Update some Kconfig comments; Add a upper limit on the lower priority worker thread for priority inheritance

include/nuttx/wqueue.h

@@ -79,8 +79,10 @@
  *   then an additional, lower-priority work queue will also be created.  This
  *   lower priority work queue is better suited for more extended processing
  *   (such as file system clean-up operations)
- * CONFIG_SCHED_LPWORKPRIORITY - The execution priority of the lower priority
+ * CONFIG_SCHED_LPWORKPRIORITY - The minimum execution priority of the lower
- *   worker thread.  Default: 50
+ *   priority worker thread.  Default: 50
+ * CONFIG_SCHED_LPWORKPRIOMAX - The maximum execution priority of the lower
+ *   priority worker thread.  Default: 176
  * CONFIG_SCHED_LPWORKPERIOD - How often the lower priority worker thread
  *  checks for work in units of microseconds.  Default: 50*1000 (50 MS).
  * CONFIG_SCHED_LPWORKSTACKSIZE - The stack size allocated for the lower
@@ -175,6 +177,27 @@
 #    define CONFIG_SCHED_LPWORKPRIORITY 50
 #  endif
 
+#  ifndef CONFIG_SCHED_LPWORKPRIOMAX
+#    ifdef CONFIG_SCHED_HPWORK
+#      define CONFIG_SCHED_LPWORKPRIOMAX (CONFIG_SCHED_WORKPRIORITY-16)
+#    else
+#      define CONFIG_SCHED_LPWORKPRIOMAX 176
+#    endif
+#  endif
+
+#  ifdef CONFIG_SCHED_HPWORK
+#    if CONFIG_SCHED_LPWORKPRIORITY >= CONFIG_SCHED_WORKPRIORITY
+#      error CONFIG_SCHED_LPWORKPRIORITY >= CONFIG_SCHED_WORKPRIORITY
+#    endif
+#    if CONFIG_SCHED_LPWORKPRIOMAX >= CONFIG_SCHED_WORKPRIORITY
+#      error CONFIG_SCHED_LPWORKPRIOMAX >= CONFIG_SCHED_WORKPRIORITY
+#    endif
+#  endif
+
+#  if CONFIG_SCHED_LPWORKPRIORITY > CONFIG_SCHED_LPWORKPRIOMAX
+#    error CONFIG_SCHED_LPWORKPRIORITY > CONFIG_SCHED_LPWORKPRIOMAX
+#  endif
+
 #  ifndef CONFIG_SCHED_LPWORKPERIOD
 #    define CONFIG_SCHED_LPWORKPERIOD (50*1000) /* 50 milliseconds */
 #  endif

libc/Kconfig

@@ -412,9 +412,10 @@ config SCHED_HPWORK
 	bool "High priority (kernel) worker thread"
 	default y
 	---help---
-		If SCHED_WORKQUEUE is defined, then a single, high priority work queue is
+		If SCHED_WORKQUEUE is defined, then a single, high priority work
-		created by default.  This high priority worker thread is intended to serve
+		queue is created by default.  This high priority worker thread is
-		as the "bottom half" for driver interrupt handling.
+		intended to serve as the "bottom half" for driver interrupt
+		handling.
 
 if SCHED_HPWORK
 
@@ -422,7 +423,20 @@ config SCHED_WORKPRIORITY
 	int "High priority worker thread priority"
 	default 192
 	---help---
-		The execution priority of the worker thread.  Default: 192
+		The execution priority of the higher priority worker thread.
+
+		The higher priority worker thread is intended to serve as the
+		"bottom" half for device drivers.  As a consequence it must run at
+		a very high, fixed priority.  Typically, it should be the highest
+		priority thread in your system.  Default: 192
+
+		For lower priority, application oriented worker thread support,
+		please consider enabling the lower priority work queue.  The lower
+		priority work queue runs at a lower priority, of course, but has
+		the added advantage that it supports "priority inheritance" (if
+		PRIORITY_INHERITANCE is also selected):  The priority of the lower
+		priority worker thread can then be adjusted to match the highest
+		priority client.
 
