/* SPDX-License-Identifier: GPL-2.0 */

/* adaptation of kernel's <asm-generic/rwonce.h>
 * See https://www.kernel.org/doc/Documentation/memory-barriers.txt
 */
/*
 * Prevent the compiler from merging or refetching reads or writes. The
 * compiler is also forbidden from reordering successive instances of
 * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
 * particular ordering. One way to make the compiler aware of ordering is to
 * put the two invocations of READ_ONCE or WRITE_ONCE in different C
 * statements.
 *
 * These two macros will also work on aggregate data types like structs or
 * unions.
 *
 * Their two major use cases are: (1) Mediating communication between
 * process-level code and irq/NMI handlers, all running on the same CPU,
 * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
 * mutilate accesses that either do not require ordering or that interact
 * with an explicit memory barrier or atomic instruction that provides the
 * required ordering.
 */
#ifndef __BR_RWONCE_H
#define __BR_RWONCE_H

/************ originally in <include/linux/compiler_attributes.h> */
#if __has_attribute(__error__)
# define __compiletime_error(msg)       __attribute__((__error__(msg)))
#else
# define __compiletime_error(msg)
#endif

/************ originally in <include/linux/compiler_types.h> */
/*
 * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
 *                             non-scalar types unchanged.
 */
/*
 * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
 * is not type-compatible with 'signed char', and we define a separate case.
 */
#define __scalar_type_to_expr_cases(type)                               \
    unsigned type:  (unsigned type)0,                                   \
    signed type:    (signed type)0

#define __unqual_scalar_typeof(x)                                       \
    typeof(_Generic((x),                                                \
                    char:  (char)0,                                     \
                    __scalar_type_to_expr_cases(char),                  \
                    __scalar_type_to_expr_cases(short),                 \
                    __scalar_type_to_expr_cases(int),                   \
                    __scalar_type_to_expr_cases(long),                  \
                    __scalar_type_to_expr_cases(long long),             \
                    default: (x)))

/* Is this type a native word size -- useful for atomic operations */
#define __native_word(t) \
    (sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
     sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))

#ifdef __OPTIMIZE__
# define __compiletime_assert(condition, msg, prefix, suffix)           \
    do {                                                                \
        extern void prefix ## suffix(void) __compiletime_error(msg);    \
        if (!(condition))                                               \
            prefix ## suffix();                                         \
    } while (0)
#else
# define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
#endif

#define _compiletime_assert(condition, msg, prefix, suffix) \
    __compiletime_assert(condition, msg, prefix, suffix)

/**
 * compiletime_assert - break build and emit msg if condition is false
 * @condition: a compile-time constant condition to check
 * @msg:       a message to emit if condition is false
 *
 * In tradition of POSIX assert, this macro will break the build if the
 * supplied condition is *false*, emitting the supplied error message if the
 * compiler has support to do so.
 */
#define compiletime_assert(condition, msg)                              \
    _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)

#define compiletime_assert_atomic_type(t)                               \
    compiletime_assert(__native_word(t),                                \
                       "Need native word sized stores/loads for atomicity.")

/************ originally in <asm-generic/rwonce.h> */
/*
 * Yes, this permits 64-bit accesses on 32-bit architectures. These will
 * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
 * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
 * (e.g. a virtual address) and a strong prevailing wind.
 */
#define compiletime_assert_rwonce_type(t)                                  \
    compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \
                       "Unsupported access size for {READ,WRITE}_ONCE().")

/*
 * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
 * atomicity. Note that this may result in tears!
 */
#ifndef __READ_ONCE
#define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
#endif

#define READ_ONCE(x)                                                    \
({                                                                      \
    compiletime_assert_rwonce_type(x);                                  \
    __READ_ONCE(x);                                                     \
})

#define __WRITE_ONCE(x, val)                                            \
do {                                                                    \
    *(volatile typeof(x) *)&(x) = (val);                                \
} while (0)

#define WRITE_ONCE(x, val)                                              \
do {                                                                    \
    compiletime_assert_rwonce_type(x);                                  \
    __WRITE_ONCE(x, val);                                               \
} while (0)

#endif  /* __BR_RWONCE_H */