QLogic 7104-1M-CABLE User Manual

B–Write Combining
PAT and Write Combining

B-2

IB0056101-00  H

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If you do not see any of these messages on your console, but suspect this 
problem, check the /var/log/messages file. Some systems suppress driver 

load messages but still output them to the log file.

Methods for enabling and disabling the two write combining mechanisms are 
described in the following sections. There are no conflicts between the two 
methods.

PAT is the default mechanism for allocating WC mappings for the PIO buffers. It is 
set as a parameter in /etc/modprobe.conf (on Red Hat systems) or 
/etc/modprobe.conf.local (on SLES systems). The default is:

option ib_ipath wc_pat=1

If PAT is unavailable or PAT initialization fails, the code generates a message in 
the log and falls back to the MTRR mechanism. To use MTRR, disable PAT by 
setting this module parameter to 0 (as a root user):

Then, revert to using the MTRR-only behavior by following one of the two 
suggestions in 

The driver will need to be restarted after the changes have been made.

The following sections describe two methods for fixing MTRR issues.

See the Troubleshooting section of the QLogic Host Channel Adapter and QLogic 
OFED Software Users Guide for more details on a related performance issue.

You can edit the BIOS setting for MTRR mapping. The BIOS setting looks similar 
to:

MTRR Mapping          [Discrete]

There is no WC entry in /proc/mtrr when using PAT.