clarify usage expectations for cnt32_to_63()

[linux-2.6-omap-h63xx.git] / include / linux / cnt32_to_63.h

diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h
index 8c0f9505b48cbed9cab8b268ec67519df0f01ddc..7605fdd1eb65de1fffd5f1bf5b47428e16475133 100644 (file)
--- a/include/linux/cnt32_to_63.h
+++ b/include/linux/cnt32_to_63.h
@@ -16,6 +16,7 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <asm/byteorder.h>
+#include <asm/system.h>
 
 /* this is used only to give gcc a clue about good code generation */
 union cnt32_to_63 {
@@ -53,11 +54,19 @@ union cnt32_to_63 {
  * needed increment.  And any race in updating the value in memory is harmless
  * as the same value would simply be stored more than once.
  *
- * The only restriction for the algorithm to work properly is that this
- * code must be executed at least once per each half period of the 32-bit
- * counter to properly update the state bit in memory. This is usually not a
- * problem in practice, but if it is then a kernel timer could be scheduled
- * to manage for this code to be executed often enough.
+ * The restrictions for the algorithm to work properly are:
+ *
+ * 1) this code must be called at least once per each half period of the
+ *    32-bit counter;
+ *
+ * 2) this code must not be preempted for a duration longer than the
+ *    32-bit counter half period minus the longest period between two
+ *    calls to this code.
+ *
+ * Those requirements ensure proper update to the state bit in memory.
+ * This is usually not a problem in practice, but if it is then a kernel
+ * timer should be scheduled to manage for this code to be executed often
+ * enough.
  *
  * Note that the top bit (bit 63) in the returned value should be considered
  * as garbage.  It is not cleared here because callers are likely to use a
@@ -68,9 +77,10 @@ union cnt32_to_63 {
  */
 #define cnt32_to_63(cnt_lo) \
 ({ \
-       static volatile u32 __m_cnt_hi; \
+       static u32 __m_cnt_hi; \
        union cnt32_to_63 __x; \
        __x.hi = __m_cnt_hi; \
+       smp_rmb(); \
        __x.lo = (cnt_lo); \
        if (unlikely((s32)(__x.hi ^ __x.lo) < 0)) \
                __m_cnt_hi = __x.hi = (__x.hi ^ 0x80000000) + (__x.hi >> 31); \