diff options
| author | Vineet Gupta <vgupta@synopsys.com> | 2015-08-19 11:53:58 (GMT) |
|---|---|---|
| committer | Vineet Gupta <vgupta@synopsys.com> | 2015-08-20 13:35:49 (GMT) |
| commit | 090749502ff20d7d9ec244036fe636b6bf0433b6 (patch) | |
| tree | 52ce66c6c0a525b4a9eff0ee5917c52f72479ead /arch/arc/include | |
| parent | 6de6066c0d24a66df465cf87a4041ef7ef35ba6f (diff) | |
| download | linux-090749502ff20d7d9ec244036fe636b6bf0433b6.tar.xz | |
ARC: add/fix some comments in code - no functional change
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Diffstat (limited to 'arch/arc/include')
| -rw-r--r-- | arch/arc/include/asm/cmpxchg.h | 22 | ||||
| -rw-r--r-- | arch/arc/include/asm/perf_event.h | 2 |
2 files changed, 12 insertions, 12 deletions
diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h index 44fd531..af7a2db 100644 --- a/arch/arc/include/asm/cmpxchg.h +++ b/arch/arc/include/asm/cmpxchg.h @@ -110,18 +110,18 @@ static inline unsigned long __xchg(unsigned long val, volatile void *ptr, sizeof(*(ptr)))) /* - * On ARC700, EX insn is inherently atomic, so by default "vanilla" xchg() need - * not require any locking. However there's a quirk. - * ARC lacks native CMPXCHG, thus emulated (see above), using external locking - - * incidently it "reuses" the same atomic_ops_lock used by atomic APIs. - * Now, llist code uses cmpxchg() and xchg() on same data, so xchg() needs to - * abide by same serializing rules, thus ends up using atomic_ops_lock as well. + * xchg() maps directly to ARC EX instruction which guarantees atomicity. + * However in !LLSC config, it also needs to be use @atomic_ops_lock spinlock + * due to a subtle reason: + * - For !LLSC, cmpxchg() needs to use that lock (see above) and there is lot + * of kernel code which calls xchg()/cmpxchg() on same data (see llist.h) + * Hence xchg() needs to follow same locking rules. * - * This however is only relevant if SMP and/or ARC lacks LLSC - * if (UP or LLSC) - * xchg doesn't need serialization - * else <==> !(UP or LLSC) <==> (!UP and !LLSC) <==> (SMP and !LLSC) - * xchg needs serialization + * Technically the lock is also needed for UP (boils down to irq save/restore) + * but we can cheat a bit since cmpxchg() atomic_ops_lock() would cause irqs to + * be disabled thus can't possibly be interrpted/preempted/clobbered by xchg() + * Other way around, xchg is one instruction anyways, so can't be interrupted + * as such */ #if !defined(CONFIG_ARC_HAS_LLSC) && defined(CONFIG_SMP) diff --git a/arch/arc/include/asm/perf_event.h b/arch/arc/include/asm/perf_event.h index 2b8880e..e2eaf6f 100644 --- a/arch/arc/include/asm/perf_event.h +++ b/arch/arc/include/asm/perf_event.h @@ -95,7 +95,7 @@ static const char * const arc_pmu_ev_hw_map[] = { /* counts condition */ [PERF_COUNT_HW_INSTRUCTIONS] = "iall", - [PERF_COUNT_HW_BRANCH_INSTRUCTIONS] = "ijmp", + [PERF_COUNT_HW_BRANCH_INSTRUCTIONS] = "ijmp", /* Excludes ZOL jumps */ [PERF_COUNT_ARC_BPOK] = "bpok", /* NP-NT, PT-T, PNT-NT */ [PERF_COUNT_HW_BRANCH_MISSES] = "bpfail", /* NP-T, PT-NT, PNT-T */ |