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-rw-r--r--include/linux/cnt32_to_63.h20
1 files changed, 19 insertions, 1 deletions
diff --git a/include/linux/cnt32_to_63.h b/include/linux/cnt32_to_63.h
index 7605fdd1eb6..e3d8bf26e5e 100644
--- a/include/linux/cnt32_to_63.h
+++ b/include/linux/cnt32_to_63.h
@@ -61,13 +61,31 @@ union cnt32_to_63 {
*
* 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.
+ * 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.
*
+ * And finally:
+ *
+ * 3) the cnt_lo argument must be seen as a globally incrementing value,
+ * meaning that it should be a direct reference to the counter data which
+ * can be evaluated according to a specific ordering within the macro,
+ * and not the result of a previous evaluation stored in a variable.
+ *
+ * For example, this is wrong:
+ *
+ * u32 partial = get_hw_count();
+ * u64 full = cnt32_to_63(partial);
+ * return full;
+ *
+ * This is fine:
+ *
+ * u64 full = cnt32_to_63(get_hw_count());
+ * return full;
+ *
* 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
* multiplier on the returned value which can get rid of the top bit