Interrupt Controller Technical Specification

Overview

This document specifies the Interrupt Controller (RV_PLIC) functionality. This module conforms to the Comportable guideline for peripheral functionality. See that document for integration overview within the broader top level system.

Features

• RISC-V Platform-Level Interrupt Controller (PLIC) compliant interrupt controller
• Support arbitrary number of interrupt vectors (up to 255) and targets
• Support interrupt enable, interrupt status registers
• Memory-mapped MSIP register per HART for software interrupt control.

Description

The RV_PLIC module is designed to manage various interrupt sources from the peripherals. It receives interrupt events as either edge or level of the incoming interrupt signals (intr_src_i) and can notify multiple targets.

Compatibility

The RV_PLIC is compatible with any RISC-V core implementing the RISC-V privilege specification.

Theory of Operations

Block Diagram

Hardware Interfaces

• Interface Tables

Design Details

Identifier

Each interrupt source has a unique ID assigned based upon its bit position within the input intr_src_i. ID ranges from 0 to N, the number of interrupt sources. ID 0 is reserved and represents no interrupt. The bit 0 of intr_src_i shall be tied to 0 from the outside of RV_PLIC. The intr_src_i[i] bit has an ID of i. This ID is used when targets “claim” the interrupt and to “complete” the interrupt event.

Priority and Threshold

Interrupt sources have configurable priority values. The maximum value of the priority is configurable through the localparam MAX_PRIO in the rv_plic top-level module. For each target there is a threshold value (THRESHOLD0 for target 0). RV_PLIC notifies a target of an interrupt only if it‘s priority is strictly greater than the target’s threshold. Note this means an interrupt with a priority is 0 is effectively prevented from causing an interrupt at any target and a target can suppress all interrupts by setting it's threshold to the max priority value.

MAX_PRIO parameter is most area contributing option in RV_PLIC. If MAX_PRIO is big, then finding the highest priority in Process module may consume a lot of logic gates.

Interrupt Gateways

The Gateway observes incoming interrupt sources and converts them to a common interrupt format used internally by RV_PLIC. It can be parameterized to detect interrupts events on an edge (when the signal changes from 0 to 1) or level basis (where the signal remains at 1). The choice is a system-integration decision and can be configured via the design parameter LevelEdgeTrig for each interrupt request.

When the gateway detects an interrupt event it raises the interrupt pending bit (IP) for that interrupt source. When an interrupt is claimed by a target the relevant bit of IP is cleared. A bit in IP will not be reasserted until the target signals completion of the interrupt. Any new interrupt event between a bit in IP asserting and completing that interrupt is ignored. In particular this means that for edge triggered interrupts if a new edge is seen after the source's IP bit is asserted but before completion, that edge will be ignored (counting missed edges as discussed in the RISC-V PLIC specification is not supported).

Note that there is no ability for a level triggered interrupt to be cancelled. If the interrupt drops after the gateway has set a bit in IP, the bit will remain set until the interrupt is completed. The SW handler should be conscious of this and check the interrupt still requires handling in the handler if this behaviour is possible.

Interrupt Enables

Each target has a set of Interrupt Enable (IE0 for target 0) registers. Each bit in the IE0 registers controls the corresponding interrupt source. If an interrupt source is disabled for a target, then interrupt events from that source won‘t trigger an interrupt at the target. RV_PLIC doesn’t have a global interrupt disable feature.

Interrupt Claims

“Claiming” an interrupt is done by a target reading the associated Claim/Completion register for the target (CC0 for target 0). The return value of the CC0 read represents the ID of the pending interrupt that has the highest priority. If two or more pending interrupts have the same priority, RV_PLIC chooses the one with lowest ID. Only interrupts that that are enabled for the target can be claimed. The target priority threshold doesn't matter (this only factors into whether an interrupt is signalled to the target) so lower priority interrupt IDs can be returned on a read from CC0. If no interrupt is pending (or all pending interrupts are disabled for the target) a read of CC0 returns an ID of 0.

Interrupt Completion

After an interrupt is claimed, the relevant bit of interrupt pending (IP) is cleared, regardless of the status of the intr_src_i input value. Until a target “completes” the interrupt, it won't be re-asserted if a new event for the interrupt occurs. A target completes the interrupt by writing the ID of the interrupt to the Claim/Complete register (CC0 for target 0). The write event is forwarded to the Gateway logic, which resets the interrupt status to accept a new interrupt event. The assumption is that the processor has cleaned up the originating interrupt event during the time between claim and complete such that intr_src_i[ID] will have de-asserted (unless a new interrupt has occurred).

