bootloader/pureboot/pureboot.py

#!/usr/bin/env python3
"""pureboot host tool — the smart half of the pureboot protocol (README.md).

The device exposes primitives; this tool composes them: image loading (raw
binary or Intel HEX), flash programming with read-back verification, erase as
writing 0xff, EEPROM programming, fuse and info readout, the hand-over jump,
and — on chips without a hardware boot section — the reset-vector surgery
that re-homes the application's entry through the trampoline word below the
loader. Page 0 and the trampoline are written first, so every interruption
point of a flash leaves the chip reset-recoverable into the loader.

It also updates the loader itself (--update-loader): pureboot's image is
position-independent, so the tool installs the identical binary one 512-byte
slot below the resident loader, jumps into that staging copy, lets it rewrite
the resident slot, and restores what the staging slot held — resumable at
every phase from the flash state plus a host-side state file carrying the
saved bytes.

Python standard library only; the serial port is driven with termios on POSIX
and the Win32 serial API (through ctypes) on Windows, so any tty or COM port
works — a USB adapter as well as a simavr pty.
"""

import argparse
import json
import os
import sys
import time

if os.name == "nt":
    import ctypes
    from ctypes import wintypes
else:
    import select
    import termios

PROMPT = b"+"
PROTOCOL_VERSION = 1
SLOT = 512  # the loader slot size; also the self-update staging distance


class Error(Exception):
    pass


# ---------------------------------------------------------------- serial ---


class PosixPort:
    """A raw serial port with deadline-based reads, over termios."""

    def __init__(self, path, baud):
        self.fd = os.open(path, os.O_RDWR | os.O_NOCTTY)
        attrs = termios.tcgetattr(self.fd)
        attrs[0] = 0  # iflag
        attrs[1] = 0  # oflag
        attrs[2] = termios.CREAD | termios.CLOCAL | termios.CS8  # cflag
        attrs[3] = 0  # lflag
        try:
            speed = getattr(termios, f"B{baud}")
        except AttributeError:
            raise Error(f"unsupported baud rate {baud}") from None
        attrs[4] = attrs[5] = speed
        attrs[6][termios.VMIN] = 0
        attrs[6][termios.VTIME] = 0
        termios.tcsetattr(self.fd, termios.TCSANOW, attrs)

    def close(self):
        os.close(self.fd)

    def write(self, data):
        os.write(self.fd, data)

    def flush_input(self):
        termios.tcflush(self.fd, termios.TCIFLUSH)

    def read_available(self, wait):
        """Everything that arrives within `wait` seconds of quiet start."""
        ready, _, _ = select.select([self.fd], [], [], wait)
        return os.read(self.fd, 4096) if ready else b""

    def read_exact(self, count, timeout):
        data = b""
        deadline = time.monotonic() + timeout
        while len(data) < count:
            remaining = deadline - time.monotonic()
            if remaining <= 0:
                raise Error(f"timeout: got {len(data)} of {count} bytes")
            ready, _, _ = select.select([self.fd], [], [], remaining)
            if ready:
                data += os.read(self.fd, count - len(data))
        return data


if os.name == "nt":
    # The same port, over the Win32 serial API — kernel32 through ctypes, so
    # the tool stays standard-library only. Timeouts live in the driver
    # (COMMTIMEOUTS) rather than in a readiness call: Windows has no select()
    # for a COM handle, so each read asks the driver for its own deadline.

    _GENERIC_READ, _GENERIC_WRITE = 0x80000000, 0x40000000
    _OPEN_EXISTING, _PURGE_RXCLEAR = 3, 0x0008
    _INVALID_HANDLE = wintypes.HANDLE(-1).value
    # A gap this long ends a read_available(): longer than the coalescing a
    # USB-serial adapter's latency timer imposes (16 ms on FTDI parts), so a
    # burst is not split, short enough to stay responsive.
    _GAP_MS = 30

    class _DCB(ctypes.Structure):
        _fields_ = [
            ("DCBlength", wintypes.DWORD),
            ("BaudRate", wintypes.DWORD),
            ("fBits", wintypes.DWORD),  # the packed flag bitfield, set below
            ("wReserved", wintypes.WORD),
            ("XonLim", wintypes.WORD),
            ("XoffLim", wintypes.WORD),
            ("ByteSize", wintypes.BYTE),
            ("Parity", wintypes.BYTE),
            ("StopBits", wintypes.BYTE),
            ("XonChar", ctypes.c_char),
            ("XoffChar", ctypes.c_char),
            ("ErrorChar", ctypes.c_char),
            ("EofChar", ctypes.c_char),
            ("EvtChar", ctypes.c_char),
            ("wReserved1", wintypes.WORD),
        ]

    class _COMMTIMEOUTS(ctypes.Structure):
        _fields_ = [
            ("ReadIntervalTimeout", wintypes.DWORD),
            ("ReadTotalTimeoutMultiplier", wintypes.DWORD),
            ("ReadTotalTimeoutConstant", wintypes.DWORD),
            ("WriteTotalTimeoutMultiplier", wintypes.DWORD),
            ("WriteTotalTimeoutConstant", wintypes.DWORD),
        ]

