Pi Auto-Discovery: TFTP fallback for unknown serial numbers

[mrveiss]

Overview

Enable automatic discovery of Raspberry Pi devices without pre-registration. Currently, Pi network boot fails if the serial number directory doesn't exist in TFTP.

Problem

1. Pi powers on, requests /<serial>/start4.elf via TFTP
2. If /<serial>/ directory doesn't exist, TFTP returns "file not found"
3. Pi fails to boot - no discovery possible

Proposed Solution

Option A: TFTP Fallback Directory (Recommended)

Create a pi-discovery/ directory that serves as fallback for unknown serials:

/srv/tftp/
├── pi-nodes/
│   ├── d83add36/     # Known Pi (registered)
│   └── a1b2c3d4/     # Known Pi
└── pi-discovery/     # Fallback for unknown
    ├── bootcode.bin
    ├── start4.elf
    ├── start.elf
    ├── fixup*.dat
    ├── *.dtb
    ├── kernel8.img
    ├── initramfs.img
    ├── config.txt    # Generic discovery config
    └── cmdline.txt   # pureboot.mode=discover

Modify TFTP server to:

1. Check if /<serial>/ exists
2. If not, serve files from pi-discovery/
3. Pi boots, runs deploy environment, calls API to register

Option B: Dynamic Directory Creation

TFTP server creates directory on-the-fly when unknown serial requests files:

1. Unknown serial requests /<serial>/start4.elf
2. TFTP server creates /<serial>/ with default files
3. Serves the requested file
4. Pi boots and registers via API

Implementation Tasks

• Create pi-discovery/ directory structure
• Add TFTP fallback logic for unknown serials
• Create generic config.txt for discovery mode
• Create cmdline.txt with pureboot.mode=discover
• Update deploy environment to handle discover mode
• Auto-register Pi via API on first boot
• Create node directory after successful registration
• Add setting to enable/disable auto-discovery
• Document setup process

Discovery Boot Flow

Pi powers on
    ↓
DHCP provides TFTP IP (192.168.168.66)
    ↓
Pi requests /<serial>/start4.elf
    ↓
TFTP checks: /<serial>/ exists?
    ├── Yes → Serve from node directory
    └── No → Serve from pi-discovery/
    ↓
Pi boots kernel + initramfs
    ↓
Deploy env runs, detects pureboot.mode=discover
    ↓
Calls POST /api/v1/nodes/register-pi
    ↓
PureBoot creates /<serial>/ directory
    ↓
Node appears in UI as "discovered"

Test Hardware Available

• Raspberry Pi 3B+
• Raspberry Pi 4

Related

• Issue Raspberry Pi Network Boot #6 - Raspberry Pi Network Boot (closed)
• Commit b9c154a - Pi 3 support with bootcode.bin

Acceptance Criteria

• Unknown Pi can network boot without pre-registration
• Pi appears in PureBoot UI after first boot
• Existing registered nodes continue to work
• Works with Pi 3B+, Pi 4, Pi 5
• Auto-discovery can be disabled via settings