Cortex R5 Debug

Cortex-R5 Debug

Overview

Use this skill to debug Cortex-R5 (ARMv7-R, PMSA) systems by separating core mode, boot source, memory map, exception vectors, TCM, MPU/cache policy, interrupt controller, and safety configuration. Cortex-R5 uses an MPU (not an MMU) and CP15-based control; failures usually come from memory attributes, ECC/TCM setup, or lockstep configuration rather than C code.

When To Use

Use this skill when:

• The user is bringing up or debugging Cortex-R5/R5F, Cortex-R4, or Cortex-R7 firmware (Xilinx Zynq UltraScale+ RPU, TI Hercules/Sitara, and similar).
• The issue involves early boot, Undefined/Prefetch Abort/Data Abort exceptions, IRQ/FIQ, ATCM/BTCM, MPU regions, caches, ECC/parity, lockstep, split mode, or JTAG attach.
• The platform is an SoC with real-time cores, safety islands, motor/control firmware, storage controllers, or heterogeneous Linux + R5 systems.

Do not use this skill for Cortex-M microcontrollers. Use cortex-m-debug when the target has NVIC, VTOR, and M-profile exception behavior instead of CP15/CPSR.

First Questions

Ask for:

• SoC/board, exact core, boot owner, toolchain, debugger, and whether the R5 runs bare metal or an RTOS.
• Core mode: single core, split (performance) mode, or lock-step (DCLS); which core the debugger is attached to.
• Memory map: boot ROM, flash, DDR, OCM/SRAM, ATCM/BTCM base and size, stacks, heaps, and linker script.
• MPU/cache policy, TCM enable state (INITRAMA/INITRAMB reset straps), ECC init, and DMA/coherency paths.
• Exception symptom plus a dump of CPSR/SPSR, DFSR/DFAR (data abort) or IFSR/IFAR (prefetch abort), LR, and disassembly around PC/LR.

Debug Workflow

1. Prove debugger attach and reset control. Confirm JTAG target and core selection, reset type, and halt behavior. Read MIDR and MPIDR to confirm you are on the expected R5 core, and check whether the sibling R5 or an A-core changes state.

2. Verify boot entry. Check the reset vector and vector base. Vectors sit at 0x00000000 or at 0xFFFF0000 when SCTLR.V (bit 13) is set. Verify per-mode banked stacks are set for SVC, IRQ, FIQ, ABT, UND, and SYS before C runtime init.

3. Stabilize memory first. Enable and scrub ATCM/BTCM before use — set the base+enable in the ATCM/BTCM Region Registers (MRC/MCR p15, 0, Rt, c9, c1, 0 for ATCM, c9, c1, 1 for BTCM; bit 0 is Enable). Initialize ECC-protected RAM (write to establish valid ECC) before any read. Init BSS/data and stacks before touching DDR or cached regions.

4. Configure MPU/cache deliberately. Read MPUIR (MRC p15, 0, Rt, c0, c0, 4) for region count. Per region: select with RGNR (c6, c2, 0), set DRBAR (c6, c1, 0), DRSR (c6, c1, 2, size+enable), and DRACR (c6, c1, 4, AP/TEX/S/C/B/XN). Mark peripherals as Device/Strongly-ordered with XN, TCM/SRAM/DDR as Normal. Enable via SCTLR.M (bit 0); enable caches with SCTLR.C (bit 2) and SCTLR.I (bit 12). Use DSB/ISB after CP15 writes.

5. Decode exceptions. Data Abort: read DFSR (c5, c0, 0) and DFAR (c6, c0, 0). Prefetch Abort: read IFSR (c5, c0, 1) and IFAR (c6, c0, 2). ECC/parity events also surface in ADFSR/AIFSR (c5, c1, 0 / c5, c1, 1). Decode the DFSR status field (short-descriptor FS = bit 10 + bits [3:0]): 0b00001 alignment, 0b00000 background (no MPU region), 0b01101 permission, 0b01000 synchronous external abort, 0b10110 asynchronous external abort, 0b11001/0b11000 synchronous/asynchronous parity/ECC. Check WnR (bit 11) for write vs read.

6. Bring up interrupts separately. Validate routing before load. For a GIC: GICD_CTLR, GICD_ISENABLERn, GICD_IPRIORITYRn, GICD_ITARGETSRn, then CPU interface GICC_CTLR, GICC_PMR, and the GICC_IAR/GICC_EOIR acknowledge/EOI pair. For a PL192-style VIC or TI VIM, validate the vector table and enable/priority registers. Confirm the FIQ/IRQ mode has its own stack and that CPSR.I/CPSR.F/CPSR.A masks are cleared as intended.

7. Add multicore/safety features last. Prove single-core execution before enabling lock-step comparison, split-mode messaging, watchdogs, or safety monitors. Note lock-step vs split is a hardware/reset configuration (e.g., DCLS, INITRAM* straps), not a runtime software toggle.

Common Failures

• Linker script places vectors, stacks, or .data in ATCM/BTCM or DDR not yet enabled/initialized by the boot stage.
• ATCM/BTCM assumed enabled: the region Enable bit (c9, c1, x bit 0) is clear, or INITRAMA/INITRAMB reset base does not match the linker map.
• ECC-protected RAM read before being written, producing a synchronous parity/ECC Data Abort (DFSR = 0b11001) with the address in DFAR.
• Peripheral registers left in a Normal/cacheable MPU region instead of Device with XN, causing reordering or speculative-access aborts.
• DMA buffers cached without DCCMVAC/DCIMVAC/DCCIMVAC maintenance plus DSB around ownership handoff.
• Missing banked IRQ/FIQ stack, so the first interrupt corrupts memory once CPSR.I/CPSR.F is cleared.
• MPU enabled (SCTLR.M) with no DRSR-enabled region and background region off (SCTLR.BR, bit 17), so every access takes a background fault (DFSR = 0b00000).
• Debugger attached to the wrong core, or expecting two cores while the group is in lock-step (only one core is visible).

Verification

Before claiming Cortex-R5 firmware works:

• State SoC, core, mode (single/split/lock-step), boot chain, memory map, and debugger path.
• Confirm reset-to-main with vectors (SCTLR.V/base), per-mode stacks, TCM enabled, and C runtime initialized.
• Confirm MPU regions (DRBAR/DRSR/DRACR) and cache state (SCTLR.M/.C/.I) for code, data, Device/peripheral, DMA, and shared memory.
• Confirm at least one timer/interrupt path (GIC/VIC ack + EOI) and one exception dump path (DFSR/DFAR or IFSR/IFAR).
• Confirm watchdog, lock-step/split, and inter-core communication behavior if used.

Example

User:

Cortex-R5 一开 D-cache 就随机 data abort。

Agent:

1. Reads DFSR/DFAR at the abort to classify the fault, e.g. via OpenOCD after halt:

   arm mrc 15 0 5 0 0   ;# DFSR  (p15, op1=0, CRn=c5, CRm=c0, op2=0)
   arm mrc 15 0 6 0 0   ;# DFAR  (p15, op1=0, CRn=c6, CRm=c0, op2=0)
   

   (Xilinx XSDB equivalent: select the RPU core with targets, then rrd for CPSR/banked regs and mrd for the faulting address.)

2. Checks whether the faulting DFAR lands in a peripheral or shared buffer marked Normal/cacheable, and whether the status is permission (0b01101), external (0b01000), or parity/ECC (0b11001).

3. Fixes the MPU attributes (DRACR: Device + XN for peripherals) and adds DCCIMVAC/DSB cache maintenance around DMA/shared memory before changing application logic.