    _k32 = ctypes.WinDLL("kernel32", use_last_error=True)
    _LPDWORD = ctypes.POINTER(wintypes.DWORD)
    # Declared, not inferred: a HANDLE is a pointer, and a defaulted int
    # return would truncate it on 64-bit.
    _k32.CreateFileW.restype = wintypes.HANDLE
    _k32.CreateFileW.argtypes = [wintypes.LPCWSTR, wintypes.DWORD, wintypes.DWORD,
                                 wintypes.LPVOID, wintypes.DWORD, wintypes.DWORD, wintypes.HANDLE]
    _k32.ReadFile.argtypes = [wintypes.HANDLE, wintypes.LPVOID, wintypes.DWORD, _LPDWORD, wintypes.LPVOID]
    _k32.WriteFile.argtypes = [wintypes.HANDLE, wintypes.LPCVOID, wintypes.DWORD, _LPDWORD, wintypes.LPVOID]
    _k32.GetCommState.argtypes = [wintypes.HANDLE, ctypes.POINTER(_DCB)]
    _k32.SetCommState.argtypes = [wintypes.HANDLE, ctypes.POINTER(_DCB)]
    _k32.SetCommTimeouts.argtypes = [wintypes.HANDLE, ctypes.POINTER(_COMMTIMEOUTS)]
    _k32.PurgeComm.argtypes = [wintypes.HANDLE, wintypes.DWORD]
    _k32.CloseHandle.argtypes = [wintypes.HANDLE]

    def _fail(what):
        code = ctypes.get_last_error()
        raise Error(f"{what}: {ctypes.FormatError(code).strip()} (Windows error {code})")

    class WindowsPort:
        """A raw serial port with deadline-based reads, over Win32."""

        def __init__(self, path, baud):
            # Win32 takes the rate as a plain integer, so unlike termios any
            # rate the hardware can divide down to is available — but a driver
            # may also accept one it cannot produce (an FT232R takes a baud of
            # 3, reports it back, and goes on using the previous divisor).
            # Only obvious nonsense is refusable; the rest is the driver's word.
            if baud < 50:
                raise Error(f"unsupported baud rate {baud}")
            # \\.\COM6: the device-namespace form. A bare COMn resolves only
            # for n < 10, and double-digit ports are routine on Windows.
            if path.lower().startswith("com") and path[3:].isdigit():
                path = rf"\\.\{path}"
            self.handle = _k32.CreateFileW(
                path, _GENERIC_READ | _GENERIC_WRITE, 0, None, _OPEN_EXISTING, 0, None
            )
            if self.handle == _INVALID_HANDLE:
                _fail(f"cannot open {path}")
            self.timeouts = None
            try:
                dcb = _DCB()
                dcb.DCBlength = ctypes.sizeof(_DCB)
                if not _k32.GetCommState(self.handle, ctypes.byref(dcb)):
                    _fail(f"cannot read the state of {path}")
                dcb.BaudRate, dcb.ByteSize, dcb.Parity, dcb.StopBits = baud, 8, 0, 0  # 8N1
                # fBinary, and DTR/RTS asserted (fDtrControl and fRtsControl,
                # two bits each, = _ENABLE); every other flag clear, so no
                # parity and no flow control. Raising both matches what opening
                # a POSIX tty does — including the reset pulse on the boards
                # that wire DTR to it.
                dcb.fBits = 0x1 | (1 << 4) | (1 << 12)
                if not _k32.SetCommState(self.handle, ctypes.byref(dcb)):
                    _fail(f"cannot configure {path} for {baud} baud 8N1")
                # Arm them once here too: reads re-arm per call, but the write
                # timeout would otherwise stay at the driver's default — which
                # may be "wait forever" — until the first read.
                self._deadline(_GAP_MS, 1000)
            except Error:
                # An open port outlives the exception otherwise, and a COM
                # handle is exclusive: the next attempt would meet its own
                # leftover as "Access is denied".
                self.close()
                raise

        def close(self):
            _k32.CloseHandle(self.handle)

