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