cpumask: add a few comments of cpumask functions

Current few cpumask functions' purposes are not quite clear. Stupid
user like myself needs to dig into details for clear function
purpose and return value.
Add few explanation for them is helpful.

Thanks for Srivatsa's comments and correction!

Signed-off-by: Alex Shi <alex.shi@intel.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/cpumask.h

@@ -272,6 +272,8 @@ static inline void cpumask_clear_cpu(int cpu, struct cpumask *dstp)
  * @cpu: cpu number (< nr_cpu_ids)
  * @cpumask: the cpumask pointer
  *
+ * Returns 1 if @cpu is set in @cpumask, else returns 0
+ *
  * No static inline type checking - see Subtlety (1) above.
  */
 #define cpumask_test_cpu(cpu, cpumask) \
@@ -282,6 +284,8 @@ static inline void cpumask_clear_cpu(int cpu, struct cpumask *dstp)
  * @cpu: cpu number (< nr_cpu_ids)
  * @cpumask: the cpumask pointer
  *
+ * Returns 1 if @cpu is set in old bitmap of @cpumask, else returns 0
+ *
  * test_and_set_bit wrapper for cpumasks.
  */
 static inline int cpumask_test_and_set_cpu(int cpu, struct cpumask *cpumask)
@@ -294,6 +298,8 @@ static inline int cpumask_test_and_set_cpu(int cpu, struct cpumask *cpumask)
  * @cpu: cpu number (< nr_cpu_ids)
  * @cpumask: the cpumask pointer
  *
+ * Returns 1 if @cpu is set in old bitmap of @cpumask, else returns 0
+ *
  * test_and_clear_bit wrapper for cpumasks.
  */
 static inline int cpumask_test_and_clear_cpu(int cpu, struct cpumask *cpumask)
@@ -324,6 +330,8 @@ static inline void cpumask_clear(struct cpumask *dstp)
  * @dstp: the cpumask result
  * @src1p: the first input
  * @src2p: the second input
+ *
+ * If *@dstp is empty, returns 0, else returns 1
  */
 static inline int cpumask_and(struct cpumask *dstp,
 			       const struct cpumask *src1p,
@@ -365,6 +373,8 @@ static inline void cpumask_xor(struct cpumask *dstp,
  * @dstp: the cpumask result
  * @src1p: the first input
  * @src2p: the second input
+ *
+ * If *@dstp is empty, returns 0, else returns 1
  */
 static inline int cpumask_andnot(struct cpumask *dstp,
 				  const struct cpumask *src1p,
@@ -414,6 +424,8 @@ static inline bool cpumask_intersects(const struct cpumask *src1p,
  * cpumask_subset - (*src1p & ~*src2p) == 0
  * @src1p: the first input
  * @src2p: the second input
+ *
+ * Returns 1 if *@src1p is a subset of *@src2p, else returns 0
  */
 static inline int cpumask_subset(const struct cpumask *src1p,
 				 const struct cpumask *src2p)