Management Component Transport Protocol (MCTP) LPC Transport Binding Specification for ASPEED BMC Systems

Scope

This design provides an efficient method to transfer MCTP packets between the host and BMC over the LPC bus on ASPEED BMC platforms.

References

The following referenced documents are indispensable for the application of this document.

1. DMTF DSP0236, Management Component Transport Protocol (MCTP) Base Specification 1.0,
   http://www.dmtf.org/standards/published_documents/DSP0236_1.0.pdf

2. Intel (R) Low Pin Count (LPC) Interface Specification 1.1,
   https://www.intel.com/content/dam/www/program/design/us/en/documents/low-pin-count-interface-specification.pdf

3. IPMI Consortium, Intelligent Platform Management Interface Specification, v1.5 Revision 1.1 February 20, 2002,
   http://download.intel.com/design/servers/ipmi/IPMIv1_5rev1_1.pdf

Definitions

BTU: Baseline Transmission Unit

Defined by the MCTP base specification as the smallest maximum packet size all MCTP-compliant endpoints must accept.

IBF: Input Buffer Full

A hardware-defined flag bit in a KCS device's Status Register (STR). The IBF flag indicates that a value has been written by the host to the corresponding Input Data Register (IDR).

IDR: Input Data Register

One of the three register interfaces exposed by a KCS device. The IDR is a one byte buffer which is written by the host and read by the BMC.

KCS: Keyboard-Controller-Style

A set of bit definitions and operation of the registers typically used in keyboard microcontrollers and embedded controllers. The term "Keyboard Controller Style" reflects that the register definition was originally used as the legacy "8742" keyboard controller interface in PC architecture computer systems. This interface is available built-in to several commercially available microcontrollers. Data is transferred across the KCS interface using a per-byte handshake.

LPC Bus: Low Pin Count Bus

A bus specification that implements ISA bus in a reduced physical form while extending ISA's capabilities.

LPC FW: LPC Firmware Cycles

LPC firmware cycles allow separate boot BIOS firmware memory cycles and application memory cycles with respect to the LPC bus. The ASPEED BMCs allow remapping of the LPC firmware cycles onto arbitrary regions of the BMC's physical address space, including RAM.

MTU: Maximum Transmission Unit

The largest payload the link will accept for a packet. The Maximum Transmission Unit represents a value that is at least as large as the BTU. Negotiation of MTU values larger than the BTU may improve throughput for data-intensive transfers.

OBF: Output Buffer Full

A hardware-defined flag bit in a KCS device's Status Register (STR). The OBF flag indicates that a value has been written by the BMC to the corresponding Output Data Register (ODR).

ODR: Output Data Register

One of the three register interfaces exposed by a KCS device. The ODR is a one byte buffer which is written by the BMC and read by the host.

STR: Status Register

One of the three register interfaces exposed by a KCS device. STR is a BMC-controlled, eight-bit register exposed to both the BMC and the host for indication of IBF and OBF events on the input (IDR) and output (ODR) buffers. Bits that are not defined by hardware can be software-controlled in a manner defined by a platform-specific ABI.

Conventions

Where unspecified, state, command and sequence descriptions apply to all versions of the protocol unless marked otherwise.

MCTP over LPC Transport

Concepts

The basic components used for the transfer are:

• An interrupt mechanism using the IPMI KCS interface
• A window of the LPC FW address space, where reads and writes are forwarded to BMC memory, using the LPC2AHB hardware

In order to transfer a packet, either side of the channel (BMC or host) will:

1. Write the packet to the LPC FW window
   • The BMC will perform writes by writing to the memory backing the LPC window
   • The host will perform writes by writing to the LPC bus, at predefined addresses
2. Trigger an interrupt on the remote side, by writing to the KCS data buffer

On this indication, the remote side will:

1. Read from the KCS status register, which shows that the single-byte KCS data buffer is full
2. Read the provided command from the KCS data buffer, acknowledging the interrupt
3. Read the MCTP packet from the LPC FW window

Scope

The document limits itself to describing the operation of the binding protocol. The following issues of protocol ABI are considered out of scope:

1. The LPC IO address and Serial IRQ parameters of the KCS device
2. The concrete location of the control region in the LPC FW address space

KCS Interface

The KCS hardware on the ASPEED BMCs is used as a method of indicating, to the remote side, that a packet is ready to be transferred through the LPC FW mapping.