        def _deadline(self, interval, total):
            """Arm the driver's read timeouts: `interval` ms of quiet ends a
            read once bytes have arrived, `total` ms ends it regardless."""
            if self.timeouts == (interval, total):
                return
            spec = _COMMTIMEOUTS()
            spec.ReadIntervalTimeout = interval
            spec.ReadTotalTimeoutConstant = total
            spec.WriteTotalTimeoutConstant = 5000
            if not _k32.SetCommTimeouts(self.handle, ctypes.byref(spec)):
                _fail("cannot set the port timeouts")
            self.timeouts = (interval, total)

        def _read(self, count):
            buffer = ctypes.create_string_buffer(count)
            got = wintypes.DWORD()
            if not _k32.ReadFile(self.handle, buffer, count, ctypes.byref(got), None):
                _fail("read failed")
            return buffer.raw[: got.value]

        def write(self, data):
            written = wintypes.DWORD()
            if not _k32.WriteFile(self.handle, data, len(data), ctypes.byref(written), None):
                _fail("write failed")
            if written.value != len(data):
                raise Error(f"short write: {written.value} of {len(data)} bytes")

        def flush_input(self):
            if not _k32.PurgeComm(self.handle, _PURGE_RXCLEAR):
                _fail("cannot flush the input buffer")

        def read_available(self, wait):
            """Everything that arrives within `wait` seconds of quiet start."""
            # A zero total means *no* timeout to the driver, so never round
            # down to it — the same trap on the deadline below.
            self._deadline(_GAP_MS, max(1, round(wait * 1000)))
            return self._read(4096)

        def read_exact(self, count, timeout):
            data = b""
            deadline = time.monotonic() + timeout
            while len(data) < count:
                remaining = deadline - time.monotonic()
                if remaining <= 0:
                    raise Error(f"timeout: got {len(data)} of {count} bytes")
                # No interval timeout here: only the count or the deadline
                # ends the read, so a gap mid-reply is simply waited out.
                self._deadline(0, max(1, round(remaining * 1000)))
                data += self._read(count - len(data))
            return data


Port = WindowsPort if os.name == "nt" else PosixPort


# -------------------------------------------------------------- protocol ---


class Info:
    """The 12-byte info block."""

    def __init__(self, raw):
        if len(raw) != 12 or raw[0:2] != b"PB":
            raise Error(f"bad info block: {raw.hex()}")
        if raw[2] != PROTOCOL_VERSION:
            raise Error(f"protocol version {raw[2]}, tool speaks {PROTOCOL_VERSION}")
        self.raw = bytes(raw)
        self.signature = raw[3:6]
        self.page = raw[6]
        self.base = raw[7] | (raw[8] << 8)
        self.eeprom_size = raw[9] | (raw[10] << 8)
        self.patch_vector = bool(raw[11] & 1)
        self.flash_size = self.base + SLOT
        self.stage = self.base - SLOT  # where a staging copy of the loader goes
        # The hand-over target, as the word address 'J' takes: the trampoline
        # below the loader (tinies), or word 0 (mega — the application's own
        # reset vector; BOOTRST re-vectors a reset into the loader instead).
        self.app_entry_word = (self.base - 2) // 2 if self.patch_vector else 0

    def describe(self):
        sig = " ".join(f"{b:02x}" for b in self.signature)
        vector = "host-patched reset vector" if self.patch_vector else "hardware boot section"
        return (
            f"signature {sig}, page {self.page} B, "
            f"app flash {self.base} B (loader at {self.base:#06x}), "
            f"EEPROM {self.eeprom_size} B, {vector}"
        )


class Loader:
    """A pureboot session. Between commands the loader has prompted `+` and
    awaits a command byte; every method restores that invariant — except
    jump(), after which the target must be knocked afresh."""

    def __init__(self, port):
        self.port = port
        self.info = None

    def connect(self, wait):
        """Knock until the activation window answers, then read the info
        block. Also converges when the loader already sits in its command
        loop: the knock bytes are ignored-or-executed there, and the drain
        absorbs whatever they produced."""
        self.port.flush_input()
        deadline = time.monotonic() + wait
        while True:
            self.port.write(b"pb")
            if PROMPT in self.port.read_available(0.4):
                break
            if time.monotonic() > deadline:
                raise Error("no answer — reset the device within its activation window")
        while self.port.read_available(0.3):
            pass
        self.port.write(b"b")
        self.info = Info(self.port.read_exact(12, 2.0))
        self._expect_prompt()
        return self.info

    def _expect_prompt(self, timeout=2.0):
        byte = self.port.read_exact(1, timeout)
        if byte != PROMPT:
            raise Error(f"expected prompt, got {byte.hex()}")

