bootloader/tsb/tsb_tricks.cpp

// TinySafeBoot on libavr — tier 2: C++ with compiler trickery.
//
// Same protocol and libavr surface as the pure variant (tsb_pure.cpp), but the
// readable one-handler-per-command shape is traded for size: flash and EEPROM
// share a single code path selected by a *runtime* flag decoded from the
// command byte, so the compiler cannot constant-propagate it into two clones.
// Attributes pin that sharing down (noinline/noclone) and the hot page pointer
// and byte counter are pinned to call-saved registers to erase the prologue
// push/pop that C++ function decomposition otherwise pays. No inline assembly.

#include <libavr/libavr.hpp>

#include <avr/io.h> // SP / RAMEND for the crt-free boot entry

using namespace avr::literals;
namespace spm = avr::spm;
namespace ee = avr::eeprom;

using dev = avr::device<{.clock = 16_MHz}>;
using serial_t = dev::uart0<{.baud = 115200_Bd, .max_baud_error = 3_pct}>;
inline constexpr serial_t serial{};

namespace tsb {

constexpr auto off = avr::irq::guard_policy::unused;

constexpr std::uint8_t confirm = '!';
constexpr std::uint8_t request = '?';

constexpr std::uint16_t page = spm::page_bytes;
constexpr std::uint16_t boot_bytes = 1024;
constexpr std::uint16_t app_end = spm::flash_bytes - boot_bytes - page;
constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;

constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 19;

// clang-format off
[[gnu::progmem]] constexpr std::uint8_t info[16] = {
    'T', 'S', 'B',
    build_date & 0xFF, build_date >> 8,
    0xF3,
    0x1E, 0x95, 0x0F,
    page / 2,
    (app_end / 2) & 0xFF, (app_end / 2) >> 8,
    eeprom_end & 0xFF, eeprom_end >> 8,
    0xAA, 0xAA,
};
// clang-format on

[[gnu::section(".noinit")]] std::uint8_t buffer[page];

// The hot page walk lives in call-saved global registers, TSB-style: g_addr is
// the running flash/EEPROM byte address, g_cnt the byte countdown. Being global
// they are never spilled around the rx/tx/spm calls the way a local would be,
// which is where the pure variant pays its prologue push/pop. r4-r7 are
// call-saved, so the library's UART/SPM helpers preserve them across calls.
register std::uint16_t g_addr asm("r4");
register std::uint8_t g_cnt asm("r6");

std::uint8_t rx()
{
	for (;;)
		if (auto byte = serial.read())
			return *byte;
}

void tx(std::uint8_t byte)
{
	serial.write(byte);
}

const std::uint8_t *flash_ptr(std::uint16_t addr)
{
	return reinterpret_cast<const std::uint8_t *>(addr);
}

// One page transfer, memory selected at run time. noinline + noclone keep it a
// single shared body: the `flash` flag arrives from the command byte, so the
// optimiser cannot split it back into a flash copy and an EEPROM copy. All of
// these walk g_addr / g_cnt, set by the caller.

// Stream g_cnt bytes to the host from flash (LPM) or EEPROM, advancing g_addr
// so a caller can send consecutive pages without re-seeding it.
[[gnu::noinline, gnu::noclone]] void send(bool flash)
{
	do {
		tx(flash ? avr::flash_load(flash_ptr(g_addr)) : ee::read(g_addr));
		++g_addr;
	} while (--g_cnt);
}

// Take one page from the host into the SRAM buffer.
[[gnu::noinline]] void get_page()
{
	g_cnt = 0;
	do {
		buffer[g_cnt] = rx();
	} while (++g_cnt != page);
}

[[gnu::noinline]] bool request_confirm()
{
	tx(request);
	return rx() == confirm;
}

// Program the SRAM buffer into the already-erased page at g_addr (flash) or into
// EEPROM. g_addr is left on the page base for the caller to advance.
[[gnu::noinline, gnu::noclone]] void write_page(bool flash)
{
	g_cnt = 0;
	if (flash) {
		do {
			spm::fill<off>(g_addr + g_cnt, static_cast<std::uint16_t>(buffer[g_cnt] | (buffer[g_cnt + 1] << 8)));
			g_cnt += 2;
		} while (g_cnt != page);
		spm::write_page<off>(g_addr);
		spm::wait();
	} else {
		do {
			ee::write<off>(g_addr + g_cnt, buffer[g_cnt]);
		} while (++g_cnt != page);
	}
}

[[noreturn]] void appjump()
{
	spm::wait();
	reinterpret_cast<void (*)()>(0)();
	__builtin_unreachable();
}

// 'f'/'e': stream memory back one page per host '!'. send advances g_addr, so
// flash self-terminates at the application boundary; EEPROM runs until the host
// stops.
[[gnu::noinline]] void read_mem(bool flash)
{
	g_addr = 0;
	for (;;) {
		if (rx() != confirm)
			return;
		g_cnt = page;
		send(flash);
		if (flash && g_addr >= app_end)
			return;
	}
}

// 'F'/'E': flash erases the whole application first, then both take the pages
// the host offers behind '?'.
[[gnu::noinline]] void write_mem(bool flash)
{
	if (flash) {
		g_addr = 0;
		do {
			spm::erase_page<off>(g_addr);
			spm::wait();
			g_addr += page;
		} while (g_addr < app_end);
	}
	g_addr = 0;
	while (request_confirm()) {
		get_page();
		write_page(flash);
		g_addr += page;
	}
	if (flash)
		spm::rww_enable<off>();
}

// 'C': replace the config page, then echo it back for the host to verify.
void write_config()
{
	if (!request_confirm())
		return;
	get_page();
	g_addr = app_end;
	spm::erase_page<off>(g_addr);
	spm::wait();
	write_page(true);
	spm::rww_enable<off>();
	g_cnt = page;
	send(true); // g_addr is still app_end
}

[[noreturn]] void run()
{
	if (avr::hw::reg<"MCUSR">::read() & avr::hw::field<"MCUSR", "WDRF">{}(1).value)
		appjump();

	avr::init<serial_t>();

	std::uint8_t knocks = 0;
	std::uint32_t idle = 4000000;
	while (knocks < 3) {
		if (auto byte = serial.read())
			knocks = *byte == '@' ? knocks + 1 : 0;
		else if (--idle == 0)
			appjump();
	}

	for (const std::uint8_t *pw = flash_ptr(app_end + 3); avr::flash_load(pw) != 0xff; ++pw)
		if (rx() != avr::flash_load(pw))
			for (;;) {
			}

	g_addr = reinterpret_cast<std::uint16_t>(&info[0]);
	g_cnt = sizeof(info);
	send(true);

	for (;;) {
		tx(confirm);
		// Decode the command arithmetically so `flash`/`write` stay runtime
		// values: bit 5 is the case bit (upper = write), and the folded-lower
		// letter picks the memory. A single unified path serves f/F/e/E.
		std::uint8_t cmd = rx();
		std::uint8_t lower = cmd | 0x20;
		bool write = (cmd & 0x20) == 0;
		if (lower == 'f' || lower == 'e') {
			bool flash = lower == 'f';
			if (write)
				write_mem(flash);
			else
				read_mem(flash);
		} else if (lower == 'c') {
			if (write) {
				write_config();
			} else {
				g_addr = app_end;
				g_cnt = page;
				send(true);
			}
		} else {
			appjump();
		}
	}
}

} // namespace tsb

extern "C" [[gnu::naked, gnu::used, gnu::section(".vectors")]] void __boot_entry()
{
	SP = RAMEND;
	tsb::run();
}