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Date: Sat, 31 May 2014 16:14:45 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Hans-Christian Egtvedt <egtvedt@...fundet.no>
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
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.
---
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))
/*
* 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"
--
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