    def _command(self, tx, reply_len=0, timeout=2.0):
        self.port.write(tx)
        reply = self.port.read_exact(reply_len, timeout) if reply_len else b""
        self._expect_prompt(timeout)
        return reply

    def _stream_read(self, command, address, count):
        data = b""
        while count:
            chunk = min(count, 256)
            head = bytes((ord(command), address & 0xFF, address >> 8, chunk & 0xFF))
            data += self._command(head, chunk, 5.0)
            address += chunk
            count -= chunk
        return data

    def read_flash(self, address, count):
        return self._stream_read("R", address, count)

    def read_eeprom(self, address, count):
        return self._stream_read("r", address, count)

    def write_page(self, address, data):
        assert len(data) == self.info.page and address % self.info.page == 0
        head = bytes((ord("W"), address & 0xFF, address >> 8))
        self._command(head + data, 0, 2.0)

    def write_eeprom(self, address, data):
        offset = 0
        while offset < len(data):
            chunk = data[offset : offset + 256]
            head = bytes((ord("w"), address & 0xFF, address >> 8, len(chunk) & 0xFF))
            self.port.write(head)
            for byte in chunk:
                self.port.write(bytes((byte,)))
                self._expect_prompt()  # per-byte ack: the write has begun
            self._expect_prompt()  # the next command prompt
            address += len(chunk)
            offset += len(chunk)

    def read_fuses(self):
        return self._command(b"F", 4, 2.0)

    def jump(self, word_address):
        """'J': the device acks, then execution continues at the word
        address — a loader slot's base (whose copy must then be knocked
        afresh) or the application entry."""
        self.port.write(bytes((ord("J"), word_address & 0xFF, word_address >> 8)))
        self._expect_prompt()

    def enter_copy(self, byte_address, wait):
        """Jump into the loader copy at `byte_address` and knock it. Ending
        up in the copy addressed is guaranteed by construction: a jump to a
        slot base lands in that slot's entry stub."""
        self.jump(byte_address // 2)
        return self.connect(wait)

    def run_application(self):
        self.jump(self.info.app_entry_word)


# ---------------------------------------------------------------- images ---


def load_image(path):
    """Raw binary, or Intel HEX by extension (.hex/.ihx/.ihex)."""
    data = open(path, "rb").read()
    if not path.lower().endswith((".hex", ".ihx", ".ihex")):
        if not data:
            raise Error(f"{path}: empty image")
        return data
    memory = {}
    for number, line in enumerate(data.decode("ascii", "replace").splitlines(), 1):
        line = line.strip()
        if not line:
            continue
        if not line.startswith(":"):
            raise Error(f"{path}:{number}: not an Intel HEX record")
        record = bytes.fromhex(line[1:])
        if sum(record) & 0xFF:
            raise Error(f"{path}:{number}: checksum mismatch")
        count, address, kind = record[0], (record[1] << 8) | record[2], record[3]
        payload = record[4 : 4 + count]
        if kind == 0:
            for i, byte in enumerate(payload):
                memory[address + i] = byte
        elif kind == 1:
            break
        elif kind in (2, 4) and not any(payload):
            continue  # a zero base extends nothing
        elif kind in (3, 5):
            continue  # start address: irrelevant, reset is the entry
        else:
            raise Error(f"{path}:{number}: record type {kind} reaches beyond the 16-bit space")
    if not memory:
        raise Error(f"{path}: empty image")
    return bytes(memory.get(i, 0xFF) for i in range(max(memory) + 1))


# --------------------------------------------------------------- surgery ---


def rjmp_target(word_address, opcode, flash_words):
    return (word_address + 1 + (opcode & 0x0FFF)) % flash_words


def rjmp_to(word_address, destination, flash_words):
    return 0xC000 | ((destination - word_address - 1) % flash_words % 0x1000)


def plan_flash(image, info):
    """The pages to program, as {page_address: bytes}, already carrying the
    reset-vector surgery where the chip needs it."""
    page = info.page
    limit = info.base - (2 if info.patch_vector else 0)
    if len(image) > limit:
        raise Error(f"image is {len(image)} B, application flash ends at {limit}")
    final = bytearray(image) + bytearray([0xFF] * (-len(image) % page))