{ signal: [
  { name: 'clk',           wave: 'p...........' },
  { name: 'intr_src_i[i]', wave: '01....0.1...', node:'.a....e.f...'},
  { name: 'irq_o',         wave: '0.1.0......1', node:'..b.d......h'},
  { name: 'irq_id_o',      wave: '=.=.=......=',
                           data: ["0","i","0","i"] },
  { name: 'claim',         wave: '0..10.......', node:'...c........'},
  { name: 'complete',      wave: '0.........10', node:'..........g.'},
  ],
  head:{
    text: 'Interrupt Flow',
    tick: 0,
  },
}

In the example above an interrupt for source ID i is configured as a level interrupt and is raised at a, this results in the target being notified of the interrupt at b. The target claims the interrupt at c (reading i from it‘s Claim/Complete register) so irq_o deasserts though intr_src_i[i] remains raised. The SW handles the interrupt and it drops at e. However a new interrupt quickly occurs at f. As complete hasn’t been signaled yet irq_o isn‘t asserted. At g the interrupt is completed (by writing i to it’s Claim/Complete register) so at h irq_o is asserted due to the new interrupt.

Programmers Guide

Initialization

After reset, RV_PLIC doesn't generate any interrupts to any targets even if interrupt sources are set, as all priorities and thresholds are 0 by default and all IE values are 0. Software should configure the above three registers.

PRIO0 .. PRIO31 registers are unique. So, only one of the targets shall configure them.

// Pseudo-code below
void plic_init() {
  // Configure priority
  // Note that PRIO0 register doesn't affect as intr_src_i[0] is tied to 0.
  for (int i = 0; i < N_SOURCE; ++i) {
    *(PRIO + i) = value(i);
  }
}

void plic_threshold(tid, threshold) {
  *(THRESHOLD + tid) = threshold;
}

void plic_enable(tid, iid) {
  // iid: 0-based ID
  int offset = ceil(N_SOURCE / 32) * tid + (iid >> 5);

  *(IE + offset) = *(IE + offset) | (1 << (iid % 32));
}

Handling Interrupt Request Events

If software receives an interrupt request, it is recommended to follow the steps shown below (assuming target 0 which uses CC0 for claim/complete).

1. Claim the interrupts right after entering to the interrupt service routine by reading the CC0 register.
2. Determine which interrupt should be serviced based on the values read from the CC0 register.
3. Execute ISR, clearing the originating peripheral interrupt.
4. Write Interrupt ID to CC0
5. Repeat as necessary for other pending interrupts.

It is possible to have multiple interrupt events claimed. If software claims one interrupt request, then the process module advertises any pending interrupts with lower priority unless new higher priority interrupt events occur. If a higher interrupt event occurs after previous interrupt is claimed, the RV_PLIC IP advertises the higher priority interrupt. Software may utilize an event manager inside a loop so that interrupt claiming and completion can be separated.

void interrupt_service() {
  uint32_t tid = /* ... */;
  uint32_t iid = *(CC + tid);
  if (iid == 0) {
    // Interrupt is claimed by one of other targets.
    return;
  }

  do {
    // Process interrupts...
    // ...

    // Finish.
    *(CC + tid) = iid;
    iid = *(CC + tid);
  } while (iid != 0);
}

As a reference, default interrupt service routines are auto-generated for each IP, and are documented here.

Device Interface Functions (DIFs)

• Device Interface Functions

Registers

The RV_PLIC in the top level is generated by topgen tool so that the number of interrupt sources may be different.

• IE: CEILING(N_SOURCE / DW) X N_TARGET Each bit enables corresponding interrupt source. Each target has IE set.
• PRIO: N_SOURCE Universal set across all targets. Lower n bits are valid. n is determined by MAX_PRIO parameter
• THRESHOLD: N_TARGET Priority threshold per target. Only priority of the interrupt greater than threshold can raise interrupt notification to the target.
• IP: CEILING(N_SOURCE / DW) Pending bits right after the gateways. Read-only
• CC: N_TARGET Claim by read, complete by write

• Register Table