFIG_CPU_LOONGSON3
is not suitable there.