From: arighi <arighi@38d2e660-2303-0410-9eaa-f027e97ec537>
Date: Fri, 4 Feb 2011 10:15:57 +0000 (+0000)
Subject: timer: fix a build warning and add a comment
X-Git-Tag: 2.7.0~285
X-Git-Url: https://codewiz.org/gitweb?a=commitdiff_plain;h=9ce278eab99540113abad1e55026fb9d3aa3c2e2;p=bertos.git

timer: fix a build warning and add a comment

Fix the following build warning:
 bertos/drv/timer.c:292: warning: signed and unsigned type in conditional expression

And comment the formula used to evaluate the timer delta.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4681 38d2e660-2303-0410-9eaa-f027e97ec537
---

diff --git a/bertos/drv/timer.c b/bertos/drv/timer.c
index 4d130946..d75f15ab 100644
--- a/bertos/drv/timer.c
+++ b/bertos/drv/timer.c
@@ -289,7 +289,27 @@ void timer_busyWait(hptime_t delay)
 	for (;;)
 	{
 		now = timer_hw_hpread();
-		delta = (now < prev) ? (TIMER_HW_CNT - prev + now) : (now - prev);
+		/*
+		 * The timer counter may wrap here and "prev" can become
+		 * greater than "now". So, be sure to always evaluate a
+		 * coherent timer difference:
+		 *
+		 * 0     prev            now   TIMER_HW_CNT
+		 * |_____|_______________|_____|
+		 *        ^^^^^^^^^^^^^^^
+		 * delta = now - prev
+		 *
+		 * 0     now             prev  TIMER_HW_CNT
+		 * |_____|_______________|_____|
+		 *  ^^^^^                 ^^^^^
+		 * delta = (TIMER_HW_CNT - prev) + now
+		 *
+		 * NOTE: TIMER_HW_CNT can be any value, not necessarily a power
+		 * of 2. For this reason the "%" operator is not suitable for
+		 * the generic case.
+		 */
+		delta = (now < prev) ? ((hptime_t)TIMER_HW_CNT - prev + now) :
+						(now - prev);
 		if (delta >= delay)
 			break;
 		delay -= delta;