OpenTitan Darjeeling is a system-on-a-chip Secure Execution Environment, capable of serving as a root of trust (RoT) for measurement and attestation among other applications, for instantiation within a larger system. It can serve as the SoC root of trust, a platform root of trust, or even be integrated and leveraged for individual chiplet RoTs.
Darjeeling’s block diagram shows the system configuration, including the RISC-V Ibex processor and all of the memories and comportable peripheral IPs. The system is split into a high speed domain (e.g. 1 GHz clock in a recent process node) and a peripheral domain (e.g. 250 MHz). The system has support for light sleep only, since the entire root-of-trust is expected to be always-on in an integrated context.
The SoC integration wrapper contains shared infrastructure that can be adapted based on the integrator’s needs. It contains a control network (CTN) crossbar for attaching shared SoC-level peripherals, as well as a large, shared CTN SRAM.
Communication with the SoC is mainly via the mailboxes, DMA and SoC proxy module. The SoC proxy module serves as a comportable IP frontend for incoming IRQs, reset requests, wake up requests, alerts and the TL-UL egress port into the CTN network. Egress TL-UL requests are filtered by address checking and binary address translation (BAT) logic that provides flexibility and isolation in the CTN space. Code can be executed from both internal memories (ROM partitions 1 and 2, main SRAM) and CTN SRAM.
Debug access is established via the JTAG TAP attached to a debug TL-UL crossbar. Through that, the debug module, life cycle controller, and a JTAG mailbox can be accessed. The JTAG mailbox can be used to implement firmware-driven SoC-level debug authorization. Infrastructure signals such as clocks, resets and the entropy source are provided by the analog sensor top (AST) block, which is connected to the Darjeeling-internal power, clock and reset manager blocks. The sensor control block provides a comportable IP front-end for the AST block that the Ibex processor can interact with.
The following table provides a more detailed summary of the supported features:
|OpenTitan Darjeeling Features|
The Darjeeling configuration derived from the OpenTitan’s discrete “Earl Grey” has been extended to meet the requirements for an SoC-integrated RoT. The main processing elements and cryptographic features are significantly similar, while several unneeded IO peripherals in an integrated context have been removed. A set of new IP blocks have been developed to enable integration into a larger SoC. These blocks are highlighted with a blue in the block diagram, and include:
- An extended key manager block with support for TCG’s DICE Protection Environment (DPE)
- A DMA controller facilitating data exchange between the OpenTitan IP and the SoC
- A mailbox IP with TL-UL bus interface and configurable shared memory region
- A SoC proxy module that serves as a comportable fronted for external interrupts, alerts and the like
Certain functions and blocks remain under active development. Contributions are welcome, but this design is provided as-is.
- DV and and SoC wrapper infrastructure is being developed to enable the debug crossbar.
- The binary translation logic and the SoC DFx control module are under development. The former will allow for granular filtering and remapping of TL-UL requests, while the latter will provide SoC level gating functionality for DFT and debug infrastructure.
- Crossbar generation support at the SoC-integration is not yet mature, so some TL-UL connections have not been made at this point.
Since Darjeeling was derived from the discrete “Earl Grey” chip, several legacy structures are temporarily present.
- The pinmux and padring will eventually be removed.
- The always-on / non-always-on power and clock domain split from “Earl Grey” will eventually be removed. This is evident in the current clock and reset tree organization.
- The JTAG TAP has not been fully unified yet (i.e., the life cycle controller and debug module still have two separate TAPs).