tsb: reimplement TinySafeBoot on libavr in three size tiers
The native-UART fixed-baud TinySafeBoot protocol, ported onto libavr as a
crt-free boot-section loader, in three variants that trade clarity for size:
tsb_pure 740 B idiomatic C++: SRAM page buffer, separate flash/EEPROM
leaves, shared framing; the polled `unused` guard posture.
tsb_tricks 658 B unified runtime-flag paths (noinline/noclone), call-saved
global-register page walk — attributes only, no asm.
tsb_asm 508 B streaming store + hand-rolled UART/SPM/EEPROM/erase loops;
fits the 512 B boot section (BOOTSZ=11). Trims the optional
password gate and WDT-reset bail — unreachable in C++ with
both (hand-asm is ~15 % denser). Tiers 1-2 keep them and
live in the 1 KB section they fit.
All three are .text byte-identical across libavr's generated and reflect modes.
The CMake build strips the leaked -O3 (a Release build is silently -O3, not the
-Os this loader is measured against) and gates each variant's size against its
section. A simavr harness (test/device.c + test/tsbtest.py) drives the real wire
protocol over a pty and flashes the device; the size and protocol tests run in
ctest. Verified byte-for-byte against the reference tsbloader_adv (C#/mono):
activate, read info, flash write + verify.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
266
tsb/tsb_pure.cpp
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266
tsb/tsb_pure.cpp
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// TinySafeBoot on libavr — tier 1: pure, idiomatic C++.
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//
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// A ≤512-byte serial flash bootloader for the ATmega328P boot section,
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// reimplementing the TinySafeBoot wire protocol (native-UART fixed-baud
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// lineage) on libavr. This variant is written for clarity: well-factored
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// functions, no compiler-specific size hacks, no inline assembly. The only
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// attributes are the ones the task inherently needs — [[gnu::progmem]] for the
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// flash-resident info block and the naked reset entry that stands in for the
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// absent C runtime.
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//
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// The structure follows the hand-written reference: one SRAM page buffer that
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// every page transfer shares, separate flash/EEPROM leaf routines (so nothing
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// is duplicated by constant propagation), and the polled `unused` interrupt
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// posture so every SPM/EEPROM lock folds away.
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#include <libavr/libavr.hpp>
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#include <avr/io.h> // SP / RAMEND for the crt-free boot entry
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using namespace avr::literals;
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namespace spm = avr::spm;
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namespace ee = avr::eeprom;
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using dev = avr::device<{.clock = 16_MHz}>;
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using serial_t = dev::uart0<{.baud = 115200_Bd, .max_baud_error = 3_pct}>;
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inline constexpr serial_t serial{};
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namespace tsb {
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// The loader is purely polled — it never enables interrupts — so every SPM and
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// EEPROM lock folds to nothing under this posture.
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constexpr auto off = avr::irq::guard_policy::unused;
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// The two handshake bytes, identical across every TSB host.
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constexpr std::uint8_t confirm = '!';
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constexpr std::uint8_t request = '?';
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// Boot geometry for the ATmega328P 512-byte boot section (BOOTSZ=11). The page
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// size, flash and EEPROM extents are the chip database's to know. app_end is
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// both the first byte the loader protects and the config page (the LASTPAGE
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// holding app-jump vector, timeout and password), one page below the boot
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// section.
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constexpr std::uint16_t page = spm::page_bytes;
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constexpr std::uint16_t boot_bytes = 1024;
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constexpr std::uint16_t app_end = spm::flash_bytes - boot_bytes - page;
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constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
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// Firmware version stamp: YY*512 + MM*32 + DD, the encoding the host decodes.
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constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 19;
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// The 16-byte device-info block the host reads on activation. Flash-resident so
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// it needs no .data (there is no crt to copy it).
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// clang-format off
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[[gnu::progmem]] constexpr std::uint8_t info[16] = {
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'T', 'S', 'B',
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build_date & 0xFF, build_date >> 8,
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0xF3, // status byte (native-UART fixed-baud lineage)
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0x1E, 0x95, 0x0F, // ATmega328P signature
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page / 2, // page size in words
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(app_end / 2) & 0xFF, (app_end / 2) >> 8, // app-flash boundary, words
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eeprom_end & 0xFF, eeprom_end >> 8,
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0xAA, 0xAA, // ATmega processor-type marker (bytes 14 == 15)
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};
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// clang-format on
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// One page staged in SRAM. Scratch that is always filled before it is read, so
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// it lives in .noinit — no startup clear (there is no crt to run one) and no
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// bytes in .text, which is the only thing the boot-section budget counts.
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[[gnu::section(".noinit")]] std::uint8_t buffer[page];
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std::uint8_t rx()
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{
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for (;;)
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if (auto byte = serial.read())
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return *byte;
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}
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void tx(std::uint8_t byte)
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{
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serial.write(byte);
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}
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const std::uint8_t *flash_ptr(std::uint16_t addr)
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{
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return reinterpret_cast<const std::uint8_t *>(addr);
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}
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// Stream `count` bytes to the host, from flash (LPM) or from EEPROM.