    if info.patch_vector:
        flash_words = info.flash_size // 2
        word0 = final[0] | (final[1] << 8)
        if word0 & 0xF000 != 0xC000:
            raise Error(
                "the image's reset vector is not an rjmp — pureboot's vector "
                "surgery cannot re-home it (crt-less entry at address 0?)"
            )
        entry = rjmp_target(0, word0, flash_words)
        if entry >= info.base // 2:
            raise Error(
                "the image's reset vector already targets the loader — this "
                "is a read-back of a patched image; flash the original"
            )
        trampoline_word = (info.base - 2) // 2
        patch = rjmp_to(0, info.base // 2, flash_words)
        final[0], final[1] = patch & 0xFF, patch >> 8
        trampoline_page = info.base - page
        if len(final) < trampoline_page + page:
            final += bytearray([0xFF] * (trampoline_page + page - len(final)))
        jump = rjmp_to(trampoline_word, entry, flash_words)
        final[info.base - 2], final[info.base - 1] = jump & 0xFF, jump >> 8

    pages = {a: bytes(final[a : a + page]) for a in range(0, len(final), page)}
    return pages


def covered(pages, info, skip_blank):
    """Pages in programming order; optionally dropping all-0xff pages (sound
    only over erased flash) — never a load-bearing one.

    With a patched vector (tinies), the patched page 0 goes first and the
    trampoline page second: from the first write on, a reset lands in the
    loader and the loader's own fall-through lands on the application entry,
    so every interruption point of the flash is recoverable. With a hardware
    boot section a reset re-vectors to the loader regardless; ascending
    order, page 0 last, maximizes what an interrupted image retains."""
    trampoline_page = info.base - info.page if info.patch_vector else None
    first = [0, trampoline_page] if info.patch_vector else []
    rest = [a for a in sorted(pages) if a not in first]
    if skip_blank:
        rest = [a for a in rest if pages[a].count(0xFF) != len(pages[a])]
    order = [a for a in first if a in pages] + rest
    if not info.patch_vector:
        order = [a for a in order if a != 0] + ([0] if 0 in pages else [])
    return order


# ----------------------------------------------------------------- fuses ---


def mega_boot(high_fuse):
    """Decode the ATmega328P high fuse's boot configuration (DS40002061B
    §27.3, Table 27-13/27-16): BOOTSZ1:0 in bits 2:1 select the boot-section
    words, BOOTRST in bit 0 (programmed = 0) re-vectors reset to its start.
    Returns (bootrst_programmed, boot_section_start_byte)."""
    bootsz = (high_fuse >> 1) & 0x03
    words = {0b11: 256, 0b10: 512, 0b01: 1024, 0b00: 2048}[bootsz]
    return (high_fuse & 1) == 0, 0x8000 - words * 2


# ---------------------------------------------------------- loader update ---


def image_info(image):
    """The info block embedded in a pureboot binary, or None."""
    at = image.find(b"PB" + bytes((PROTOCOL_VERSION,)))
    return Info(image[at : at + 12]) if 0 <= at <= len(image) - 12 else None


def loader_image(path):
    """A loader update image, as the slot's own content. A raw binary is that
    already; an Intel HEX links the loader at its base inside an otherwise
    blank flash image, and load_image() anchors every image at zero, so the
    blank below the base is dropped here. The base comes from the image's own
    info block rather than the device's, so an image built for somewhere else
    survives intact and the preflight can say so."""
    image = load_image(path)
    embedded = image_info(image)
    if embedded and len(image) > embedded.base:
        image = image[embedded.base :]
    return image


def staging_content(image, info):
    """The 512-byte staging-slot content: the image, padding, and — on
    chips whose hand-over jumps through the word below the resident loader —
    that word, which for a staging copy is the slot's own last word: an rjmp
    to the resident base. The staging copy's fall-through and 'J'-free exit
    both land in a loader instead of garbage."""
    if len(image) > (SLOT - 2 if info.patch_vector else SLOT):
        raise Error(f"loader image is {len(image)} B, the slot holds {SLOT - 2 if info.patch_vector else SLOT}")
    content = bytearray(image) + bytearray([0xFF] * (SLOT - len(image)))
    if info.patch_vector:
        through = rjmp_to((info.base - 2) // 2, info.base // 2, info.flash_size // 2)
        content[SLOT - 2], content[SLOT - 1] = through & 0xFF, through >> 8
    return bytes(content)


