include/linux/cnt32_to_63.h

1 /* SPDX-License-Identifier: GPL-2.0-only */
3  *  Extend a 32-bit counter to 63 bits
31  * cnt32_to_63 - Expand a 32-bit counter to a 63-bit counter
34  * Many hardware clock counters are only 32 bits wide and therefore have
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
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
56  *    32-bit counter;
59  *    32-bit counter half period minus the longest period between two
85  * Note that the top bit (bit 63) in the returned value should be considered
89  * clear-bit instruction. Otherwise caller must remember to clear the top