A/B Partition Strategies

# HPC eMMC partition layout for A/B OTA
storage:
  device: /dev/mmcblk0
  total_size_gb: 64

partitions:
  - name: boot_a
    size_mb: 256
    content: kernel + device tree (partition A)
    mountpoint: /boot_a
  - name: system_a
    size_mb: 8192
    content: root filesystem (OS + middleware) partition A
    filesystem: ext4 (read-only when active)
    mountpoint: /system
  - name: boot_b
    size_mb: 256
    content: kernel + device tree (partition B)
    mountpoint: /boot_b
  - name: system_b
    size_mb: 8192
    content: root filesystem (OS + middleware) partition B
    filesystem: ext4
    mountpoint: /system_b  # mounted only during OTA write
  - name: userdata
    size_mb: 32768
    content: apps, maps, user preferences, logs
    filesystem: ext4 (read-write)
    note: NOT updated by A/B OTA; persists across updates
  - name: factory
    size_mb: 4096
    content: factory recovery image
    note: read-only; used for factory reset only
  - name: metadata
    size_mb: 64
    content: boot flag, update status, anti-rollback counter
    filesystem: raw (no filesystem; direct structure write)

/* Simplified A/B bootloader partition selection */
#include "partition_metadata.h"
#include "secure_boot.h"

typedef struct {
    uint32_t magic;          /* 0xAB2024AB */
    uint8_t  active_slot;    /* 0=A, 1=B */
    uint8_t  boot_attempt;   /* retry counter */
    uint8_t  max_retries;    /* 3: after 3 failures, revert */
    uint8_t  health_ok;      /* set by OS after validation */
    uint32_t version_a;      /* anti-rollback counter */
    uint32_t version_b;
    uint32_t crc32;          /* metadata integrity */
} BootMetadata_t;

uint8_t bootloader_select_partition(BootMetadata_t *meta) {
    /* Check if last boot was health-validated */
    if (meta->health_ok == 0 && meta->boot_attempt > 0) {
        /* Previous boot did not confirm health -- count attempts */
        meta->boot_attempt++;
        if (meta->boot_attempt > meta->max_retries) {
            /* Too many failures: revert to other slot */
            meta->active_slot ^= 1;  /* toggle A<->B */
            meta->boot_attempt = 0;
            meta->health_ok = 0;
        }
    } else {
        meta->boot_attempt = 1;
        meta->health_ok = 0;  /* cleared until OS sets it */
    }

    /* Verify signature of selected partition before booting */
    if (!secure_boot_verify_partition(meta->active_slot)) {
        /* Signature invalid: try other slot */
        meta->active_slot ^= 1;
        if (!secure_boot_verify_partition(meta->active_slot)) {
            bootloader_panic("Both partitions failed signature check");
        }
    }

    return meta->active_slot;
}