Specifications

QSSC-S4R Technical Product Specification Processor Presence and Population Check
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x During runtime, if BIOS needs to return bus ownership to the BMC, it must first try to do this using the IPMI OEM
command method. Only if the BMC doesn’t respond to the IPMI OEM command, then BIOS must reset the
associated semaphore bit to indicate that the BMC now owns the bus.
x During runtime, if BIOS needs to return bus ownership to the BMC, it must first try to do this using the IPMI OEM
command method. Only if the BMC doesn’t respond to the IPMI OEM command, then BIOS must reset the
associated semaphore bit to indicate that the BMC now owns the bus.
x The BMC FW checks that it is owner of the bus prior to initiating any transaction on the bus by inspecting the state
of the associated mailbox semaphore bit.
24.20.2 Sequence of Operations during Memory Hot Plug
The BIOS/BMC interactions are as follows:
x BIOS owns all the bus segments until completion of POST. After POST completes, the BMC becomes the default
owner.
x When a memory hot-plug or memory on-line operation is initiated, BIOS must request access of the applicable
SMBus segment from the BMC using the Acquire System Resource OEM IPMI command. If the BMC is in the
middle of an SMBus transaction, it must respond to the BIOS with an appropriate response code and halt any
further transactions on that bus. After waiting to allow the BMC to finish its transaction for 250ms, BIOS must retry
its request for bus ownership. It is recommended that BIOS should attempt a minimum of 2 retries. .If the BMC
doesn’t relinquish the bus or is not responding to the command request after BIOS has completed its retry attempts
then BIOS may assume ownership of the bus segment by forcibly clearing the semaphore bit in order to complete
the hot-plug operation.
x Once BIOS has gained ownership of the bus segment, BMC will no longer poll on that bus until it regains
ownership.
x Once BIOS has completed the memory operation, BIOS sends new DIMM population mapping data to the BMC.
x BIOS must relinquish ownership to the BMC by resending the command after it has completed all bus accesses
required for the operation. Note that if BIOS hangs and doesn’t return the semaphore, the BMC will eventually
detect an SMI timeout and reset the system.
x Once BMC has regained ownership of the SMBus segment and there are no pending BIOS requests for access to
the segment, then BMC begins polling temperature sensors that are present on that bus.
When BIOS has ownership of a bus segment, then the BMC can no longer poll the DIMM temperature sensors on that
bus. The associated IPMI aggregate sensors, the DIMM Thermal Margin and Memory Buffer Thermal Margin sensors,
will then enter and remain in the “reading unavailable” sensor state as defined by the IPMI 2.0 Specification until the
BMC once again gains ownership of that bus and resumes polling.
The diagram below illustrates the BIOS/BMC interactions for memory hot-plug from the BMC perspective.