The KCS hardware consists of two single-byte buffers: the Output Data Register (ODR) and the Input Data Register (IDR). The ODR is written by the BMC and read by the host. The IDR is the obverse.

The KCS unit also contains a status register (STR), allowing both host and BMC to determine if there is data in the ODR or IDR. These are single-bit flags, designated Input/Output Buffer Full (IBF/OBF), and are automatically set by hardware when data has been written to the corresponding ODR/IDR buffer (and cleared when data has been read).

While the IBF and OBF flags are managed in hardware, the remaining software-defined bits in the status register are used to carry other required protocol state. A problematic feature of the KCS status register is described in the IPMI specification, which states that an interrupt may be triggered on writes to the KCS status register but hardware implementations are not required to do so. Comparatively, writes to the data registers must set the corresponding buffer-full flag and invoke an interrupt.

To ensure interrupts are generated for status updates, we exploit the OBF interrupt to signal a status update by writing a dummy command to ODR after updating the status register, as outlined below.

LPC FW Window

The window of BMC-memory-backed LPC FW address space has a predefined format, consisting of:

• A control descriptor, describing static data about the rest of the window
• A receive area for BMC-to-host packets
• A transmit area, for host-to-BMC packets

The control descriptor contains a version, and offset and size data for the transmit and receive areas. These offsets are relative to the start of the LPC FW window.

Full definition of the control area is defined below, and it will be the base for all future versions.

struct mctp_lpcmap_hdr {
   uint32_t magic;

   uint16_t bmc_ver_min;
   uint16_t bmc_ver_cur;
   uint16_t host_ver_min;
   uint16_t host_ver_cur;
   uint16_t negotiated_ver;
   uint16_t pad0;

   uint32_t rx_offset;
   uint32_t rx_size;
   uint32_t tx_offset;
   uint32_t tx_size;
} __attribute__((packed));

The magic value marking the beginning of the control area is the ASCII encoding of "MCTP":

#define LPC_MAGIC 0x4d435450

All medium-specific metadata is in big-endian format. This includes:

1. Control area data
2. Medium-specific packet header fields
3. Medium-specific packet trailer fields

MCTP packet data is transferred exactly as is presented, and no data escaping is performed.

In all versions of the protocol, the transmit and receive areas contain a medium-specific header comprising a 32-bit payload length field, followed immediately by the MCTP packet data to be transferred. The full MCTP packet, including MCTP header, is considered to be the payload for the purpose of the header's length field.

struct mctp_lpcbuf_hdr {
   uint32_t length;
} __attribute__((packed));

A medium-specific packet trailer must immediately follow the payload. The length of the trailer is not accounted for in the length field of the medium-specific packet header: The length of the trailer is implied by the negotiated protocol version.

For protocol versions 1 and 2, the medium-specific trailer length is zero.

For protocol version 3, the medium-specific trailer comprises a CRC-32 checksum of the payload.

struct mctp_lpcbuf_tlr {
   uint32_t crc32;
} __attribute__((packed));

Where the CRC-32 implementation is defined by the following characteristics (or equivalent):

1. The polynomial x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
2. Initialising the remainder state to 2^32 - 1
3. Incrementally shifting and XORing data bytes through the reversed polynomial representation 0xEDB88320
4. XORing the calculated remainder with 2^32 - 1

For all defined versions, only a single MCTP packet is present in the Rx and Tx areas. This may change for future versions of the protocol.

Negotiation of the Maximum Transmission Unit

Version 1 of the protocol offers no mechanism for negotiation of the maximum transmission unit. The Rx and Tx buffers must be sized to accommodate packets up to the Baseline Transmission Unit, and the implementation assumes that the MTU is set to the BTU regardless of the values of rx_size and tx_size.

Version 2 of the protocol exploits the rx_size and tx_size fields in the control region to negotiate the link MTU. Note that at the time that the MTU is under negotiation the protocol version has not been finalised, so the process is necessarily backwards-compatible.

