mmc-mailbox

The MMC mailbox is a data interface between the DMMC-STAMP and the AMC payload (typically a Linux system running on a FPGA/SoC).

It is implemented as a dual-port RAM on the DMMC-STAMP CPLD which emulates an at24-like I²C interface.

Scope

This repository implements the userspace tools on the Linux side to access the mailbox, as well as a daemon to conduct an orderly Linux shutdown when a user pulls the hot swap handle of the AMC.

Mailbox contents

The DMMC-STAMP provides following information on the mailbox:

• MMC information (SW version, HW revision, slot number, etc)
• MMC sensors (names & readings)
• For all FRUs:
  • Status (present, compatible, powered, error)
  • FRU information (vendor, product, serial number, etc) if available
  • Temperature sensors (if available)

It also implements FPGA/SoC status and control flags that are primarily used to implement an orderly Linux shutdown when the MMC powers down the payload.

There is a 256 byte area reserved for application-specific usage.

See memory map for details.

Locking

To avoid race conditions, the MMC mailbox uses double-buffering. The uppermost byte has a "lock" flag preventing the STAMP from switching the page. This lock flag is transparently handled at driver level, as soon as more than one byte is read or written.

Block diagram

On the Linux software side, there are following components:

• I²C Adapter Driver: This can be any I²C adapter driver, but in order for the shutdown signaling to work, it needs to implement master_xfer_atomic(). See i2c-xiic-atomic on GitHub for a patched version of the Xilinx i2c-xiic driver.
• mmc-mailbox-driver: This is a I²C peripheral driver which is selected from the device tree with compatible = "desy,mmcmailbox".
• libmmcmb: This is a user-space library implementing high-level access to the mailbox data structures.
• mmcinfo: This is a console application to show MMC mailbox information in plain text.
• mmcctrld: This is a daemon polling the FPGA control flags, triggering a Linux system shutdown as soon as the shutdown request flag is set.

Linux system shutdown

This sequence diagram illustrates how the MMC mailbox is used to conduct the Linux shutdown:

sequenceDiagram
    participant M as MCH
    participant S as STAMP
    participant D as mmcctrld
    participant R as mmc-mailbox-driver
    participant L as Linux system

    Note over M: Hotplug event or shutdown command
    M ->> S: Send QUIESCE command
    S ->> D: Set fpga_ctrl.req_shutdown
    D ->> L: Run shutdown -h now
    L ->> L: Shutdown system
    L ->> R: Call pm_power_off callback
    R ->> S: Set fpga_status.app_shutdown_finished
    S ->> S: Perform board-specific power-down sequence
    S ->> M: Send QUIESCED event
    Note over M: Disable 12V

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Example device tree configuration

This example .dts code sets up a DMMC-STAMP mailbox at I²C address 0x2a, connected to a Xilinx I2C interface named iic_axi_iic_mmc:

&iic_axi_iic_mmc {
  mmcmailbox@2a {
      compatible = "desy,mmcmailbox";
      reg = <0x2a>;
  };
};