QLogic OFED+ Host Software User Guide, Rev. B

H–Write Combining
PAT and Write Combining
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 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). The default is:
option ib_qib wc_pat=1
If PAT is unavailable or PAT initialization fails, the code generates a message in
the log and falls back to the Memory Type Range Registers (MTRR) mechanism.
To use MTRR, disable PAT by setting this module parameter to 0 (as a root user):
option ib_qib wc_pat=0
Then, revert to using MTRR-only behavior by following one of the two suggestions
in “MTRR Mapping and Write Combining” on page H-2.
The driver must be restarted after the changes have been made.
NOTE:
There will be no WC entry in /proc/mtrr when using PAT.
MTRR Mapping and Write Combining
.
Two suggestions for properly enabling MTRR mapping for write combining are
described in the following sections.
See “Performance Issues” on page F-9 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]
For systems with very large amounts of memory (32GB or more), it may also be
necessary to adjust the BIOS setting for the PCI hole granularity to 2GB. This
setting allows the memory to be mapped with fewer MTRRs, so that there will be
one or more unused MTRRs for the InfiniPath driver.
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