embed(esp): use esptool to convert elf to img

[luoliwoshang]

Tinygo's convert logic is only pick some segment,will loss lot's segment at LLGo, we have owner segment layout(some because our linkscript not same with tinygo)

https://github.com/tinygo-org/tinygo/blob/db9f1182f5f2a64ea496752899626578d2b313a7/builder/esp.go#L44-L47

at #1435, because the convert logic,not convert the init_array to bin,so the libc init have lot's problem

[luoliwoshang]

@xgopilot Analyze this question

[xgopilot]

Analysis of Issue #1565: Use esptool for ELF to Image Conversion

• Gather context about the issue
• Understand the current ESP binary conversion implementation
• Analyze the TinyGo approach and its limitations
• Research esptool as an alternative solution
• Provide analysis and recommendations

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Problem Summary

The current LLGo ESP firmware conversion (internal/firmware/esp.go) is ported from TinyGo and only extracts SHT_PROGBITS sections, missing critical sections like:

• SHT_INIT_ARRAY (.init_array) - Constructor function pointers
• SHT_FINI_ARRAY (.fini_array) - Destructor function pointers
• SHT_PREINIT_ARRAY (.preinit_array) - Pre-initialization arrays

This causes libc initialization failures when constructors in .init_array (like board_init()) aren't loaded.

Current Implementation Analysis

File: internal/firmware/esp.go:39-42

for _, section := range inf.Sections {
    if section.Type != elf.SHT_PROGBITS || section.Size == 0 || section.Flags&elf.SHF_ALLOC == 0 {
        continue  // SKIPS init_array, fini_array, etc.
    }
    // ...
}

This filtering logic explicitly skips any section that isn't SHT_PROGBITS, which is the root cause.

Analysis of PR #1435 Workaround

PR #1435 implemented a linker script workaround by merging .init_array into .rodata in targets/esp32-riscv.app.elf.ld. While this works for ESP32-C3, it:

1. Only addresses ESP32-C3 (RISC-V), not ESP32 (Xtensa)
2. Requires maintaining custom linker scripts
3. Is a workaround, not a proper fix

Proposed Solution: Use esptool

Option 1: Invoke esptool externally (Recommended)

Replace the Go-based conversion with a call to esptool's elf2image:

esptool.py --chip esp32c3 elf2image --flash-mode dio --flash-size 2MB input.elf -o output.bin

Pros:

• Uses ESP's official tool with proper segment handling
• Explicitly supports SEC_TYPE_INITARRAY, SEC_TYPE_FINIARRAY, SEC_TYPE_PREINITARRAY
• Has --use-segments option to use ELF program headers instead of sections
• Automatically maintained by Espressif
• Handles all ESP chip variants correctly

Cons:

• Adds Python/esptool as a runtime dependency
• Slightly more complex build process

Option 2: Extend the current implementation

Modify makeESPFirmareImage() to include additional section types:

// Include PROGBITS, INIT_ARRAY, FINI_ARRAY, PREINIT_ARRAY
validTypes := map[elf.SectionType]bool{
    elf.SHT_PROGBITS:       true,
    elf.SHT_INIT_ARRAY:     true,
    elf.SHT_FINI_ARRAY:     true,
    elf.SHT_PREINIT_ARRAY:  true,
}

for _, section := range inf.Sections {
    if !validTypes[section.Type] || section.Size == 0 || section.Flags&elf.SHF_ALLOC == 0 {
        continue
    }
    // ...
}

Pros:

• No additional dependencies
• Minimal code change
• Keeps existing infrastructure

Cons:

• May miss other edge cases that esptool handles
• Requires ongoing maintenance to match ESP's evolving format

Recommendation

I recommend Option 1 (esptool) for the following reasons:

1. Correctness: esptool is the reference implementation maintained by Espressif
2. Completeness: Handles all section types and chip-specific quirks
3. Future-proof: Updates automatically with new chip support
4. Already available: ESP development typically requires esptool anyway

For development environments where esptool isn't available, Option 2 can serve as a fallback.

Implementation Outline

1. Add esptool detection in the build process
2. Modify internal/firmware/esp.go to call esptool when available
3. Keep the current Go implementation as fallback
4. Update documentation about esptool dependency for ESP targets

Related References

• Current implementation: internal/firmware/esp.go
• PR fix(esp32c3): use newlib startup and merge init_array to rodata for println support #1435 workaround: Merged .init_array into .rodata via linker script
• ESP32-C3 linker script: targets/esp32-riscv.app.elf.ld
• ESP32 linker script: targets/esp32.app.elf.ld
• esptool docs: https://docs.espressif.com/projects/esptool

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Analysis complete. Let me know if you'd like me to implement either solution!

[luoliwoshang]

This plan will process when esp32's test ready