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-rw-r--r--include/linux/cnt32_to_63.h22
1 files changed, 16 insertions, 6 deletions
diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h
index 8c0f9505b48c..7605fdd1eb65 100644
--- a/include/linux/cnt32_to_63.h
+++ b/include/linux/cnt32_to_63.h
@@ -16,6 +16,7 @@
16#include <linux/compiler.h> 16#include <linux/compiler.h>
17#include <linux/types.h> 17#include <linux/types.h>
18#include <asm/byteorder.h> 18#include <asm/byteorder.h>
19#include <asm/system.h>
19 20
20/* this is used only to give gcc a clue about good code generation */ 21/* this is used only to give gcc a clue about good code generation */
21union cnt32_to_63 { 22union cnt32_to_63 {
@@ -53,11 +54,19 @@ union cnt32_to_63 {
53 * needed increment. And any race in updating the value in memory is harmless 54 * needed increment. And any race in updating the value in memory is harmless
54 * as the same value would simply be stored more than once. 55 * as the same value would simply be stored more than once.
55 * 56 *
56 * The only restriction for the algorithm to work properly is that this 57 * The restrictions for the algorithm to work properly are:
57 * code must be executed at least once per each half period of the 32-bit 58 *
58 * counter to properly update the state bit in memory. This is usually not a 59 * 1) this code must be called at least once per each half period of the
59 * problem in practice, but if it is then a kernel timer could be scheduled 60 * 32-bit counter;
60 * to manage for this code to be executed often enough. 61 *
62 * 2) this code must not be preempted for a duration longer than the
63 * 32-bit counter half period minus the longest period between two
64 * calls to this code.
65 *
66 * Those requirements ensure proper update to the state bit in memory.
67 * This is usually not a problem in practice, but if it is then a kernel
68 * timer should be scheduled to manage for this code to be executed often
69 * enough.
61 * 70 *
62 * Note that the top bit (bit 63) in the returned value should be considered 71 * Note that the top bit (bit 63) in the returned value should be considered
63 * as garbage. It is not cleared here because callers are likely to use a 72 * as garbage. It is not cleared here because callers are likely to use a
@@ -68,9 +77,10 @@ union cnt32_to_63 {
68 */ 77 */
69#define cnt32_to_63(cnt_lo) \ 78#define cnt32_to_63(cnt_lo) \
70({ \ 79({ \
71 static volatile u32 __m_cnt_hi; \ 80 static u32 __m_cnt_hi; \
72 union cnt32_to_63 __x; \ 81 union cnt32_to_63 __x; \
73 __x.hi = __m_cnt_hi; \ 82 __x.hi = __m_cnt_hi; \
83 smp_rmb(); \
74 __x.lo = (cnt_lo); \ 84 __x.lo = (cnt_lo); \
75 if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \ 85 if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
76 __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \ 86 __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \