73 lines
2.2 KiB
C
73 lines
2.2 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __BARRIER_H
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#define __BARRIER_H
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#include <asm/compiler.h>
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#define mb() __asm__ __volatile__("mb": : :"memory")
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#define rmb() __asm__ __volatile__("mb": : :"memory")
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#define wmb() __asm__ __volatile__("wmb": : :"memory")
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/**
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* read_barrier_depends - Flush all pending reads that subsequents reads
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* depend on.
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*
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* No data-dependent reads from memory-like regions are ever reordered
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* over this barrier. All reads preceding this primitive are guaranteed
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* to access memory (but not necessarily other CPUs' caches) before any
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* reads following this primitive that depend on the data return by
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* any of the preceding reads. This primitive is much lighter weight than
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* rmb() on most CPUs, and is never heavier weight than is
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* rmb().
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*
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* These ordering constraints are respected by both the local CPU
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* and the compiler.
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*
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* Ordering is not guaranteed by anything other than these primitives,
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* not even by data dependencies. See the documentation for
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* memory_barrier() for examples and URLs to more information.
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*
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* For example, the following code would force ordering (the initial
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* value of "a" is zero, "b" is one, and "p" is "&a"):
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*
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* <programlisting>
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* CPU 0 CPU 1
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*
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* b = 2;
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* memory_barrier();
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* p = &b; q = p;
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* read_barrier_depends();
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* d = *q;
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* </programlisting>
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*
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* because the read of "*q" depends on the read of "p" and these
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* two reads are separated by a read_barrier_depends(). However,
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* the following code, with the same initial values for "a" and "b":
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*
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* <programlisting>
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* CPU 0 CPU 1
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*
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* a = 2;
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* memory_barrier();
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* b = 3; y = b;
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* read_barrier_depends();
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* x = a;
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* </programlisting>
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*
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* does not enforce ordering, since there is no data dependency between
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* the read of "a" and the read of "b". Therefore, on some CPUs, such
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* as Alpha, "y" could be set to 3 and "x" to 0. Use rmb()
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* in cases like this where there are no data dependencies.
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*/
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#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
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#ifdef CONFIG_SMP
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#define __ASM_SMP_MB "\tmb\n"
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#else
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#define __ASM_SMP_MB
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#endif
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#include <asm-generic/barrier.h>
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#endif /* __BARRIER_H */
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