From 89a0040f1b5e83eaab116d04283e241543c9b4e2 Mon Sep 17 00:00:00 2001
From: Robin Gareus <robin@gareus.org>
Date: Thu, 10 Jun 2021 03:16:35 +0200
Subject: [PATCH] Allow to override rt priority for internal backends

---
 libs/pbd/pbd/pthread_utils.h | 20 +++++++++--------
 libs/pbd/pthread_utils.cc    | 43 ++++++++++++++++++++++++++++++++----
 2 files changed, 50 insertions(+), 13 deletions(-)

diff --git a/libs/pbd/pbd/pthread_utils.h b/libs/pbd/pbd/pthread_utils.h
index c6e809157a..47f8357b9e 100644
--- a/libs/pbd/pbd/pthread_utils.h
+++ b/libs/pbd/pbd/pthread_utils.h
@@ -54,15 +54,9 @@
 /* these are relative to sched_get_priority_max()
  * see pbd_absolute_rt_priority()
  */
-#ifdef PLATFORM_WINDOWS
-# define PBD_RT_PRI_MAIN -1
-# define PBD_RT_PRI_MIDI -2
-# define PBD_RT_PRI_PROC -2
-#else
-# define PBD_RT_PRI_MAIN -20
-# define PBD_RT_PRI_MIDI -21
-# define PBD_RT_PRI_PROC -22
-#endif
+# define PBD_RT_PRI_MAIN pbd_pthread_priority (THREAD_MAIN)
+# define PBD_RT_PRI_MIDI pbd_pthread_priority (THREAD_MIDI)
+# define PBD_RT_PRI_PROC pbd_pthread_priority (THREAD_PROC)
 
 LIBPBD_API int  pthread_create_and_store (std::string name, pthread_t  *thread, void * (*start_routine)(void *), void * arg);
 LIBPBD_API void pthread_cancel_one (pthread_t thread);
@@ -71,6 +65,14 @@ LIBPBD_API void pthread_kill_all (int signum);
 LIBPBD_API const char* pthread_name ();
 LIBPBD_API void pthread_set_name (const char* name);
 
+enum PBDThreadClass {
+	THREAD_MAIN, // main audio I/O thread
+	THREAD_MIDI, // MIDI I/O threads
+	THREAD_PROC // realtime worker
+};
+
+LIBPBD_API int pbd_pthread_priority (PBDThreadClass);
+
 LIBPBD_API int pbd_pthread_create (
 		const size_t stacksize,
 		pthread_t *thread,
diff --git a/libs/pbd/pthread_utils.cc b/libs/pbd/pthread_utils.cc
index 78bf942171..69f1072148 100644
--- a/libs/pbd/pthread_utils.cc
+++ b/libs/pbd/pthread_utils.cc
@@ -277,6 +277,42 @@ pbd_pthread_create (
 	return rv;
 }
 
+int
+pbd_pthread_priority (PBDThreadClass which)
+{
+	/* fall back to use values relative to max */
+#ifdef PLATFORM_WINDOWS
+	switch (which) {
+		case THREAD_MAIN:
+			return -1;
+		case THREAD_MIDI:
+			return -2;
+		default:
+		case THREAD_PROC:
+			return -2;
+	}
+#else
+	int base = -20;
+	const char* p = getenv ("ARDOUR_SCHED_PRI");
+	if (p && *p) {
+		base = atoi (p);
+		if (base > -5 && base < 5) {
+			base = -20;
+		}
+	}
+
+	switch (which) {
+		case THREAD_MAIN:
+			return base;
+		case THREAD_MIDI:
+			return base - 1;
+		default:
+		case THREAD_PROC:
+			return base - 2;
+	}
+#endif
+}
+
 int
 pbd_absolute_rt_priority (int policy, int priority)
 {
@@ -285,13 +321,12 @@ pbd_absolute_rt_priority (int policy, int priority)
 	const int p_max = sched_get_priority_max (policy); // Linux: 99
 
 	if (priority == 0) {
-		/* use default. XXX this should be relative to audio (JACK) thread,
-		 * internal backends use -20 (Audio), -21 (MIDI), -22 (compuation)
-		 */
 		priority = (p_min + p_max) / 2;
 	} else if (priority > 0) {
-		priority += p_min;
+		/* value relative to minium */
+		priority += p_min - 1;
 	} else {
+		/* value relative maximum */
 		priority += p_max + 1;
 	}