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     imcsmb — Intel integrated Memory Controller (iMC) SMBus controller driver


     device pci
     device smbus
     device imcsmb

     Alternatively, to load the driver as a module at boot time, place the following line in



     The imcsmb driver provides smbus(4) support for the SMBus controller functionality in the
     integrated Memory Controllers (iMCs) embedded in Intel Sandybridge-Xeon, Ivybridge-Xeon,
     Haswell-Xeon, and Broadwell-Xeon CPUs.  Each CPU implements one or more iMCs, depending on
     the number of cores; each iMC implements two SMBus controllers (iMC-SMBs).  The iMC-SMBs are
     used by the iMCs to read configuration information from the DIMMs during POST.  They may
     also be used, by motherboard firmware or a BMC, to monitor the temperature of the DIMMs.

     The iMC-SMBs are not general-purpose SMBus controllers.  By their nature, they are only ever
     attached to DIMMs, so they implement only the SMBus operations need for communicating with
     DIMMs.  Specifically:

           -   READB
           -   READW
           -   WRITEB
           -   WRITEW

     A more detailed discussion of the hardware and driver architecture can be found at the top
     of sys/dev/imcsmb/imcsmb_pci.c.


     As mentioned above, firmware might use the iMC-SMBs to read DIMM temperatures.  The public
     iMC documentation does not describe any sort of coordination mechanism to prevent requests
     from different sources -- such as the motherboard firmware, a BMC, or the operating system
     -- from interfering with each other.

     Therefore, it is highly recommended that developers contact the motherboard vendor for any
     board-specific instructions on how to disable and re-enable DIMM temperature monitoring.

     DIMM temperature monitoring should be disabled before returning from
     imcsmb_pci_request_bus(), and re-enabled before returning from imcsmb_pci_release_bus().
     The driver includes comments to that effect at the appropriate locations.  The driver has
     been tested and shown to work, with only that type of modification, on certain motherboards
     from Intel.  (Unfortunately, those modifications were based on material covered under a non-
     disclosure agreement, and therefore are not included in this driver.)  The driver has also
     been tested and shown to work as-is on various motherboards from SuperMicro.

     The smb(4) driver will connect to the smbus(4) instances created by imcsmb.  However, since
     the IMC-SMBs are not general-purpose SMBus controllers, using smbmsg(8) with those smb(4)
     devices is not supported.


     jedec_dimm(4), smbus(4)


     The imcsmb driver first appeared in FreeBSD 12.0.


     The imcsmb driver was originally written for Panasas by Joe Kloss.  It was substantially
     refactored, and this manual page was written, by
     Ravi Pokala <>