AVR-GCC中如何在.init3段可靠调用C函数？

Step 1: Move Validation Logic to a Normal C Function #

Remove the naked attribute from your validation function and place it in a custom section to ensure it’s flashed early. This function handles the magic number check and failure behavior without compiler compatibility issues.

#include <avr/io.h>
#include <stdint.h>

// Place this function in a custom section to prioritize it in .text
__attribute__((section(".app_early")))
void app_valid_check(void) {
    // Pre-build generated magic numbers (adjust names to match your setup)
    extern const uint32_t TEXT_START_MAGIC;
    extern const uint32_t TEXT_END_MAGIC;
    const uint32_t EXPECTED_MAGIC = 0x12345678; // Replace with your magic value

    // Check firmware integrity
    if (TEXT_START_MAGIC != EXPECTED_MAGIC || TEXT_END_MAGIC != EXPECTED_MAGIC) {
        // Configure LED pin (adjust port/pin to match your hardware)
        DDRB |= (1 << PB0);
        // Configure UART (adjust baud rate/settings for your device)
        UCSR0B |= (1 << TXEN0);
        UBRR0 = 103; // Example for 9600 baud at 16MHz

        // Infinite loop: blink LED and send 0xAA via UART
        while (1) {
            PORTB ^= (1 << PB0); // Toggle LED
            // Simple delay (adjust count for ~500ms blink)
            for (uint32_t i = 0; i < 150000; i++) {
                asm volatile("nop");
            }
            UDR0 = 0xAA; // Send failure signal
        }
    }
    // If validation passes, return to continue initialization
}

Step 2: Create a Naked Assembly Wrapper in .init3 #

Write a tiny naked function (pure assembly) in the .init3 section to call your validation function. This avoids compiler warnings about non-ASM code in naked C functions.

Option A: Inline C with Pure ASM #

// Declare the wrapper as naked and place it in .init3
__attribute__((naked, section(".init3")))
void _init3_validation_wrapper(void) {
    asm volatile (
        "call app_valid_check\n" // Call the C validation function
        "ret\n"                  // Return to continue init sequence
    );
}

Option B: Separate Assembly File (init3_wrapper.S) #

.section .init3,"ax",@progbits
.global _init3_validation_wrapper

_init3_validation_wrapper:
    call app_valid_check
    ret

Step 3: Update the Linker Script #

Modify your linker script to ensure the .app_early section (containing your validation function) is placed at the start of .text, so it’s flashed early within your 500ms window.

Add this to your linker script’s .text section definition:

SECTIONS {
    .text : {
        KEEP(*(.app_early))       /* Prioritize validation code first */
        *(.text)                  /* Rest of the firmware code */
        /* Add your TEXT_END_MAGIC placement here as per pre-build step */
    }
    /* Keep other existing sections (.data, .bss, etc.) */
}

Why This Works #

• Stack Safety: The .init3 section runs after the AVR runtime initializes the stack pointer, so calling a normal C function is safe (no stack corruption risks).
• No Compiler Warnings: The only naked code is pure assembly, which is fully supported by AVR-GCC.
• Early Execution: Placing app_valid_check in .app_early ensures it’s the first code in .text, so it’s flashed early during DFU.
• Failure Handling: If validation fails, the C function enters an infinite loop (never returning to the wrapper), so the device never reaches main(). If valid, the function returns, and the init sequence proceeds normally.

内容的提问来源于stack exchange，提问作者Tom MacDonald