P-FP: Clarify meaning of priorities

Add a comment to explain how priorities are
interpreted, and provide some useful macros for
userspace.

1 files changed, 19 insertions, 1 deletions

diff --git a/include/litmus/rt_param.h b/include/litmus/rt_param.h
index 1ce96af51287..c4669a276e6f 100644
--- a/include/litmus/rt_param.h
+++ b/include/litmus/rt_param.h
@@ -33,7 +33,25 @@ typedef enum {
         PRECISE_ENFORCEMENT  /* budgets are enforced with hrtimers */
 } budget_policy_t;
 
-#define LITMUS_MAX_PRIORITY 512
+/* We use the common priority interpretation "lower index == higher priority",
+ * which is commonly used in fixed-priority schedulability analysis papers.
+ * So, a numerically lower priority value implies higher scheduling priority,
+ * with priority 1 being the highest priority. Priority 0 is reserved for
+ * priority boosting. LITMUS_MAX_PRIORITY denotes the maximum priority value
+ * range.
+ */
+
+#define LITMUS_MAX_PRIORITY     512
+#define LITMUS_HIGHEST_PRIORITY   1
+#define LITMUS_LOWEST_PRIORITY    (LITMUS_MAX_PRIORITY - 1)
+
+/* Provide generic comparison macros for userspace,
+ * in case that we change this later. */
+#define litmus_higher_fixed_prio(a, b)  (a < b)
+#define litmus_lower_fixed_prio(a, b)   (a > b)
+#define litmus_is_valid_fixed_prio(p)           \
+        ((p) >= LITMUS_HIGHEST_PRIORITY &&      \
+         (p) <= LITMUS_LOWEST_PRIORITY)
 
 struct rt_task {
         lt_t            exec_cost;