The relevant property that each endpoint must control is the MTU of packets it will receive, as this governs how the remote endpoint's packetisation impacts memory pressure at the local endpoint. As such while the BMC MUST populate rx_size for backwards compatibility with version 1, the host MAY write rx_size without regard for its current value if the host supports version 2. The BMC controls the value of tx_size, and MAY choose to adjust it in response to the host's proposed rx_size value. As such, when Channel Active is set by the BMC, the host MUST read both rx_size and tx_size in response to ensure both the BMC and the host have a consistent understanding of the MTU in each direction. It is convention for rx_size and tx_size to be set to the same value by the BMC as part of finalising the channel, though it is not invalid to have asymmetric MTUs.

For all protocol versions, the following properties must be upheld for the Rx and Tx buffers to be considered valid:

• Intersect neither eachother nor the control region
• Not extend beyond the window allocated to MCTP in the LPC FW address space
• Must accommodate at least BTU-sized payloads

The BMC MAY choose to fail channel initialisation if these properties are violated in the negotiation process.

KCS Status and Control Sequences

The KCS status flags and command set govern the state of the protocol, defining the ability to send and receive packets on the LPC bus.

KCS Status Register Layout

Bit	Managed By	Description
7 (MSB)	Software	BMC Active
6	Software	Channel active, version negotiated
5	Software	Unused
4	Software	Unused
3	Hardware	Command / Data
2	Software	Unused
1	Hardware	Input Buffer Full
0 (LSB)	Hardware	Output Buffer Full

KCS Data Register Commands

Command	Description
0x00	Initialise
0x01	Tx Begin
0x02	Rx Complete
0xff	Dummy Value

Host Command to BMC Sequence

The host sends commands to the BMC to signal channel initialisation, begin transmission of a packet, or to complete reception of a packet.

Step	Description
1	The host writes a command value to IDR
2	The hardware sets IBF, which triggers a BMC interrupt
3	The BMC reads the status register for IBF
4	If IBF is set, the BMC reads the host command from IDR
5	The interrupt is acknowledged by the data register read

BMC Command to Host Sequence

The BMC sends commands to the host to begin transmission of a packet or to complete reception of a packet.

Step	Description
1	The BMC writes a command value to ODR
2	The hardware sets OBF, which triggers a host interrupt
3	The host reads the status register for OBF
4	If OBF is set, the host reads the BMC command from ODR
5	The interrupt is acknowledged by the data register read

BMC Status Update Sequence

The BMC sends status updates to the host to signal loss of function, loss of channel state, or the presence of a command in the KCS data register.

Step	Description
1	The BMC writes the status value to the status register
2	The BMC writes the dummy command to ODR
3	The hardware sets OBF, which triggers a host interrupt
4	If OBF is set, the host reads the BMC command from ODR
5	The interrupt is acknowledged by the data register read
6	The host observes the command is the dummy command
7	The host reads the status register to capture the state change

LPC Window Ownership and Synchronisation

Because the LPC FW window is shared between the host and the BMC we need strict rules on which entity is allowed to access it at specific times.

Firstly, we have rules for modification:

• The control data is only written during initialisation. The control area is never modified once the channel is active.
• Only the BMC may write to the Rx buffer described in the control area
• Only the host may write to the Tx buffer described in the control area

During packet transmission, the follow sequence occurs:

1. The Tx side writes the packet to its Tx buffer
2. The Tx side sends a Tx Begin message, indicating that the buffer ownership is transferred
3. The Rx side now owns the buffer, and reads the message from its Rx area
4. The Rx side sends a Rx Complete once done, indicating that the buffer ownership is transferred back to the Tx side.

LPC Binding Operation

The binding operation is not symmetric as the BMC is the only side that can drive the status register. Each side's initialisation sequence is outlined below.

The sequences below contain steps where the BMC updates the channel status and where commands are sent between the BMC and the host. The act of updating status or sending a command invokes the behaviour outlined in KCS Control.

The packet transmission sequences assume that BMC Active and Channel Active are set.