def update_preflight(image, info, fuse_bytes):
    """Errors and warnings before any flash is touched. Returns warnings."""
    embedded = image_info(image)
    if embedded is None:
        raise Error("no pureboot info block in the update image — not a pureboot binary?")
    if embedded.raw[3:] != info.raw[3:]:
        raise Error(
            f"update image is for another target: it declares "
            f"[{embedded.describe()}], the device says [{info.describe()}]"
        )
    warnings = []
    if not info.patch_vector:
        if fuse_bytes is None:
            raise Error("a loader update on this chip needs its fuses — unreadable? pass --assume-fuses")
        high = fuse_bytes[3]
        bootrst, bls_start = mega_boot(high)
        if info.stage < bls_start:
            raise Error(
                f"cannot self-update: the staging slot {info.stage:#06x} lies below the "
                f"boot section ({bls_start:#06x}, high fuse {high:#04x}) where SPM is disabled "
                f"— a boot section of at least 1 KB (BOOTSZ) is required, and only an "
                f"external programmer can change fuses"
            )
        if not bootrst:
            warnings.append(
                "BOOTRST unprogrammed: reset boots the application throughout the update; "
                "an interruption is recovered by re-running this update"
            )
        elif bls_start == info.stage:
            warnings.append(
                "BOOTRST targets the staging slot: brief unrecoverable windows exist while "
                "the staging copy itself is being installed or retired (page-write scale)"
            )
        else:
            warnings.append(
                f"BOOTRST targets {bls_start:#06x}, inside application flash: reset reaches a "
                f"loader only across erased flash from there"
            )
    return warnings


class UpdateState:
    """The host-side memory of an update in flight: what the staging slot
    held (and page 0, where the update repoints it). Losing this file after
    the staging slot was overwritten loses those saved bytes — the update
    still completes, but the staging region can then only be restored by
    reflashing the application."""

    def __init__(self, path):
        self.path = path
        self.data = None

    def load_or_save(self, loader):
        info = loader.info
        if os.path.exists(self.path):
            self.data = json.load(open(self.path))
            if bytes.fromhex(self.data["signature"]) != info.signature or self.data["base"] != info.base:
                raise Error(f"{self.path} belongs to a different device — remove it to start over")
            return
        self.data = {
            "signature": info.signature.hex(),
            "base": info.base,
            "staging": loader.read_flash(info.stage, SLOT).hex(),
            "page0": loader.read_flash(0, info.page).hex() if info.patch_vector else "",
        }
        with open(self.path, "w") as f:
            json.dump(self.data, f)

    @property
    def staging(self):
        return bytes.fromhex(self.data["staging"])

    @property
    def page0(self):
        return bytes.fromhex(self.data["page0"])

    def discard(self):
        os.unlink(self.path)


def write_differing(loader, base, content, order=None):
    """Program the pages of `content` at `base` that differ from flash —
    idempotent, so a resumed phase redoes only what an interruption left."""
    page = loader.info.page
    offsets = order if order is not None else range(0, len(content), page)
    written = 0
    for offset in offsets:
        want = content[offset : offset + page]
        if loader.read_flash(base + offset, page) != want:
            loader.write_page(base + offset, want)
            written += 1
    for at in range(0, len(content), 256):
        if loader.read_flash(base + at, min(256, len(content) - at)) != content[at : at + 256]:
            raise Error(f"verify failed at {base + at:#06x} after programming")
    return written


def patch_word0(loader, page0, target_base):
    """Rewrite page 0 with its word 0 re-aimed at `target_base` — the
    resume insurance around rewriting a loader slot the reset path uses."""
    info = loader.info
    patched = bytearray(page0)
    word = rjmp_to(0, target_base // 2, info.flash_size // 2)
    patched[0], patched[1] = word & 0xFF, word >> 8
    write_differing(loader, 0, bytes(patched))
    return bytes(patched)


def op_update_loader(loader, wait, path, state_path, fuse_bytes):
    """Replace the resident loader with `path`, using the loader itself as
    its own staging loader. Every phase is idempotent and keyed off the
    actual flash state, so a re-run after any interruption resumes; the
    state file carries the bytes the staging slot held."""
    info = loader.info
    image = loader_image(path)
    for warning in update_preflight(image, info, fuse_bytes):
        print(f"note: {warning}")
    staged = staging_content(image, info)
    resident = bytes(image) + bytes([0xFF] * (SLOT - len(image)))
    page = info.page

    state = UpdateState(state_path)
    state.load_or_save(loader)

    # Install the staging copy. On a chip whose staging slot starts at
    # address 0 (the 1 KB tiny13A), its first page carries the reset vector:
    # written last, so any earlier interruption still resets into the old
    # resident, and from then on resets enter the staging copy.
    order = list(range(0, SLOT, page))
    if info.stage == 0:
        order = order[1:] + [0]
    if write_differing(loader, info.stage, staged, order):
        print(f"staging copy installed at {info.stage:#06x}")

    # Enter it and let it rewrite the resident slot. Where a patched reset
    # vector routes through the resident (a tiny with the staging slot away
    # from page 0), word 0 is re-aimed at the staging copy around the
    # rewrite, so a power failure mid-rewrite still resets into a loader.
    loader.enter_copy(info.stage, wait)
    redirect = info.patch_vector and info.stage != 0
    if redirect:
        patch_word0(loader, state.page0, info.stage)
    if write_differing(loader, info.base, resident):
        print(f"resident loader rewritten at {info.base:#06x}")

