IPMI（Intelligent Platform Management Interface）是一种带外（out-of-band）管理接口，IPMI的核心是一个BMC（Baseboard Manager Controller），其并不依赖于服务器的处理器、BIOS或操作系统来工作，可谓非常地独立，是一个单独在系统内运行的无代理管理子系统，只要有BMC与IPMI固件其便可开始工作，而BMC通常是一个安装在服务器主板上的独立的板卡，也有服务器主板提供对IPMI支持的。（IPMI目前已不维护，逐渐由RedFish替代， RedFish目前支持IPMI2.0和PMCI）。
- IPMI的硬件连接
下图为简要的连接方案
4个主要部分为remote，BMC，SERVER，pcie card。其中BMC通过IPMB（I2C硬件接口）与PCIE板卡连接，软件上BMC OS通过ipmi driver与pcie card内部的firmware通信，主要传输控制流。PCIE card与SERVER保持原有连接, 传输数据流。BMC通过LPC（Low Pin Count Bus）连接SERVER可以对cpu进行管理，属于System interface。关于remote 部分，是指其他server远程访问BMC,主要通过LAN通信，中间经过KVM over IP(Keyboard Video Mouse)或者SOL （Serial Over LAN）转换器转换成网络通信方式。
实物图
下图为IPMI 2.0 spec中的连接结构原图
- IPMI的系统结构
如下图，其中Remote Access负责接受远端Server的控制协议，Local Access负责本地控制协议，其中system interface包括KCS（Keyboard Controller Style），BT（block transfer）等负责与system cpu通信，mgmt Busses主要负责device 比如pcie card相关的通信。
具体参考
https://www.intel.cn/content/www/cn/zh/products/docs/servers/ipmi/ipmi-second-gen-interface-spec-v2-rev1-1.html
software
- ipmitools（user mode tools）：
负责调用ipmi driver进行管理，或者通过网络进行控制，即可以再remote端使用ipmitools经过BMC里通过kipmi0进程（属于ipmitools一部分）进行数据转发。
ipmitools控制命令比如
温度查看
查看风扇
- linux ipmi driver：
位于driver/char/ipmi，包括以下几个部分
ipmi_msghandler - This is the central piece of software for the IPMI system. It handles all messages, message timing, and responses. The IPMI users tie into this, and the IPMI physical interfaces (called System Management Interfaces, or SMIs) also tie in here. This provides the kernelland interface for IPMI, but does not provide an interface for use by application processes.
ipmi_devintf - This provides a userland IOCTL interface for the IPMI driver, each open file for this device ties in to the message handler as an IPMI user.
ipmi_si - A driver for various system interfaces. This supports KCS, SMIC, and BT interfaces. Unless you have an SMBus interface or your own custom interface, you probably need to use this.
ipmi_ssif - A driver for accessing BMCs on the SMBus. It uses the I2C kernel driver’s SMBus interfaces to send and receive IPMI messages over the SMBus.
ipmi_powernv - A driver for access BMCs on POWERNV systems.
ipmi_watchdog - IPMI requires systems to have a very capable watchdog timer. This driver implements the standard Linux watchdog timer interface on top of the IPMI message handler.
ipmi_poweroff - Some systems support the ability to be turned off via IPMI commands.
bt-bmc - This is not part of the main driver, but instead a driver for accessing a BMC-side interface of a BT interface. It is used on BMCs running Linux to provide an interface to the host.
- pcie card firmware
firmware负责从IPMB（i2c）接收到command并进行处理，返回结果。
x86->pcie card的调用：
ipmi sensor path：
|++++bmc os+++++++| |++PCIE card firmware++|
application->/driver/ipmi->IPMB(i2c hardware)->firmware->method
IPMI VS Redfish
Redfish是被用于取代IPMI的新方案。Redfish目前支持IPMI2.0以及PMCI
PMCI（Platform Management Components Intercommunication）
- PMCI连接结构
如下图，中间是BMC，Redfish是运行在BMC上的网络服务。
- PMCI系统结构
具体参考https://www.dmtf.org/sites/default/files/DSP2015_2.0.0%20%20%E8%AF%91%E6%96%87.pdf
