ARCv2: Implement atomic64 based on LLOCKD/SCONDD instructions

ARCv2 ISA provides 64-bit exclusive load/stores so use them to implement
the 64-bit atomics and elide the spinlock based generic 64-bit atomics

boot tested with atomic64 self-test (and GOD bless the person who wrote
them, I realized my inline assmebly is sloppy as hell)

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-snps-arc@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

2 files changed, 260 insertions, 3 deletions

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 0d3e59f56974..073b3582544b 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -13,7 +13,7 @@ config ARC
         select CLKSRC_OF
         select CLONE_BACKWARDS
         select COMMON_CLK
-        select GENERIC_ATOMIC64
+        select GENERIC_ATOMIC64 if !ISA_ARCV2 || !(ARC_HAS_LL64 && ARC_HAS_LLSC)
         select GENERIC_CLOCKEVENTS
         select GENERIC_FIND_FIRST_BIT
         # for now, we don't need GENERIC_IRQ_PROBE, CONFIG_GENERIC_IRQ_CHIP
diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
index 4e3c1b6b0806..d0e222e3776b 100644
--- a/arch/arc/include/asm/atomic.h
+++ b/arch/arc/include/asm/atomic.h
@@ -20,6 +20,7 @@
 #ifndef CONFIG_ARC_PLAT_EZNPS
 
 #define atomic_read(v)  READ_ONCE((v)->counter)
+#define ATOMIC_INIT(i)  { (i) }
 
 #ifdef CONFIG_ARC_HAS_LLSC
 
@@ -343,10 +344,266 @@ ATOMIC_OPS(xor, ^=, CTOP_INST_AXOR_DI_R2_R2_R3)
 
 #define atomic_add_negative(i, v)       (atomic_add_return(i, v) < 0)
 
-#define ATOMIC_INIT(i)                  { (i) }
+
+#ifdef CONFIG_GENERIC_ATOMIC64
 