    # Enter the new resident and put the staging region back: page 0 first
    # where it lives in that region (word 0 then points at the new resident
    # for the rest of the restore), the saved trampoline with the rest.
    loader.enter_copy(info.base, wait)
    if redirect:
        write_differing(loader, 0, state.page0)
    order = list(range(0, SLOT, page))
    if info.stage == 0:
        order = [0] + order[1:]
    write_differing(loader, info.stage, state.staging, order)

    state.discard()
    print(f"loader updated: {len(image)} B at {info.base:#06x}, staging region restored")


def check_walk_region(pages, info, fuse_bytes, force):
    """With BOOTRST programmed but targeting below the loader, reset reaches
    the loader only by walking across erased flash from the boot-section
    start; application data in that span would divert reset into itself.
    Only checkable when the fuses are known (--fuses or --assume-fuses)."""
    if info.patch_vector or fuse_bytes is None:
        return
    bootrst, bls_start = mega_boot(fuse_bytes[3])
    if not bootrst or bls_start >= info.base:
        return
    overlap = [a for a in sorted(pages) if a >= bls_start and pages[a].count(0xFF) != len(pages[a])]
    if overlap and not force:
        raise Error(
            f"the image writes {overlap[0]:#06x}.. inside the reset walk region "
            f"[{bls_start:#06x}, {info.base:#06x}) (BOOTRST programmed): reset could no "
            f"longer reach the loader — --force to flash it anyway"
        )


# ------------------------------------------------------------ operations ---


def op_erase_flash(loader):
    """0xff over the whole application area. Descending on a patched-vector
    chip: page 0 — the patched reset vector — goes last, so an interrupted
    erase still resets into the loader, and once it is gone the whole area
    is erased and the reset walk reaches the loader anyway."""
    blank = bytes([0xFF] * loader.info.page)
    addresses = range(0, loader.info.base, loader.info.page)
    for address in reversed(addresses) if loader.info.patch_vector else addresses:
        loader.write_page(address, blank)
    print(f"erase: {loader.info.base // loader.info.page} pages")


def op_erase_eeprom(loader):
    loader.write_eeprom(0, bytes([0xFF] * loader.info.eeprom_size))
    print(f"erase: {loader.info.eeprom_size} B of EEPROM")


def op_flash(loader, path, erase, verify, fuse_bytes=None, force=False):
    image = load_image(path)
    pages = plan_flash(image, loader.info)
    check_walk_region(pages, loader.info, fuse_bytes, force)
    if erase:
        op_erase_flash(loader)
    order = covered(pages, loader.info, skip_blank=erase)
    for address in order:
        loader.write_page(address, pages[address])
    print(f"flash: {path}: {len(order)} pages")
    if verify:
        verify_pages(loader, pages)


def verify_pages(loader, pages):
    for address in sorted(pages):
        got = loader.read_flash(address, loader.info.page)
        if got != pages[address]:
            first = next(i for i in range(len(got)) if got[i] != pages[address][i])
            raise Error(
                f"verify failed at {address + first:#06x}: "
                f"wrote {pages[address][first]:02x}, read {got[first]:02x}"
            )
    print(f"verify: {len(pages)} pages ok")


def op_verify_flash(loader, path):
    verify_pages(loader, plan_flash(load_image(path), loader.info))


def op_read_flash(loader, path):
    data = loader.read_flash(0, loader.info.base)
    open(path, "wb").write(data)
    print(f"read flash: {len(data)} B -> {path}")


def op_eeprom(loader, path, erase, verify):
    image = load_image(path)
    if len(image) > loader.info.eeprom_size:
        raise Error(f"EEPROM image is {len(image)} B, device has {loader.info.eeprom_size}")
    if erase:
        op_erase_eeprom(loader)
    loader.write_eeprom(0, image)
    print(f"eeprom: {path}: {len(image)} B")
    if verify:
        got = loader.read_eeprom(0, len(image))
        if got != image:
            first = next(i for i in range(len(got)) if got[i] != image[i])
            raise Error(f"verify failed at EEPROM {first:#06x}: wrote {image[first]:02x}, read {got[first]:02x}")
        print(f"verify: {len(image)} B ok")


def op_verify_eeprom(loader, path):
    image = load_image(path)
    got = loader.read_eeprom(0, len(image))
    if got != image:
        first = next(i for i in range(len(got)) if got[i] != image[i])
        raise Error(f"verify failed at EEPROM {first:#06x}: expected {image[first]:02x}, read {got[first]:02x}")
    print(f"verify: {len(image)} B of EEPROM ok")


def op_read_eeprom(loader, path):
    data = loader.read_eeprom(0, loader.info.eeprom_size)
    open(path, "wb").write(data)
    print(f"read EEPROM: {len(data)} B -> {path}")


def op_fuses(loader):
    low, lock, extended, high = loader.read_fuses()
    print(f"fuses: low {low:02x} high {high:02x} extended {extended:02x} lock {lock:02x}")
    return bytes((low, lock, extended, high))


