linux-kernel - Re: [RFC][PATCH 14/31] locking,metag: Implement atomic_fetch

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Date:	Mon, 2 May 2016 10:15:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	James Hogan <james.hogan@...tec.com>
Cc:	torvalds@...ux-foundation.org, mingo@...nel.org,
	tglx@...utronix.de, will.deacon@....com,
	paulmck@...ux.vnet.ibm.com, boqun.feng@...il.com,
	waiman.long@....com, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	rth@...ddle.net, vgupta@...opsys.com, linux@....linux.org.uk,
	egtvedt@...fundet.no, realmz6@...il.com,
	ysato@...rs.sourceforge.jp, rkuo@...eaurora.org,
	tony.luck@...el.com, geert@...ux-m68k.org, ralf@...ux-mips.org,
	dhowells@...hat.com, jejb@...isc-linux.org, mpe@...erman.id.au,
	schwidefsky@...ibm.com, dalias@...c.org, davem@...emloft.net,
	cmetcalf@...lanox.com, jcmvbkbc@...il.com, arnd@...db.de,
	dbueso@...e.de, fengguang.wu@...el.com
Subject: Re: [RFC][PATCH 14/31] locking,metag: Implement
 atomic_fetch_{add,sub,and,or,xor}()

On Sat, Apr 30, 2016 at 01:20:31AM +0100, James Hogan wrote:
> > +	asm volatile (							\
> > +		"1:	LNKGETD %1, [%2]\n"				\
> > +		"	" #op "	%0, %1, %3\n"				\
> 
> i was hoping never to have to think about meta asm constraints again :-P

There is a solution for that: rm -rf arch/metag :-)

> and/or/xor are only available in the data units, as determined by %1 in
> this case, so the constraint for result shouldn't have "a" in it.
> 
> diff --git a/arch/metag/include/asm/atomic_lnkget.h b/arch/metag/include/asm/atomic_lnkget.h
> index 50ad05050947..def2c642f053 100644
> --- a/arch/metag/include/asm/atomic_lnkget.h
> +++ b/arch/metag/include/asm/atomic_lnkget.h
> @@ -84,7 +84,7 @@ static inline int atomic_fetch_##op(int i, atomic_t *v)			\
>  		"	ANDT	%0, %0, #HI(0x3f000000)\n"		\
>  		"	CMPT	%0, #HI(0x02000000)\n"			\
>  		"	BNZ 1b\n"					\
> -		: "=&d" (temp), "=&da" (result)				\
> +		: "=&d" (temp), "=&d" (result)				\
>  		: "da" (&v->counter), "bd" (i)				\
>  		: "cc");						\
> 
> That also ensures the "bd" constraint for %3 (meaning "an op2 register
> where op1 [%1 in this case] is a data unit register and the instruction
> supports O2R") is consistent.
> 
> So with that change this patch looks good to me:

Right, so I'd _never_ have thought to look at that,

> Acked-by: James Hogan <james.hogan@...tec.com>

Thanks!

> Note that for the ATOMIC_OP_RETURN() case (add/sub only) either address
> or data units can be used (hence the "da" for %1), but then the "bd"
> constraint on %3 is wrong as op1 [%1] may not be in data unit (sorry I
> didn't spot that at the time). I'll queue a fix, something like below
> probably ("br" means "An Op2 register and the instruction supports O2R",
> i.e. op1/%1 doesn't have to be a data unit register):
> 
> diff --git a/arch/metag/include/asm/atomic_lnkget.h b/arch/metag/include/asm/atomic_lnkget.h
> index 50ad05050947..def2c642f053 100644
> --- a/arch/metag/include/asm/atomic_lnkget.h
> +++ b/arch/metag/include/asm/atomic_lnkget.h
> @@ -61,7 +61,7 @@ static inline int atomic_##op##_return(int i, atomic_t *v)		\
>  		"	CMPT	%0, #HI(0x02000000)\n"			\
>  		"	BNZ 1b\n"					\
>  		: "=&d" (temp), "=&da" (result)				\
> -		: "da" (&v->counter), "bd" (i)				\
> +		: "da" (&v->counter), "br" (i)				\
>  		: "cc");						\
>  									\
>  	smp_mb();							\
>  									\							\

Thanks, again :-)