 config SCHED_WORKPERIOD
 	int "High priority worker thread period"
@@ -445,8 +459,9 @@ config SCHED_LPWORK
 		If SCHED_WORKQUEUE is defined, then a single work queue is created by
 		default.  If SCHED_LPWORK is also defined then an additional, lower-
 		priority work queue will also be created.  This lower priority work
-		queue is better suited for more extended processing (such as file system
+		queue is better suited for more extended, application oriented
-		clean-up operations)
+		processing (such as file system clean-up operations or asynchronous
+		I/O)
 
 if SCHED_LPWORK
 
@@ -454,7 +469,55 @@ config SCHED_LPWORKPRIORITY
 	int "Low priority worker thread priority"
 	default 50
 	---help---
-		The execution priority of the lopwer priority worker thread.  Default: 50
+		The minimum execution priority of the lower priority worker thread.
+
+		The lower priority worker thread is intended support application-
+		oriented functions.  The lower priority work queue runs at a lower
+		priority, of course, but has the added advantage that it supports
+		"priority inheritance" (if PRIORITY_INHERITANCE is also selected):
+		The priority of the lower priority worker thread can then be
+		adjusted to match the highest priority client.  Default: 50
+
+		NOTE: This priority inheritance feature is not automatic.  The
+		lower priority worker thread will always a fixed priority unless
+		you implement logic that calls lpwork_boostpriority() to raise the
+		priority of the lower priority worker thread (typically called
+		before scheduling the work) and then call the matching
+		lpwork_restorepriority() when the work is completed (typically
+		called within the work handler at the completion of the work).
+		Currently, only the NuttX asynchronous I/O logic uses this dynamic
+		prioritization feature.
+
+		The higher priority worker thread, on the other hand, is intended
+		to serve as the "bottom" half for device drivers.  As a consequence
+		it must run at a very high, fixed priority.  Typically, it should
+		be the highest priority thread in your system.
+
+config SCHED_LPWORKPRIOMAX
+	int "Low priority worker thread maximum priority"
+	default 176
+	depends on PRIORITY_INHERITANCE
+	---help---
+		The maximum execution priority of the lower priority worker thread.
+
+		The lower priority worker thread is intended support application-
+		oriented functions.  The lower priority work queue runs at a lower
+		priority, of course, but has the added advantage that it supports
+		"priority inheritance" (if PRIORITY_INHERITANCE is also selected):
+		The priority of the lower priority worker thread can then be
+		adjusted to match the highest priority client.
+
+		The higher priority worker thread, on the other hand, is intended
+		to serve as the "bottom" half for device drivers.  As a consequence
+		it must run at a very high, fixed priority.  Typically, it should
+		be the highest priority thread in your system.
+
+		This function provides an upper limit on the priority of the lower
+		priority worker thread.  This would be necessary, for example, if
+		the higher priority worker thread were to defer work to the lower
+		priority thread.  Clearly, in such a case, you would want to limit
+		the maximum priority of the lower priority work thread.  Default:
+		176
 
 config SCHED_LPWORKPERIOD
 	int "Low priority worker thread period"

sched/wqueue/work_inherit.c

@@ -78,6 +78,13 @@ void lpwork_boostpriority(uint8_t reqprio)
   irqstate_t flags;
   pid_t wpid;
 
+  /* Clip to the configured maximum priority */
+
+  if (reqprio > CONFIG_SCHED_LPWORKPRIOMAX)
+    {
+      reqprio = CONFIG_SCHED_LPWORKPRIOMAX;
+    }
+
   /* Get the process ID of the low priority worker thread from the low
    * priority work queue.  Then get the TCB of the low priority worker
    * thread from the process ID.
@@ -194,6 +201,13 @@ void lpwork_restorepriority(uint8_t reqprio)
   int index;
   int selected;
 
+  /* Clip to the configured maximum priority */
+
+  if (reqprio > CONFIG_SCHED_LPWORKPRIOMAX)
+    {
+      reqprio = CONFIG_SCHED_LPWORKPRIOMAX;
+    }
+
   /* Get the process ID of the low priority worker thread from the low
    * priority work queue.  Then get the TCB of the low priority worker
    * thread from the process ID.