Draft
Write Combining
PAT and Write Combining
B-2 Preliminary IB0056101-00 G.02
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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
given below. There are no conflicts between the two methods.
PAT and Write Combining
This is the default mechanism for allocating write-combining (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). This is the
default:
option ib_ipath wc_pat=1
If PAT is unavailable or PAT initialization fails for some reason, the code will
generate a message in the log and fall back to the Memory Type Range Registers
(MTRR) mechanism. To use MTRR, disable PAT by setting this module parameter
to 0 (as root):
option ib_ipath wc_pat=0
Then, revert to using the MTRR-only behavior by following one of the two
suggestions in “MTRR Mapping and Write Combining” on pageB-2, below.
The driver will need to be restarted after the changes have been made.
MTRR Mapping and Write Combining
Two suggestions for using MTRR mapping to fix this problem are described
below.
See the Troubleshooting section of the QLogic HCA and QLogic OFED Software
User Guide for more details on a related performance issue.

Edit BIOS Settings to Fix MTRR Issues

You can edit the BIOS setting for MTRR Mapping. The BIOS setting looks similar
to:
MTRR Mapping [Discrete]
NOTE:
There will be no write-combining entry in /proc/mtrr when using PAT.