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Date:   Tue, 27 Aug 2019 13:26:16 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Nicholas Piggin <npiggin@gmail.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] powerpc: cleanup hw_irq.h
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On Tue, Aug 27, 2019 at 07:36:35PM +0200, Christophe Leroy wrote:
> Le 27/08/2019 à 19:29, Segher Boessenkool a écrit :
> >On Tue, Aug 27, 2019 at 10:48:24PM +1000, Nicholas Piggin wrote:
> >>Christophe Leroy's on August 27, 2019 6:13 pm:
> >>>+#define wrtee(val)	asm volatile("wrtee %0" : : "r" (val) : "memory")
> >>>+#define wrteei(val)	asm volatile("wrteei %0" : : "i" (val) : 
> >>>"memory")
> >>
> >>Can you implement just one macro that uses __builtin_constant_p to
> >>select between the imm and reg versions? I forgot if there's some
> >>corner cases that prevent that working with inline asm i constraints.
> >
> >static inline void wrtee(long val)
> >{
> >	asm volatile("wrtee%I0 %0" : : "n"(val) : "memory");
> >}
> 
> Great, didn't know that possibility.
> 
> Can it be used with any insn, for instance with add/addi ?
> Or with mr/li ?

Any instruction, yes.  %I<n> simply outputs an "i" if operand n is a
constant integer, and nothing otherwise.

So
  asm("add%I2 %0,%1,%2" : "=r"(dst) : "r"(src1), "ri"(src1));
works well.  I don't see how you would use it for li/mr...  You can do
  asm("add%I1 %0,0,%1" : "=r"(dst) : "ri"(src));
I suppose, but that is not really an mr.

> >(This output modifier goes back to the dark ages, some 2.4 or something).
> 
> Hope Clang support it ...

I don't know, sorry.  But it is used all over the place, see sfp-machine.h
for example, so maybe?


Segher