Lines Matching +full:a +full:- +full:bit

1 /* SPDX-License-Identifier: GPL-2.0-only */
3  *  Extend a 32-bit counter to 63 bits
17 /* this is used only to give gcc a clue about good code generation */
31  * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
35  * a relatively short period making wrap-arounds rather frequent.  This
36  * is a problem when implementing sched_clock() for example, where a 64-bit
37  * non-wrapping monotonic value is expected to be returned.
39  * To overcome that limitation, let's extend a 32-bit counter to 63 bits
40  * in a completely lock free fashion. Bits 0 to 31 of the clock are provided
41  * by the hardware while bits 32 to 62 are stored in memory.  The top bit in
42  * memory is used to synchronize with the hardware clock half-period.  When
43  * the top bit of both counters (hardware and in memory) differ then the
44  * memory is updated with a new value, incrementing it when the hardware
47  * Because a word store in memory is atomic then the incremented value will
48  * always be in synch with the top bit indicating to any potential concurrent
56  *    32-bit counter;
58  * 2) this code must not be preempted for a duration longer than the
59  *    32-bit counter half period minus the longest period between two
62  * Those requirements ensure proper update to the state bit in memory.
63  * This is usually not a problem in practice, but if it is then a kernel
69  * 3) the cnt_lo argument must be seen as a globally incrementing value,
70  *    meaning that it should be a direct reference to the counter data which
71  *    can be evaluated according to a specific ordering within the macro,
72  *    and not the result of a previous evaluation stored in a variable.
85  * Note that the top bit (bit 63) in the returned value should be considered
86  * as garbage.  It is not cleared here because callers are likely to use a
87  * multiplier on the returned value which can get rid of the top bit
88  * implicitly by making the multiplier even, therefore saving on a runtime
89  * clear-bit instruction. Otherwise caller must remember to clear the top
90  * bit explicitly.