 #include <asm-generic/atomic64.h>
 
-#endif
+#else   /* Kconfig ensures this is only enabled with needed h/w assist */
+
+/*
+ * ARCv2 supports 64-bit exclusive load (LLOCKD) / store (SCONDD)
+ *  - The address HAS to be 64-bit aligned
+ *  - There are 2 semantics involved here:
+ *    = exclusive implies no interim update between load/store to same addr
+ *    = both words are observed/updated together: this is guaranteed even
+ *      for regular 64-bit load (LDD) / store (STD). Thus atomic64_set()
+ *      is NOT required to use LLOCKD+SCONDD, STD suffices
+ */
+
+typedef struct {
+        aligned_u64 counter;
+} atomic64_t;
+
+#define ATOMIC64_INIT(a) { (a) }
+
+static inline long long atomic64_read(const atomic64_t *v)
+{
+        unsigned long long val;
+
+        __asm__ __volatile__(
+        "       ldd   %0, [%1]  \n"
+        : "=r"(val)
+        : "r"(&v->counter));
+
+        return val;
+}
+
+static inline void atomic64_set(atomic64_t *v, long long a)
+{
+        /*
+         * This could have been a simple assignment in "C" but would need
+         * explicit volatile. Otherwise gcc optimizers could elide the store
+         * which borked atomic64 self-test
+         * In the inline asm version, memory clobber needed for exact same
+         * reason, to tell gcc about the store.
+         *
+         * This however is not needed for sibling atomic64_add() etc since both
+         * load/store are explicitly done in inline asm. As long as API is used
+         * for each access, gcc has no way to optimize away any load/store
+         */
+        __asm__ __volatile__(
+        "       std   %0, [%1]  \n"
+        :
+        : "r"(a), "r"(&v->counter)
+        : "memory");
+}
+
+#define ATOMIC64_OP(op, op1, op2)                                       \
+static inline void atomic64_##op(long long a, atomic64_t *v)            \
+{                                                                       \
+        unsigned long long val;                                         \
+                                                                        \
+        __asm__ __volatile__(                                           \
+        "1:                             \n"                             \
+        "       llockd  %0, [%1]        \n"                             \
+        "       " #op1 " %L0, %L0, %L2  \n"                             \
+        "       " #op2 " %H0, %H0, %H2  \n"                             \
+        "       scondd   %0, [%1]       \n"                             \
+        "       bnz     1b              \n"                             \
+        : "=&r"(val)                                                    \
+        : "r"(&v->counter), "ir"(a)                                     \
+        : "cc");                                                \
+}                                                                       \
+
+#define ATOMIC64_OP_RETURN(op, op1, op2)                                \
+static inline long long atomic64_##op##_return(long long a, atomic64_t *v)      \
+{                                                                       \
+        unsigned long long val;                                         \
+                                                                        \
+        smp_mb();                                                       \
+                                                                        \
+        __asm__ __volatile__(                                           \
+        "1:                             \n"                             \
+        "       llockd   %0, [%1]       \n"                             \
+        "       " #op1 " %L0, %L0, %L2  \n"                             \
+        "       " #op2 " %H0, %H0, %H2  \n"                             \
+        "       scondd   %0, [%1]       \n"                             \
+        "       bnz     1b              \n"                             \
+        : [val] "=&r"(val)                                              \
+        : "r"(&v->counter), "ir"(a)                                     \
+        : "cc");        /* memory clobber comes from smp_mb() */        \
+                                                                        \
+        smp_mb();                                                       \
+                                                                        \
+        return val;                                                     \
+}
+
+#define ATOMIC64_FETCH_OP(op, op1, op2)                                 \
+static inline long long atomic64_fetch_##op(long long a, atomic64_t *v) \
+{                                                                       \
+        unsigned long long val, orig;                                   \
+                                                                        \
+        smp_mb();                                                       \
+                                                                        \
+        __asm__ __volatile__(                                           \
+        "1:                             \n"                             \
+        "       llockd   %0, [%2]       \n"                             \
+        "       " #op1 " %L1, %L0, %L3  \n"                             \
