Q-Logic OFED, IB0056101-00 G.02 manual PAT and Write Combining, Mtrr Mapping and Write Combining

Write Combining

PAT and Write Combining

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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 page B-2, below.

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

NOTE:

There will be no write-combining entry in /proc/mtrr when using PAT.

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]

B-2 Preliminary IB0056101-00 G.02