This document describes OEM Requests to be supported using the OpenBMC OEM Number.
This table and the next briefly recap OEM Extension messages as described in the IPMI Specification. To keep it both simple and concrete, only BT Tansport is described. Please consult that specification for the full story.
Per section 11.1 of IPMI Spec
| Bytes | Bits | Spec ID | Value | Description |
|---|---|---|---|---|
| 0 | Length* | - | Number of bytes to follow in this request | |
| 1 | 2:7 | NetFn | 0x2E | OEM Request |
| 1 | 0:1 | LUN | - | Allow any LUN |
| 2 | Seq* | - | Per section 11.3 of IPMI Spec | |
| 3 | Cmd | - | Table 3 - OpenBMC Cmd Codes | |
| 4 ~ 6 | OEN | 49871 | OEM Enterprise Number | |
| 2 ~ n+6 | Data | - | n data bytes, encoding depends on Cmd |
Notes:
Length and Seq are specific to BT transport - other transports may not have them.
Serialize numbers larger than 1 byte LSB first - e.g., represent OEM Enterprise Number 49871 as {0xcf, 0xc2, 0x00}
Per section 11.2 of IPMI Spec
| Bytes | Bits | Spec ID | Value | Description |
|---|---|---|---|---|
| 0 | Length* | - | Number of bytes to follow in this response | |
| 1 | 2:7 | NetFn | 0x2F | OEM Response |
| 1 | 0:1 | LUN | - | LUN of request to which this is a response |
| 2 | Seq* | - | Seq of request to which this is a response | |
| 3 | Cmd | - | Cmd code of request to which this is a response | |
| 4 | CC | - | Completion code, Section 5.2 of IPMI Spec v2.0 | |
| 5 ~ 7 | OEN | 49871 | OEM Enterprise Number | |
| 8 ~ n+7 | Data | - | n data bytes, encoding depends on Cmd |
/* * This is the OpenBMC IANA OEM Number */ constexpr OemNumber obmcOemNumber = 49871;
These are the Command codes allocated for use with the OpenBMC OEM Number.
| Cmd | Identifier | Description |
|---|---|---|
| 0 | - | Reserved |
| 1 | gpioCmd | GPIO Access |
| 2 | i2cCmd | I2C Device Access |
| 3 | flashCmd | Flash Device Access |
| 4 | fanManualCmd | Manual Fan Controls |
| 5 ~ 255 | - | Unallocated |
The next subsections describe command and response messages supporting the I2C Device Access extension OpenBMC OEM extension (command code 2).
| Bytes | Bits | Identifier | Description |
|---|---|---|---|
| 0 | bus | Logical I2C bus. | |
| 1 | xferFlags | Flags for all steps. | |
| 7 | I2cFlagUsePec | 1 => use PEC - see note. | |
| 6:0 | Reserved(0) | ||
| 2 ~ n-1 | Step sequence - see next. |
Notes
Total length of step sequence must exactly fill request.
Intent is to handle Linux Kernel SMBus Protocol, with com generalized to m byte sequence - e.g., at24c64 uses 2 address bytes, and n bytes of received data, rather than specific operations for various sizes.
Goal is to support SMBus v2 32-byte data block length limit; but easily supports new 4 and 8 byte transfers added for SMBus v3.
PEC refers to SMBus Packet Error Check.
SMBus address resolution, alerts, and non-standard protocols not supported. So for example, there is no way to insert a stop command within a transfer.
Depending on options, i2cdetect uses either quick write or 1 byte read; default is 1-byte read for eeprom/spd memory ranges, else quick write.
| Bytes | Bits | Identifier | Description |
|---|---|---|---|
| 0 | devAndDir | ||
| 7:1 | dev | 7-bit I2C device address. | |
| 0 | isRead | 1 = read, 0 = write. | |
| 1 | stepFlags | ||
| 7 | i2cFlagRecvLen | 1 if block read, else regular; see table. | |
| 6 | i2cFlagNoStart | 1 to suppress I2C start. | |
| 5:0 | Reserved(0) | ||
| 2 | p | Count parameter; see table | |
| 3:m+2 | wrPayload | Nonempty iff p supplies nonzero m; see table. |
| is_read | RecvLen | p | Size | Description |
|---|---|---|---|---|
| 0 | 0 | 0 | 3 | Quick write 0; same as write m=0 bytes. |
| 0 | 0 | m | m+3 | Consume and write next m bytes. |
| 0 | 1 | - | 3 | Reserved. |
| 1 | 0 | 0 | 3 | Quick write 1; same as read n=0 bytes. |
| 1 | 0 | n | 3 | Read n bytes. |
| 1 | 1 | - | 3 | Read count from device. |
Notes
All other combinations are reserved.
RecvLen corresponds to Linux I2C_M_RECV_LEN message flags bit
Transfers include byte count in SMBUS block modes, and PEC byte when selected. Allows for use with SMBUS compatibility layer.
In case of write steps, wrPayload may include:
| Bytes | Identifier | Size | Description |
|---|---|---|---|
| 0:n-1 | rd_data | 0 ~ 34 | byte sequence read |
Notes
Maximum logical payload is 32.
In the unlikely event a transfer specifies multiple read steps, all bytes read are simply concatenated in the order read.
However, maximum reply limit is sized for the largest single read.
RecvLen case w/ PEC can return up to 34 bytes: count + payload + PEC