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void send_flash(std::uint16_t addr, std::uint16_t count)
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{
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while (count--)
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tx(avr::flash_load(flash_ptr(addr++)));
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}
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void send_eeprom(std::uint16_t addr, std::uint16_t count)
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{
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while (count--)
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tx(ee::read(addr++));
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}
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// Take one page from the host into the SRAM buffer.
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void get_page()
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{
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for (std::uint16_t i = 0; i < page; ++i)
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buffer[i] = rx();
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}
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// Prompt the host with '?' and report whether it answered '!'.
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bool request_confirm()
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{
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tx(request);
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return rx() == confirm;
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}
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// Program the SRAM buffer into one already-erased flash page (low byte then
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// high, as the SPM word buffer wants).
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void write_flash_page(std::uint16_t addr)
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{
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for (std::uint16_t i = 0; i < page; i += 2)
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spm::fill<off>(addr + i, static_cast<std::uint16_t>(buffer[i] | (buffer[i + 1] << 8)));
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spm::write_page<off>(addr);
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spm::wait();
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}
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// Write the SRAM buffer into EEPROM byte by byte.
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void write_eeprom_page(std::uint16_t addr)
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{
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for (std::uint16_t i = 0; i < page; ++i)
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ee::write<off>(addr + i, buffer[i]);
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}
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// Run the application: reset vector at 0x0000. Any non-command byte, a wrong
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// password, or an idle programmer port lands here.
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[[noreturn]] void appjump()
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{
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spm::wait(); // make sure any pending SPM finished before handing over
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reinterpret_cast<void (*)()>(0)();
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__builtin_unreachable();
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}
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// 'f': stream the application flash back, one page per host '!'. Self-terminates
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// at the application boundary; the host normally stops earlier with a non-'!'.
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void read_flash()
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{
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for (std::uint16_t a = 0; a < app_end; a += page) {
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if (rx() != confirm)
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return;
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send_flash(a, page);
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}
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}
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// 'e': stream EEPROM back, one page per host '!', until the host stops.
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void read_eeprom()
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{
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for (std::uint16_t a = 0;; a += page) {
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if (rx() != confirm)
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return;
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send_eeprom(a, page);
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}
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}
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// 'F': erase the whole application first (unwritten pages stay erased), then
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// take pages the host offers behind '?'.
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void write_flash()
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{
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for (std::uint16_t a = 0; a < app_end; a += page) {
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spm::erase_page<off>(a);
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spm::wait();
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}
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for (std::uint16_t a = 0; request_confirm(); a += page) {
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get_page();
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write_flash_page(a);
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}
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spm::rww_enable<off>();
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}
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// 'E': take pages the host offers behind '?' into EEPROM.
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void write_eeprom()
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{
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for (std::uint16_t a = 0; request_confirm(); a += page) {
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get_page();
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write_eeprom_page(a);
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}
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}
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// 'C': replace the config page, then echo it back for the host to verify.
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void write_config()
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{
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if (!request_confirm())
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return;
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get_page();
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spm::erase_page<off>(app_end);
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spm::wait();
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write_flash_page(app_end);
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spm::rww_enable<off>();
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send_flash(app_end, page);
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}
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[[noreturn]] void run()
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{
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// A bootloader may be entered by a watchdog reset; the reference loader
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// hands straight back to the application in that case rather than run.
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if (avr::hw::reg<"MCUSR">::read() & avr::hw::field<"MCUSR", "WDRF">{}(1).value)
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appjump();
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avr::init<serial_t>();
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// Activation: the host knocks three '@'. With no programmer attached the
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// port stays idle, so a bounded wait boots the application instead of
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// hanging forever.
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std::uint8_t knocks = 0;
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std::uint32_t idle = 4000000;
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while (knocks < 3) {
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if (auto byte = serial.read())
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knocks = *byte == '@' ? knocks + 1 : 0;
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else if (--idle == 0)
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appjump();
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}
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// Password gate: the config page holds it at app_end+3, terminated by 0xff.
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// A blank page (0xff there) means no password. A wrong byte hangs the loader
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// silently, as TSB does.
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for (const std::uint8_t *pw = flash_ptr(app_end + 3); avr::flash_load(pw) != 0xff; ++pw)
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if (rx() != avr::flash_load(pw))
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for (;;) {
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}
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send_flash(reinterpret_cast<std::uint16_t>(&info[0]), sizeof(info));
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for (;;) {
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tx(confirm); // Mainloop ready
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switch (rx()) {
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case 'f':
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read_flash();
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break;
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case 'F':
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write_flash();
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break;
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case 'e':
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read_eeprom();
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break;
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case 'E':
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write_eeprom();
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break;
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case 'c':
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send_flash(app_end, page);
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break;
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case 'C':
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write_config();
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break;
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default:
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appjump(); // 'q' or any other byte runs the application
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}
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}
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}
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} // namespace tsb
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// Reset lands here: BOOTRST vectors to the boot section base and .vectors is
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// laid first, so this is the first instruction executed. No crt ran, so set the
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// stack pointer before anything is called.
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extern "C" [[gnu::naked, gnu::used, gnu::section(".vectors")]] void __boot_entry()
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{
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SP = RAMEND;
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tsb::run();
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}
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