BMC Initialisation Sequence

Step	Description
1	The BMC initialises the control area: magic value, BMC versions and buffer parameters
2	The BMC sets the status to BMC Active

Host Initialisation Sequence

Step	v1	v2	v3	Description
1	✓	✓	✓	The host waits for the BMC Active state
2	✓	✓	✓	The host populates the its version fields
3		✓	✓	The host derives and writes to rx_size the packet size associated with its desired MTU
4	✓	✓	✓	The host sends the Initialise command
5	✓	✓	✓	The BMC observes the Initialise command
6	✓	✓	✓	The BMC calculates and writes negotiated_ver
7		✓	✓	The BMC calculates the MTUs and updates neither, one or both of rx_size and tx_size
8	✓	✓	✓	The BMC sets the status to Channel Active
9	✓	✓	✓	The host observes that Channel Active is set
10	✓	✓	✓	The host reads the negotiated version
11		✓	✓	The host reads both rx_size and tx_size to derive the negotiated MTUs

Host Packet Transmission Sequence

Step	v1	v2	v3	Description
1			✓	The host calculates the CRC-32 over the packet data
2	✓	✓	✓	The host waits on any previous Rx Complete message
3	✓	✓	✓	The host writes the packet data and medium-specific metadata to its Tx area (BMC Rx area)
4	✓	✓	✓	The host sends the Tx Begin command, transferring ownership of its Tx buffer to the BMC
5	✓	✓	✓	The BMC observes the Tx Begin command
6	✓	✓	✓	The BMC reads the packet data and medium-specific metadata from the its Rx area (host Tx area)
7	✓	✓	✓	The BMC sends the Rx Complete command, transferring ownership of its Rx buffer to the host
8	✓	✓	✓	The host observes the Rx Complete command
9			✓	The BMC validates the provided CRC-32 over the packet data

BMC Packet Transmission Sequence

Step	v1	v2	v3	Description
1			✓	The BMC calculates the CRC-32 over the packet data
2	✓	✓	✓	The BMC waits on any previous Rx Complete message
3	✓	✓	✓	The BMC writes the packet data and medium-specific metadata to its Tx area (host Rx area)
4	✓	✓	✓	The BMC sends the Tx Begin command, transferring ownership of its Tx buffer to the host
5	✓	✓	✓	The host observes the Tx Begin command
6	✓	✓	✓	The host reads the packet data and medium-specific metadata from the host Rx area (BMC Tx area)
7	✓	✓	✓	The host sends the Rx Complete command, transferring ownership of its Rx buffer to the BMC
8	✓	✓	✓	The BMC observes the Rx Complete command
9			✓	The host validates the provided CRC-32 over the packet data

Implementation Notes

On the BMC the initial prototype implementation makes use of the following components:

• An LPC KCS device exposed by a binding-specific kernel driver
• The reserved memory mapped by the LPC2AHB bridge via the aspeed-lpc-ctrl driver
• The astlpc binding found in libmctp

From the host side, the LPC Firmware and KCS IO cycles are driven by free-standing firmware. Some firmwares exploit libmctp by implementing the driver hooks for direct access to the LPC devices.

Alternatives Considered

The KCS MCTP Binding (DSP0254)

The KCS hardware (used as the full transfer channel) can be used to transfer arbitrarily-sized MCTP messages. However, there are much larger overheads in synchronisation between host and BMC for every byte transferred.

The MCTP Serial Binding (DSP0253)

We could use the VUART hardware to transfer the MCTP packets according to the existing MCTP Serial Binding. However, the VUART device is already used for console data. Multiplexing both MCTP and console would be an alternative, but the complexity introduced would make low-level debugging both more difficult and less reliable.

The BT interface

The BT interface allows for block-at-time transfers. However, the BT buffer size is only 64 bytes on the AST2500 hardware, which does not allow us to comply with the MCTP Base Specification (DSP0236) that requires a 64-byte payload size as the minimum. The 64-byte BT buffer does not allow for MCTP and transport headers.

Additionally, we would like to develop the MCTP channel alongside the existing IPMI interfaces, to allow a gradual transition from IPMI to MCTP. As the BT channel is already used on OpenPOWER systems for IPMI transfers, we would not be able to support both in parallel.

Using the AST2500 LPC Mailbox

This would require enabling the SuperIO interface, which allows the host to access the entire BMC address space, and so introduces security vulnerabilities.