+        "       " #op2 " %H1, %H0, %H3  \n"                             \
+        "       scondd   %1, [%2]       \n"                             \
+        "       bnz     1b              \n"                             \
+        : "=&r"(orig), "=&r"(val)                                       \
+        : "r"(&v->counter), "ir"(a)                                     \
+        : "cc");        /* memory clobber comes from smp_mb() */        \
+                                                                        \
+        smp_mb();                                                       \
+                                                                        \
+        return orig;                                                    \
+}
+
+#define ATOMIC64_OPS(op, op1, op2)                                      \
+        ATOMIC64_OP(op, op1, op2)                                       \
+        ATOMIC64_OP_RETURN(op, op1, op2)                                \
+        ATOMIC64_FETCH_OP(op, op1, op2)
+
+#define atomic64_andnot atomic64_andnot
+
+ATOMIC64_OPS(add, add.f, adc)
+ATOMIC64_OPS(sub, sub.f, sbc)
+ATOMIC64_OPS(and, and, and)
+ATOMIC64_OPS(andnot, bic, bic)
+ATOMIC64_OPS(or, or, or)
+ATOMIC64_OPS(xor, xor, xor)
+
+#undef ATOMIC64_OPS
+#undef ATOMIC64_FETCH_OP
+#undef ATOMIC64_OP_RETURN
+#undef ATOMIC64_OP
+
+static inline long long
+atomic64_cmpxchg(atomic64_t *ptr, long long expected, long long new)
+{
+        long long prev;
+
+        smp_mb();
+
+        __asm__ __volatile__(
+        "1:     llockd  %0, [%1]        \n"
+        "       brne    %L0, %L2, 2f    \n"
+        "       brne    %H0, %H2, 2f    \n"
+        "       scondd  %3, [%1]        \n"
+        "       bnz     1b              \n"
+        "2:                             \n"
+        : "=&r"(prev)
+        : "r"(ptr), "ir"(expected), "r"(new)
+        : "cc");        /* memory clobber comes from smp_mb() */
+
+        smp_mb();
+
+        return prev;
+}
+
+static inline long long atomic64_xchg(atomic64_t *ptr, long long new)
+{
+        long long prev;
+
+        smp_mb();
+
+        __asm__ __volatile__(
+        "1:     llockd  %0, [%1]        \n"
+        "       scondd  %2, [%1]        \n"
+        "       bnz     1b              \n"
+        "2:                             \n"
+        : "=&r"(prev)
+        : "r"(ptr), "r"(new)
+        : "cc");        /* memory clobber comes from smp_mb() */
+
+        smp_mb();
+
+        return prev;
+}
+
+/**
+ * atomic64_dec_if_positive - decrement by 1 if old value positive
+ * @v: pointer of type atomic64_t
+ *
+ * The function returns the old value of *v minus 1, even if
+ * the atomic variable, v, was not decremented.
+ */
+
+static inline long long atomic64_dec_if_positive(atomic64_t *v)
+{
+        long long val;
+
+        smp_mb();
+
+        __asm__ __volatile__(
+        "1:     llockd  %0, [%1]        \n"
+        "       sub.f   %L0, %L0, 1     # w0 - 1, set C on borrow\n"
+        "       sub.c   %H0, %H0, 1     # if C set, w1 - 1\n"
+        "       brlt    %H0, 0, 2f      \n"
+        "       scondd  %0, [%1]        \n"
+        "       bnz     1b              \n"
+        "2:                             \n"
+        : "=&r"(val)
+        : "r"(&v->counter)
+        : "cc");        /* memory clobber comes from smp_mb() */
+
+        smp_mb();
+
+        return val;
+}
+
+/**
+ * atomic64_add_unless - add unless the number is a given value
+ * @v: pointer of type atomic64_t
+ * @a: the amount to add to v...
+ * @u: ...unless v is equal to u.
+ *
+ * if (v != u) { v += a; ret = 1} else {ret = 0}
+ * Returns 1 iff @v was not @u (i.e. if add actually happened)
+ */
+static inline int atomic64_add_unless(atomic64_t *v, long long a, long long u)
+{
+        long long val;
+        int op_done;
+
+        smp_mb();
+
+        __asm__ __volatile__(
+        "1:     llockd  %0, [%2]        \n"
+        "       mov     %1, 1           \n"
+        "       brne    %L0, %L4, 2f    # continue to add since v != u \n"
+        "       breq.d  %H0, %H4, 3f    # return since v == u \n"
+        "       mov     %1, 0           \n"
+        "2:                             \n"
+        "       add.f   %L0, %L0, %L3   \n"
+        "       adc     %H0, %H0, %H3   \n"
+        "       scondd  %0, [%2]        \n"
+        "       bnz     1b              \n"
+        "3:                             \n"
+        : "=&r"(val), "=&r" (op_done)
+        : "r"(&v->counter), "r"(a), "r"(u)
+        : "cc");        /* memory clobber comes from smp_mb() */
+
+        smp_mb();
+
+        return op_done;
+}
+
+#define atomic64_add_negative(a, v)     (atomic64_add_return((a), (v)) < 0)
+#define atomic64_inc(v)                 atomic64_add(1LL, (v))
+#define atomic64_inc_return(v)          atomic64_add_return(1LL, (v))
+#define atomic64_inc_and_test(v)        (atomic64_inc_return(v) == 0)
+#define atomic64_sub_and_test(a, v)     (atomic64_sub_return((a), (v)) == 0)
+#define atomic64_dec(v)                 atomic64_sub(1LL, (v))
+#define atomic64_dec_return(v)          atomic64_sub_return(1LL, (v))
+#define atomic64_dec_and_test(v)        (atomic64_dec_return((v)) == 0)
+#define atomic64_inc_not_zero(v)        atomic64_add_unless((v), 1LL, 0LL)
+
+#endif  /* !CONFIG_GENERIC_ATOMIC64 */
+
+#endif  /* !__ASSEMBLY__ */
 
 #endif