interfaces.md

Hardware Interfaces

Referring to the Comportable guideline for peripheral device functionality, the module rom_ctrl has the following hardware interfaces defined

• Primary Clock: clk_i
• Other Clocks: none
• Bus Device Interfaces (TL-UL): regs_tl, rom_tl
• Bus Host Interfaces (TL-UL): none
• Peripheral Pins for Chip IO: none
• Interrupts: none

Inter-Module Signals

Port Name	Package::Struct	Type	Act	Width	Description
rom_cfg	prim_rom_pkg::rom_cfg	uni	rcv	1
pwrmgr_data	rom_ctrl_pkg::pwrmgr_data	uni	req	1
keymgr_data	rom_ctrl_pkg::keymgr_data	uni	req	1
kmac_data	kmac_pkg::app	req_rsp	req	1
regs_tl	tlul_pkg::tl	req_rsp	rsp	1
rom_tl	tlul_pkg::tl	req_rsp	rsp	1

Security Alerts

Alert Name	Description
fatal	A fatal error. Fatal alerts are non-recoverable and will be asserted until a hard reset.

Security Countermeasures

Countermeasure ID	Description
ROM_CTRL.CHECKER.CTR.CONSISTENCY	Once rom_ctrl has handed control of the mux to the bus, the internal FSM counter should point at the top of ROM (where we ensure the word has invalid ECC bits). The unexpected_counter_change signal in rom_ctrl_fsm goes high and generates a fatal alert if that counter is perturbed in any way.
ROM_CTRL.CHECKER.CTRL_FLOW.CONSISTENCY	The main checker FSM steps on internal 'done' signals, coming from its address counter, the KMAC response and its comparison counter. If any of these are asserted at times we don't expect, the FSM jumps to an invalid state. This triggers an alert and will not set the external 'done' signal for pwrmgr to continue boot.
ROM_CTRL.CHECKER.FSM.LOCAL_ESC	The main checker FSM moves to an invalid state on local escalation.
ROM_CTRL.COMPARE.CTRL_FLOW.CONSISTENCY	The hash comparison module triggers a fatal error if the checker FSM triggers a second comparison after a reset. This is handled by the start_alert signal in the rom_ctrl_compare module and could be triggered if the checker FSM was somehow glitched to jump backwards.
ROM_CTRL.COMPARE.CTR.CONSISTENCY	The hash comparison module has an internal count (indexing 32-bit words in the 256-bit digests). If this glitches to a nonzero value before the comparison starts or to a value other than the last index after the comparison ends then an fatal alert is generated. This is handled by the wait_addr_alert and done_addr_alert signals in rom_ctrl_compare.
ROM_CTRL.COMPARE.CTR.REDUN	The hash comparison module has an internal count (indexing 32-bit words in the 256-bit digests) implemented using a redundant counter module. In case a mismatch is detected between the redundant counters a fatal alert is generated.
ROM_CTRL.FSM.SPARSE	FSMs are sparsely encoded. There are two FSMs. The first is in rom_ctrl_fsm. The second, simpler FSM is in rom_ctrl_compare.
ROM_CTRL.MEM.SCRAMBLE	The ROM is scrambled.
ROM_CTRL.MEM.DIGEST	A cSHAKE digest is computed of the ROM contents.
ROM_CTRL.INTERSIG.MUBI	Checker FSM 'done' signal is multi-bit encoded when passed to pwrmgr. This signal is derived from the (multi-bit) sparse FSM state in the rom_ctrl_fsm module.
ROM_CTRL.BUS.INTEGRITY	TL bus control and data signals are integrity protected (using the system-wide end-to-end integrity scheme).
ROM_CTRL.BUS.LOCAL_ESC	To avoid responding to a request with erroneous data, even though an alert went out, the bus_rom_rvalid signal used to signal a response to the ROM-side TL bus can only be high if no internal consistency error has been spotted.
ROM_CTRL.MUX.MUBI	The mux that arbitrates between the checker and the bus is multi-bit encoded. An invalid value generates a fatal alert with the sel_invalid signal in the rom_ctrl_mux module.
ROM_CTRL.MUX.CONSISTENCY	The mux that arbitrates between the checker and the bus gives access to the checker at the start of time and then switches to the bus, never going back. If a glitch does cause it to switch back, a fatal alert is generated with the sel_reverted or sel_q_reverted signals in the rom_ctrl_mux module.
ROM_CTRL.CTRL.REDUN	Addresses from TL accesses are passed redundantly to the scrambled ROM module, to ensure the address lines are not independently faultable downstream of the bus integrity ECC check. See the bus_rom_prince_index and bus_rom_rom_index signals in the rom_ctrl module.
ROM_CTRL.CTRL.MEM.INTEGRITY	End-to-end data/memory integrity scheme.
ROM_CTRL.TLUL_FIFO.CTR.REDUN	The TL-UL response FIFO pointers are implemented with duplicate counters.

Parameters

Parameter	Default (Max)	Top Earlgrey	Description
RndCnstRomKey	(see RTL)	(see RTL)	Compile-time random default constant for scrambling key (used in prim_prince block).
RndCnstRomNonce	(see RTL)	(see RTL)	Compile-time random default constant for scrambling nonce (used in prim_prince block and the two S&P blocks).

Signals

The table below lists other ROM controller inter-module signals.

Signal	Type	Destination	Description
pwrmgr_data_o	rom_ctrl_pkg::pwrmgr_data_t	pwrmgr	A structure with two fields. The first, done, becomes true when the ROM check is complete and remains true until reset. The second, good, is only valid if done is true. This is true if the digest computation matched the expected value stored in the top words of ROM and false otherwise. This field stays constant when done is true.
keymgr_data_o	rom_ctrl_pkg::keymgr_data_t	keymgr	A 256-bit digest, together with a valid signal. Once the ROM check is complete, valid will become true and will then remain true until reset. The digest in data is only valid when valid is true and is be constant until reset.
kmac_data_o	kmac_pkg::app_req_t	kmac	Outgoing data to KMAC. Data is sent in 64-bit words in the data field. When a word of data is available, the valid field is true. When this is the last word of data, the last field is also true. Since we never send partial words, the strb field is always zero.
kmac_data_i	kmac_pkg::app_rsp_t	kmac	Incoming data from KMAC interface. This contains a ready signal for passing ROM data and a done signal that shows a digest has been computed. When done is true, the digest is exposed in two shares (digest_share0 and digest_share1). The error field is ignored.