CS 452/652 Spring 2025 - Lecture 9

Events, Clock, Idle

June 3, 2025 prev next

BCM - Broadcom BCM2711 data sheet
GIC - ARM Generic Interrupt Controller

Interrupt Controller

• connects the device signals to FIQ/IRQ at each core
  • interrupt propagation: device → controller → CPU
• BCM 2711 has:
  • GIC-400 (GIC v2)
  • Legacy Interrupt Controller - not used here
  • see BCM Page 85
• functions of the controller:
  • assigns ID to each input signal
  • config interrupt from each signal
    • enable/disable
    • edge-triggered, level-sensitive
  • route input signals to outputs
  • prioritize interrupts
  • manage state of each interrupt
• GIC-400 split into parts
  • Distributor (CICD)
  • CPU Interface (GICC)
  • GICD and GICC each have control registers that kernel can use to control interrupts
    • GIC_BASE = 0xFF840000 (offset 0x1840000 from 0xFE000000)
    • GICD_BASE = GIC_BASE + 0x1000
    • GICC_BASE = GIC_BASE + 0x2000
  • Control Register Maps:
    • GICD: see GIC, Table 4.1, page 4-75
    • GICC: see GIC, Table 4.2, page 4-76

• Interrupt State managed by the GIC

  • see GIC, Section 3.2.4

  • GIC tracks interrupt state along two dimensions

    • Pending/Not Pending
      • level-sensitive: is the device asserting?
      • edge-triggered: has the device asserted?

    • Active/Not Active - is the CPU handling the interrupt

      	Not Pending	Pending
      Not Active	Inactive	Pending
      Active	Active	Active+Pending

  • Inactive → Pending when device asserts interrupt signal

    • level-sensitive: pending as long as device signal is asserted
    • edge-triggered: pending until interrupt is handled
  • Pending → Active+Pending
    • for level-sensitive, when CPU reads GICC_IAR to find cause of interrupt
  • Pending → Active
    • for edge-triggered, when CPU reads GICC_IAR to find cause of interrupt
  • remove Active state
    • when handler writes to GICC_EOIR to notify GICC of end of processing

System at Boot Time

• GIC distribution and signalling enabled (GICD_CTLR, GICC_CTLR)
• all interrupts disabled at GIC
• interrupt routing ?
• interrupts masked in EL1 (via boot.S)

System Timer Interrupts

• Initialization
  • Timer has 4 interrupt signals, corresponding to C0-C3
    • C0 and C2 reserved by VC, so use C1 and/or C3
    • BCM Chap 6 says system timer interrupts are first 4 VC (video core) interrupts
    • Interrupt IDs for video core start at 96, so
      • C1 interrupt has InterruptID 97
      • C3 interrupt has InterruptID 99
  • route the interrupt to IRQ on CPU 0
    • use GICD_ITARGETSRn
      • each register defines targets for 4 interrupts
      • 1024 interrupts, 8 bits = 8 cores
  • make sure IRQ handler is set up in your kernel's exception vector
  • enable the interrupt
    • use GICD_ISENABLERn
      • 4-byte registers, with 1 bit per InterruptID
      • 1024 interrupts, 1 bit
    • use GICD_ICENABLERn to disable interrupt
  • write values into system timer C1/C3 to trigger interrupt assertion at correct time
  • might have to clear interrupt via CS status register (see below)
  • enable IRQ handling in CPU via pstate

Timer Interrupt Handling

• save running task state
• read GICC_IAR
  • returns interruptID (should be 97/99)!
  • sets interrupt state to Active in GIC
• if system timer interrupt
  • update Cn compare register - overflow?
  • clear interrupt via CS status register
  • write InterruptID to GICC_EOIR
    • marks interrupt as not active in GIC
• perform interrupt handling work
  • unblock task(s) waiting for event
• loop: read GICC_IAR again?
• schedule next task
• resume task
• note: system timer interrupts cannot be explicitly disabled at device

AwaitEvent

• synchronization: user tasks ↔ device
• AwaitEvent() argument and return code → design decision
  • eventIds are defined by the kernel, known to the application
    • example: clock tick
  • return code - event-specific interpretation
    • example: subtype, count, data
  • micro-kernel: put as much functionality as possible in user-level tasks
• AwaitEvent() blocks calling task
• matchmaking task ↔ event
  • 0, 1, N tasks waiting? store in queue (cf. SRR)?
  • N tasks wait for event? complexity? unblock one or all?
  • 0 tasks wait for interrupt? buffer in kernel?
    • multiple same interrupts without waiting task indicate performance problem
    • use task prioritity to make sure that hardware interrupts are handled promptly
• intention of AwaitEvent() is to expose device interrupts to the application
  • program periodic interrupt in kernel
  • can be decoupled: 1ms timer interrupt vs. 10ms clock tick

Clock Server

• API:
  Time(int tid) - current time (in ticks)
  Delay(tid, ticks) - delay, relative to current time
  DelayUntil(tid, ticks) - delay, to absolute time
  • DelayUntil = Time + Delay, but at server
  • convenience for real-time loop
• these are Send() wrappers; tid is tid of a Clock server
• Time measured in "ticks"

Server Implementaion

• AwaitEvent() might block; server pattern?
  • if server calls AwaitEvent(), it cannot Receive() new requests (e.g., Time())
• delegate event blocking to special task: Notifier pattern

Clock Notifier

    for (;;) {
        AwaitEvent();
        Send(); // send tick to clock server
    }

Clock Server

    for (;;) {
        Receive();
        process(); // manage tasks
        Reply(); // if applicable
    }

• needs to manage list of suspended tasks

Idle/Halt

• what to do when processor has no other work?

  • SETI@home? crypto-mining? → often not appropriate!
  • maintenance, e.g., garbage collection → not relevant for CS 452 kernel
  • diagnosis, such as computation of idle fraction
  • save energy → low-power halt! ("race-to-idle")
    • conserve battery and/or save planet

  • implement as dedicated idle task at lowest priority

    for (;;) {
        while (maintenance()) yield();
        park();
    }
    

  • ARMv8 instruction wfi (or wfe?)

  • alternative: dynamic frequency scaling with dynamic voltage scaling (not used here)

• how to measure idle time?

• idle time shoud be high: >> 90%

K3 overview

• Note: system with 1ms clock tick is fine as well