# -------------------------------------------------------------------- cli ---


def main():
    parser = argparse.ArgumentParser(
        description="pureboot host tool", epilog="operations run in the order listed above"
    )
    parser.add_argument("--port", required=True, help="serial device: COM6, /dev/ttyUSB0, or a simavr pty")
    parser.add_argument("--baud", type=int, default=115200, help="115200 mega, 57600 tinies")
    parser.add_argument("--wait", type=float, default=30.0, help="seconds to keep knocking")
    parser.add_argument("--info", action="store_true", help="print the device info block")
    parser.add_argument("--fuses", action="store_true", help="read the fuse and lock bytes")
    parser.add_argument("--update-loader", metavar="FILE", help="replace the loader with this pureboot binary")
    parser.add_argument("--state", metavar="FILE", help="update state file (default: FILE.pbstate)")
    parser.add_argument("--assume-fuses", metavar="HEX8", help="fuse bytes low,lock,ext,high as 8 hex digits "
                        "(overrides reading them — e.g. under a simulator that cannot)")
    parser.add_argument("--erase-flash", action="store_true", help="0xff over the application flash")
    parser.add_argument("--flash", metavar="FILE", help="program an application (bin or ihex)")
    parser.add_argument("--no-verify", action="store_true", help="skip read-back after writes")
    parser.add_argument("--read-flash", metavar="FILE", help="dump the application flash")
    parser.add_argument("--verify-flash", metavar="FILE", help="compare flash against an image")
    parser.add_argument("--erase-eeprom", action="store_true", help="0xff over the EEPROM")
    parser.add_argument("--eeprom", metavar="FILE", help="program the EEPROM (bin or ihex)")
    parser.add_argument("--read-eeprom", metavar="FILE", help="dump the EEPROM")
    parser.add_argument("--verify-eeprom", metavar="FILE", help="compare EEPROM against an image")
    parser.add_argument("--force", action="store_true", help="override refusable safety checks")
    parser.add_argument("--stay", action="store_true", help="leave the loader in its session")
    args = parser.parse_args()

    if args.update_loader and (args.flash or args.erase_flash):
        parser.error("--update-loader does not combine with application flash operations")
    fuse_override = None
    if args.assume_fuses:
        try:
            fuse_override = bytes.fromhex(args.assume_fuses)
            assert len(fuse_override) == 4
        except (ValueError, AssertionError):
            parser.error("--assume-fuses takes 8 hex digits: low,lock,extended,high")

    port = Port(args.port, args.baud)
    try:
        loader = Loader(port)
        info = loader.connect(args.wait)
        if args.info:
            print(f"device: {info.describe()}")
        fuse_bytes = fuse_override
        if args.fuses or (args.update_loader and not info.patch_vector and fuse_bytes is None):
            read = op_fuses(loader)
            if fuse_bytes is None:
                fuse_bytes = read
        if args.update_loader:
            state = args.state or args.update_loader + ".pbstate"
            op_update_loader(loader, args.wait, args.update_loader, state, fuse_bytes)
        if args.flash:
            op_flash(loader, args.flash, args.erase_flash, not args.no_verify, fuse_bytes, args.force)
        elif args.erase_flash:
            op_erase_flash(loader)
        if args.read_flash:
            op_read_flash(loader, args.read_flash)
        if args.verify_flash:
            op_verify_flash(loader, args.verify_flash)
        if args.eeprom:
            op_eeprom(loader, args.eeprom, args.erase_eeprom, not args.no_verify)
        elif args.erase_eeprom:
            op_erase_eeprom(loader)
        if args.read_eeprom:
            op_read_eeprom(loader, args.read_eeprom)
        if args.verify_eeprom:
            op_verify_eeprom(loader, args.verify_eeprom)
        if args.stay:
            print("loader stays in its session (reset to leave)")
        else:
            loader.run_application()
            print("application running")
    finally:
        port.close()


if __name__ == "__main__":
    try:
        main()
    except Error as error:
        print(f"error: {error}", file=sys.stderr)
        sys.exit(1)
    except KeyboardInterrupt:
        sys.exit(130)