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Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

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+OPAL <--> BMC interactions
+==========================
+
+This document provides information about some of the user-visible interactions
+that skiboot performs with the BMC.
+
+IPMI sensors
+------------
+
+OPAL will interact with a few IPMI sensors during the boot process. These
+are:
+
+  * Boot Count [type 0xc3: OEM reserved]
+  * FW Boot progress [type 0x0f: System Firmware Progress]
+
+Boot Count: assertion type. When OPAL reaches a late stage of boot, it sets the
+boot count sensor to 0x02. This is intended to allow the BMC detect a failed
+or aborted boot, for switching to a known-good firmware image.
+
+FW Boot Progress: assertion type. During boot, skiboot will update this sensor
+to one of the IPMI-defined progress codes. The codes use by skiboot are:
+
+  * PCI Resource configuration (0x01)
+     - asserted as the PCI devices have been probed and resources allocated
+
+  * Motherboard init (0x14)
+     - asserted as the platform-specific components have been initialised
+
+  * OS boot (0x13)
+     - asserted after skiboot has loaded the PAYLOAD image, and is about to
+       boot it.
+
+Chassis control messages
+------------------------
+
+OPAL uses chassis control messages to instruct the BMC to remove power from
+the host. These messages are sent during graceful reboot and shutdown processes
+initiated by the host.
+
+For a BMC-initiated graceful power-down (or reboot), the BMC is expected to send
+an OEM-defined SEL message, using a SMS_ATN to trigger a BMC-to-host
+notification. This SEL has a type of 0xc0, and command of 0x04. The data0 field
+of the SEL indicates shutdown (0x0) or reboot (0x1).
+
+
+Watchdog support
+----------------
+
+OPAL supports a BMC watchdog during the boot process. This will be disabled
+before entering the OS.
+
+
+Real-time clock
+---------------
+
+On platforms where a real-time-clock is not available, skiboot may use the
+IPMI SEL Time as a real-time-clock device.