Module Name: src
Committed By: thorpej
Date: Sat Oct 10 18:18:05 UTC 2020
Modified Files:
src/sys/kern: kern_cctr.c
Log Message:
Add some more detail/explanation to the calibration timing diagram.
To generate a diff of this commit:
cvs rdiff -u -r1.11 -r1.12 src/sys/kern/kern_cctr.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/kern_cctr.c
diff -u src/sys/kern/kern_cctr.c:1.11 src/sys/kern/kern_cctr.c:1.12
--- src/sys/kern/kern_cctr.c:1.11 Sat Oct 10 03:05:04 2020
+++ src/sys/kern/kern_cctr.c Sat Oct 10 18:18:04 2020
@@ -1,4 +1,4 @@
-/* $NetBSD: kern_cctr.c,v 1.11 2020/10/10 03:05:04 thorpej Exp $ */
+/* $NetBSD: kern_cctr.c,v 1.12 2020/10/10 18:18:04 thorpej Exp $ */
/*-
* Copyright (c) 2020 Jason R. Thorpe
@@ -43,25 +43,39 @@
* Calibration happens periodically, and works like this:
*
* Secondary CPU Primary CPU
- * T0 = local CC
* Send IPI to publish reference CC
* --------->
+ * Indicate Primary Ready
+ * <----------------------------
+ * T0 = local CC
+ * Indicate Secondary Ready
+ * ----------------->
* (assume this happens at Tavg) Publish reference CC
- * <-----------------------
- * Notice publication
+ * Indicate completion
+ * <------------------------
+ * Notice completion
* T1 = local CC
*
* Tavg = (T0 + T1) / 2
*
* Delta = Tavg - Published primary CC value
*
+ * "Notice completion" is performed by waiting for the primary to set
+ * the calibration state to FINISHED. This is a little unfortunate,
+ * because T0->Tavg involves a single store-release on the secondary, and
+ * Tavg->T1 involves a store-relaxed and a store-release. It would be
+ * better to simply wait for the reference CC to transition from 0 to
+ * non-0 (i.e. just wait for a single store-release from Tavg->T1), but
+ * if the cycle counter just happened to read back as 0 at that instant,
+ * we would never break out of the loop.
+ *
* We trigger calibration roughly once a second; the period is actually
* skewed based on the CPU index in order to avoid lock contention. The
* calibration interval does not need to be precise, and so this is fine.
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_cctr.c,v 1.11 2020/10/10 03:05:04 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_cctr.c,v 1.12 2020/10/10 18:18:04 thorpej Exp $");
#include <sys/param.h>
#include <sys/atomic.h>
