Authors: peterh@google.com
Last Updated: 2017-05-16
Provide a mechanism so that the host can manipulate I2C and SMBus devices on the associated BMC.
Recent server systems come with a secondary processing system attached for the purpose of monitoring and control, generally referred to as a BMC. There is a large effort to develop an open source framework for writing applications and control systems that will run on the BMC, known as OpenBMC. Within Google the effort has been internalized (while also providing upstream pushes) as gBMC. The primary goal of OpenBMC is to support remote and local system management through several interfaces, IPMI being the one of interest here.
Historically, a host tool provides a kind of adaptation layer, enabling many system-level functions for Google Servers. Important functions presume direct I2C access, but with a BMC, the server is wired such that only the BMC has direct access and control over many motherboard components, including substantially all I2C adapters. To resolve this inconsistency, OpenBMC will be extended to expose the needed I2C adapters for use by host tools.
IPMI allows for OEM commands to provide custom information flow or system control with a BMC. And OpenBmc now has its own IANA OEM Enterprise Number, which may be used when upstreaming such controls for general use.
The BMC IPMI daemon supports the addition of command handlers. So we will add an OEM Extension command for I2C transfer which will perform an I2C transfer and return the results.
With this piece, in theory a host tool's I2C I/O calls could be converted to the equivalent IPMI calls. However, in some cases, the host tool would be advised to instantiate an I2C device and use a higher level API. As a simple example, we should probably use the associated Linux driver, rather than raw I2C I/O, when reprogramming an at24c64d.
So we propose to create a proxy I2C adapter for each I2C bus we need to reach on the BMD. This avoids the need to convert raw host I2C, and enables regular Linux I2C device drivers at the host to connect to actual devices on the BMC.
NetFn 0x2e OemGroup 49871 Cmd 2 (openBmcI2cOemCmd)
| byte(s) | type | ID | Description |
|---|---|---|---|
| 0 | byte | bus | i2c adapater number at BMC |
| 1 | ReqFlags | reqFlags | Flags for request |
| bit 7 - PEC flag for M_RECV_LEN transfer | |||
| bits 6:0 - zero -reserved for future use | |||
| 2+ | Step | - | One per struct i2c_msg in transfer. |
| byte(s) | type | ID | Description |
|---|---|---|---|
| 0 | byte | devAndDir | bit 0: 1 if step is read, 0 if write |
| bits 7:1 i2c device address | |||
| 1 | StepFlags | stepFlags | Flags fro each step |
| bit 7: carries M_RECV_LEN bit - read steps only | |||
| bit 6: reserved for M_NOSTART | |||
| 2 | byte | parm | Transfer length (except M_RECV_LEN) |
| 3-p+2 | byte | wr_data | Write steps only: parm bytes of data to write. |
NetFn 0x2e OemGroup 49871 Cmd 2 (openBmcI2cOemCmd) cc Completion code Data Only if successful: all bytes read, in order received.
i2c-1 0x50 FRU EEPROM
Content, trailing zero bytes omitted.
# i2cdump -f -y 1 0x50 b
0 1 2 3 4 5 6 7 8 9 a b c d e f 0123456789abcdef
00: 01 00 00 01 00 00 00 fe 01 0b 19 83 6a 99 c6 51 ?..?...?????j??Q
10: 75 61 6e 74 61 d7 4d 65 6d 6f 72 79 20 52 69 73 uanta?Memory Ris
20: 65 72 20 44 44 52 34 20 42 6f 61 72 64 cf 51 54 er DDR4 Board?QT
30: 46 34 4b 31 31 35 30 37 30 30 32 33 38 cb 33 37 F4K1150700238?37
40: 53 34 4c 52 42 30 30 32 30 c9 46 52 55 20 76 30 S4LRB0020?FRU v0
50: 2e 30 31 c3 41 33 47 01 04 c1 00 00 00 00 00 99 .01?A3G???.....?
Read `Quanta' from FRU at bus=1/addr=0x50/offset=15 at BMC.
# ipmitool -I dbus raw 0x2e 2 0x79 0x2b 0x00 1 0 0xa0 0 1 15 0xa1 0 6
79 2b 00 51 75 61 6e 74 61
Read `Quanta' from FRU at bus=1/addr=0x50/offset=15 at host
# ipmitool raw 0x2e 2 0x79 0x2b 0x00 1 0 0xa0 0 1 15 0xa1 0 6
79 2b 00 51 75 61 6e 74 61
Key to bytes
netfn 0x2e for OEM extension
cmd 2 for I2C via IPMI
oem iana number (0x79 0x2b 0x00 for OpenBmc; Google OEN also works)
req hdr (bmc bus, request flags)
write step bytes
* devAndDir 0xa0 => device addr 0x50, dir = write
* flags 0
* parm 1 => send 1 data byte (data bytes follow step header)
* data byte 15 selects register 15
* read step bytes (devAndDir, flags, parm=6 => read 6 bytes)
Hopefully you can see how to modify these commands in simple ways:
More advanced ideas
Platform device attributes at /sys/devices/platform/i2c-via-ipmi/${name}