arch/riscv/include/asm/barrier.h

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2 /*
   3  * Based on arch/arm/include/asm/barrier.h
   4  *
   5  * Copyright (C) 2012 ARM Ltd.
   6  * Copyright (C) 2013 Regents of the University of California
   7  * Copyright (C) 2017 SiFive
   8  */
   9
  10 #ifndef _ASM_RISCV_BARRIER_H
  11 #define _ASM_RISCV_BARRIER_H
  12
  13 #ifndef __ASSEMBLY__
  14 #include <asm/cmpxchg.h>
  15 #include <asm/fence.h>
  16
  17 #define nop()           __asm__ __volatile__ ("nop")
  18 #define __nops(n)       ".rept  " #n "\nnop\n.endr\n"
  19 #define nops(n)         __asm__ __volatile__ (__nops(n))
  20
  21
  22 /* These barriers need to enforce ordering on both devices or memory. */
  23 #define __mb()          RISCV_FENCE(iorw, iorw)
  24 #define __rmb()         RISCV_FENCE(ir, ir)
  25 #define __wmb()         RISCV_FENCE(ow, ow)
  26
  27 /* These barriers do not need to enforce ordering on devices, just memory. */
  28 #define __smp_mb()      RISCV_FENCE(rw, rw)
  29 #define __smp_rmb()     RISCV_FENCE(r, r)
  30 #define __smp_wmb()     RISCV_FENCE(w, w)
  31
  32 /*
  33  * This is a very specific barrier: it's currently only used in two places in
  34  * the kernel, both in the scheduler.  See include/linux/spinlock.h for the two
  35  * orderings it guarantees, but the "critical section is RCsc" guarantee
  36  * mandates a barrier on RISC-V.  The sequence looks like:
  37  *
  38  *    lr.aq lock
  39  *    sc    lock <= LOCKED
  40  *    smp_mb__after_spinlock()
  41  *    // critical section
  42  *    lr    lock
  43  *    sc.rl lock <= UNLOCKED
  44  *
  45  * The AQ/RL pair provides a RCpc critical section, but there's not really any
  46  * way we can take advantage of that here because the ordering is only enforced
  47  * on that one lock.  Thus, we're just doing a full fence.
  48  *
  49  * Since we allow writeX to be called from preemptive regions we need at least
  50  * an "o" in the predecessor set to ensure device writes are visible before the
  51  * task is marked as available for scheduling on a new hart.  While I don't see
  52  * any concrete reason we need a full IO fence, it seems safer to just upgrade
  53  * this in order to avoid any IO crossing a scheduling boundary.  In both
  54  * instances the scheduler pairs this with an mb(), so nothing is necessary on
  55  * the new hart.
  56  */
  57 #define smp_mb__after_spinlock()        RISCV_FENCE(iorw, iorw)
  58
  59 #define __smp_store_release(p, v)                                       \
  60 do {                                                                    \
  61         compiletime_assert_atomic_type(*p);                             \
  62         RISCV_FENCE(rw, w);                                             \
  63         WRITE_ONCE(*p, v);                                              \
  64 } while (0)
  65
  66 #define __smp_load_acquire(p)                                           \
  67 ({                                                                      \
  68         typeof(*p) ___p1 = READ_ONCE(*p);                               \
  69         compiletime_assert_atomic_type(*p);                             \
  70         RISCV_FENCE(r, rw);                                             \
  71         ___p1;                                                          \
  72 })
  73
  74 #ifdef CONFIG_RISCV_ISA_ZAWRS
  75 #define smp_cond_load_relaxed(ptr, cond_expr) ({                        \
  76         typeof(ptr) __PTR = (ptr);                                      \
  77         __unqual_scalar_typeof(*ptr) VAL;                               \
  78         for (;;) {                                                      \
  79                 VAL = READ_ONCE(*__PTR);                                \
  80                 if (cond_expr)                                          \
  81                         break;                                          \
  82                 __cmpwait_relaxed(ptr, VAL);                            \
  83         }                                                               \
  84         (typeof(*ptr))VAL;                                              \
  85 })
  86 #endif
  87
  88 #include <asm-generic/barrier.h>
  89
  90 #endif /* __ASSEMBLY__ */
  91
  92 #endif /* _ASM_RISCV_BARRIER_H */