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Date: Fri, 6 Jun 2014 08:25:29 +0200
From: Hans-Christian Egtvedt <egtvedt@...fundet.no>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
mingo@...nel.org, will.deacon@....com, paulmck@...ux.vnet.ibm.com,
Haavard Skinnemoen <hskinnemoen@...il.com>
Subject: Re: [PATCH 06/20] arch,avr32: Fold atomic_ops
Around Sat 31 May 2014 16:14:45 +0200 or thereabout, Peter Zijlstra wrote:
> On Tue, May 13, 2014 at 10:40:32PM +0200, Hans-Christian Egtvedt wrote:
>> Probably found the reason why we want to use sub with the signed 21-bit
>> limit, it uses one less register than the add instruction that can add up to
>> 32-bit values.
>>
>> Both instructions are 32-bit, to use a 16-bit instruction the immediate is
>> very small; 4 bit.
>>
>> sub 32-bit, type IV, takes a register and subtracts a 21-bit immediate.
>> add 32-bit, type II, adds two register values together.
>
> How about something like so? It fixes the problem that your
> atomic_sub*() can only deal with a 21bit offset, while the general
> kernel interface assumes it can deal with the full 32 bits.
>
> It also adds the above as a comment, hopefully explaining the 21bit
> magic to the next person stumbling over this.
Looks great, I only have one comment.
> ---
> arch/avr32/include/asm/atomic.h | 121 +++++++++++++++++++---------------------
> 1 file changed, 60 insertions(+), 61 deletions(-)
>
> --- a/arch/avr32/include/asm/atomic.h
> +++ b/arch/avr32/include/asm/atomic.h
> @@ -22,30 +22,43 @@
> #define atomic_read(v) (*(volatile int *)&(v)->counter)
> #define atomic_set(v, i) (((v)->counter) = i)
>
> +#define ATOMIC_OP_RETURN(op, asm_op, asm_con) \
> +static inline int __atomic_##op##_return(int i, atomic_t *v) \
> +{ \
> + int result; \
> + \
> + asm volatile( \
> + "/* atomic_" #op "_return */\n" \
> + "1: ssrf 5\n" \
> + " ld.w %0, %2\n" \
> + " " #asm_op " %0, %3\n" \
> + " stcond %1, %0\n" \
> + " brne 1b" \
> + : "=&r" (result), "=o" (v->counter) \
> + : "m" (v->counter), #asm_con (i) \
> + : "cc"); \
> + \
> + return result; \
> +}
> +
> +ATOMIC_OP_RETURN(sub, sub, rKs21)
> +ATOMIC_OP_RETURN(add, add, i)
> +
> +#undef ATOMIC_OP_RETURN
> +
> /*
> - * atomic_sub_return - subtract the atomic variable
> - * @i: integer value to subtract
> - * @v: pointer of type atomic_t
> + * Probably found the reason why we want to use sub with the signed 21-bit
> + * limit, it uses one less register than the add instruction that can add up to
> + * 32-bit values.
> *
> - * Atomically subtracts @i from @v. Returns the resulting value.
> + * Both instructions are 32-bit, to use a 16-bit instruction the immediate is
> + * very small; 4 bit.
> + *
> + * sub 32-bit, type IV, takes a register and subtracts a 21-bit immediate.
> + * add 32-bit, type II, adds two register values together.
> */
> -static inline int atomic_sub_return(int i, atomic_t *v)
> -{
> - int result;
> -
> - asm volatile(
> - "/* atomic_sub_return */\n"
> - "1: ssrf 5\n"
> - " ld.w %0, %2\n"
> - " sub %0, %3\n"
> - " stcond %1, %0\n"
> - " brne 1b"
> - : "=&r"(result), "=o"(v->counter)
> - : "m"(v->counter), "rKs21"(i)
> - : "cc");
> -
> - return result;
> -}
> +#define IS_21BIT_CONST(i) \
> + (__builtin_constant_p(i) && ((i) >= -1048575) && ((i) <= 1048576))
Perhaps undef this macro once we're done using it?
>
> /*
> * atomic_add_return - add integer to atomic variable
> @@ -56,51 +69,25 @@ static inline int atomic_sub_return(int
> */
> static inline int atomic_add_return(int i, atomic_t *v)
> {
> - int result;
> + if (IS_21BIT_CONST(i))
> + return __atomic_sub_return(-i, v);
>
> - if (__builtin_constant_p(i) && (i >= -1048575) && (i <= 1048576))
> - result = atomic_sub_return(-i, v);
> - else
> - asm volatile(
> - "/* atomic_add_return */\n"
> - "1: ssrf 5\n"
> - " ld.w %0, %1\n"
> - " add %0, %3\n"
> - " stcond %2, %0\n"
> - " brne 1b"
> - : "=&r"(result), "=o"(v->counter)
> - : "m"(v->counter), "r"(i)
> - : "cc", "memory");
> -
> - return result;
> + return __atomic_add_return(i, v);
> }
>
> /*
> - * atomic_sub_unless - sub unless the number is a given value
> + * atomic_sub_return - subtract the atomic variable
> + * @i: integer value to subtract
> * @v: pointer of type atomic_t
> - * @a: the amount to subtract from v...
> - * @u: ...unless v is equal to u.
> *
> - * Atomically subtract @a from @v, so long as it was not @u.
> - * Returns the old value of @v.
> -*/
> -static inline void atomic_sub_unless(atomic_t *v, int a, int u)
> + * Atomically subtracts @i from @v. Returns the resulting value.
> + */
> +static inline int atomic_sub_return(int i, atomic_t *v)
> {
> - int tmp;
> + if (IS_21BIT_CONST(i))
> + return __atomic_sub_return(i, v);
>
> - asm volatile(
> - "/* atomic_sub_unless */\n"
> - "1: ssrf 5\n"
> - " ld.w %0, %2\n"
> - " cp.w %0, %4\n"
> - " breq 1f\n"
> - " sub %0, %3\n"
> - " stcond %1, %0\n"
> - " brne 1b\n"
> - "1:"
> - : "=&r"(tmp), "=o"(v->counter)
> - : "m"(v->counter), "rKs21"(a), "rKs21"(u)
> - : "cc", "memory");
> + return __atomic_add_return(-i, v);
> }
>
> /*
> @@ -116,9 +103,21 @@ static inline int __atomic_add_unless(at
> {
> int tmp, old = atomic_read(v);
>
> - if (__builtin_constant_p(a) && (a >= -1048575) && (a <= 1048576))
> - atomic_sub_unless(v, -a, u);
> - else {
> + if (IS_21BIT_CONST(a)) {
> + asm volatile(
> + "/* __atomic_sub_unless */\n"
> + "1: ssrf 5\n"
> + " ld.w %0, %2\n"
> + " cp.w %0, %4\n"
> + " breq 1f\n"
> + " sub %0, %3\n"
> + " stcond %1, %0\n"
> + " brne 1b\n"
> + "1:"
> + : "=&r"(tmp), "=o"(v->counter)
> + : "m"(v->counter), "rKs21"(-a), "rKs21"(u)
> + : "cc", "memory");
> + } else {
> asm volatile(
> "/* __atomic_add_unless */\n"
> "1: ssrf 5\n"
+#undef IS_21BIT_CONST
--
Hans-Christian Egtvedt
--
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