[openfirmware] [commit] r2454 - cpu/x86/pc

repository service svn at openfirmware.info
Wed Aug 10 00:58:17 CEST 2011


Author: wmb
Date: Wed Aug 10 00:58:17 2011
New Revision: 2454
URL: http://tracker.coreboot.org/trac/openfirmware/changeset/2454

Log:
x86 timestamp counter calibration loop needs a slightly longer sensing window to prevent miscalibration.

Modified:
   cpu/x86/pc/tsccal1.fth

Modified: cpu/x86/pc/tsccal1.fth
==============================================================================
--- cpu/x86/pc/tsccal1.fth	Wed Aug 10 00:42:06 2011	(r2453)
+++ cpu/x86/pc/tsccal1.fth	Wed Aug 10 00:58:17 2011	(r2454)
@@ -24,7 +24,12 @@
       al ah mov
       h# 40 # al in
       al ah xchg
-      d# 5 #  ax  cmp
+      \ The number 10 below gives a sufficient window to ensure that a count
+      \ value in the range from 0 to 9 is seen.  The process of latching the
+      \ timer and reading the value is time-consuming because I/O port access
+      \ is slow, comparable to the clock that drives the ticker.  For many
+      \ systems, a value of 5 is enough, but I have seen systems that need 8.
+      d# 10 #  ax  cmp
    < until
    
    h# f c,  h# 31 c,	\ Get time-stamp counter value into DX,AX



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