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:
245
tsb/tsb_tricks.cpp
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245
tsb/tsb_tricks.cpp
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// TinySafeBoot on libavr — tier 2: C++ with compiler trickery.
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//
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// Same protocol and libavr surface as the pure variant (tsb_pure.cpp), but the
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// readable one-handler-per-command shape is traded for size: flash and EEPROM
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// share a single code path selected by a *runtime* flag decoded from the
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// command byte, so the compiler cannot constant-propagate it into two clones.
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// Attributes pin that sharing down (noinline/noclone) and the hot page pointer
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// and byte counter are pinned to call-saved registers to erase the prologue
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// push/pop that C++ function decomposition otherwise pays. No inline assembly.
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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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constexpr auto off = avr::irq::guard_policy::unused;
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constexpr std::uint8_t confirm = '!';
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constexpr std::uint8_t request = '?';
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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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constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 19;
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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,
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0x1E, 0x95, 0x0F,
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page / 2,
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(app_end / 2) & 0xFF, (app_end / 2) >> 8,
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eeprom_end & 0xFF, eeprom_end >> 8,
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0xAA, 0xAA,
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};
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// clang-format on
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[[gnu::section(".noinit")]] std::uint8_t buffer[page];
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// The hot page walk lives in call-saved global registers, TSB-style: g_addr is
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// the running flash/EEPROM byte address, g_cnt the byte countdown. Being global
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// they are never spilled around the rx/tx/spm calls the way a local would be,
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// which is where the pure variant pays its prologue push/pop. r4-r7 are
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// call-saved, so the library's UART/SPM helpers preserve them across calls.
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register std::uint16_t g_addr asm("r4");
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register std::uint8_t g_cnt asm("r6");
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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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// One page transfer, memory selected at run time. noinline + noclone keep it a
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// single shared body: the `flash` flag arrives from the command byte, so the
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// optimiser cannot split it back into a flash copy and an EEPROM copy. All of
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// these walk g_addr / g_cnt, set by the caller.
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// Stream g_cnt bytes to the host from flash (LPM) or EEPROM, advancing g_addr
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// so a caller can send consecutive pages without re-seeding it.
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[[gnu::noinline, gnu::noclone]] void send(bool flash)
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{
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do {
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tx(flash ? avr::flash_load(flash_ptr(g_addr)) : ee::read(g_addr));
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++g_addr;
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} while (--g_cnt);
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}
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// Take one page from the host into the SRAM buffer.
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[[gnu::noinline]] void get_page()
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{
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g_cnt = 0;
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do {
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buffer[g_cnt] = rx();
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} while (++g_cnt != page);
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}
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[[gnu::noinline]] 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 the already-erased page at g_addr (flash) or into
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// EEPROM. g_addr is left on the page base for the caller to advance.
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[[gnu::noinline, gnu::noclone]] void write_page(bool flash)
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{
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g_cnt = 0;
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if (flash) {
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do {
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spm::fill<off>(g_addr + g_cnt, static_cast<std::uint16_t>(buffer[g_cnt] | (buffer[g_cnt + 1] << 8)));
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g_cnt += 2;
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} while (g_cnt != page);
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spm::write_page<off>(g_addr);
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spm::wait();
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} else {
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do {
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ee::write<off>(g_addr + g_cnt, buffer[g_cnt]);
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} while (++g_cnt != page);
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}
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}
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[[noreturn]] void appjump()
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{
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spm::wait();
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reinterpret_cast<void (*)()>(0)();
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__builtin_unreachable();
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}
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// 'f'/'e': stream memory back one page per host '!'. send advances g_addr, so
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// flash self-terminates at the application boundary; EEPROM runs until the host
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// stops.
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[[gnu::noinline]] void read_mem(bool flash)
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{
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g_addr = 0;
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for (;;) {
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if (rx() != confirm)
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return;
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g_cnt = page;
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send(flash);
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if (flash && g_addr >= app_end)
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return;
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}
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}
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// 'F'/'E': flash erases the whole application first, then both take the pages
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// the host offers behind '?'.
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[[gnu::noinline]] void write_mem(bool flash)
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{
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if (flash) {
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g_addr = 0;
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do {
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spm::erase_page<off>(g_addr);
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spm::wait();
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g_addr += page;
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} while (g_addr < app_end);
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}
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g_addr = 0;
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while (request_confirm()) {
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get_page();
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write_page(flash);
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g_addr += page;
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}
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if (flash)
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spm::rww_enable<off>();
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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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g_addr = app_end;
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spm::erase_page<off>(g_addr);
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spm::wait();
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write_page(true);
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spm::rww_enable<off>();
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g_cnt = page;
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send(true); // g_addr is still app_end
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}
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[[noreturn]] void run()
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{
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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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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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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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g_addr = reinterpret_cast<std::uint16_t>(&info[0]);
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g_cnt = sizeof(info);
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send(true);
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for (;;) {
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tx(confirm);
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// Decode the command arithmetically so `flash`/`write` stay runtime
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// values: bit 5 is the case bit (upper = write), and the folded-lower
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// letter picks the memory. A single unified path serves f/F/e/E.
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std::uint8_t cmd = rx();
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std::uint8_t lower = cmd | 0x20;
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bool write = (cmd & 0x20) == 0;
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if (lower == 'f' || lower == 'e') {
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bool flash = lower == 'f';
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if (write)
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write_mem(flash);
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else
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read_mem(flash);
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} else if (lower == 'c') {
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if (write) {
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write_config();
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} else {
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g_addr = app_end;
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g_cnt = page;
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send(true);
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}
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} else {
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appjump();
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}
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}
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}
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} // namespace tsb
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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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