merge net into eth lib; ipv4 owns its IP via ipv4::init/get_ip; drop state struct in favor of eth::get_mac/ipv4::get_ip

eth/eth.h

@@ -0,0 +1,47 @@
+#pragma once
+#include <array>
+#include <cstdint>
+#include <span>
+#include "callback_list.h"
+#include "span_writer.h"
+
+namespace eth {
+
+using mac_addr = std::array<uint8_t, 6>;
+
+static constexpr mac_addr MAC_BROADCAST = {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
+static constexpr uint16_t ETH_ARP = __builtin_bswap16(0x0806);
+static constexpr uint16_t ETH_IPV4 = __builtin_bswap16(0x0800);
+
+struct __attribute__((packed)) header {
+    mac_addr dst;
+    mac_addr src;
+    uint16_t ethertype;
+};
+static_assert(sizeof(header) == 14);
+
+template <typename Buf>
+void prepend(Buf& buf, const mac_addr& dst, const mac_addr& src, uint16_t ethertype) {
+    auto* h = buf.template prepend<header>();
+    h->dst = dst;
+    h->src = src;
+    h->ethertype = ethertype;
+}
+
+using frame_callback = bool (*)(std::span<const uint8_t> frame);
+using frame_cb_list = callback_list<frame_callback, 16>;
+using frame_cb_handle = frame_cb_list::node*;
+
+using ethertype_handler = void (*)(std::span<const uint8_t> frame, span_writer& tx);
+using mac_filter = bool (*)(const mac_addr& dst);
+
+bool init();
+const mac_addr& get_mac();
+frame_cb_handle add_frame_callback(frame_callback cb);
+void remove_frame_callback(frame_cb_handle h);
+void poll(std::span<uint8_t> tx);
+void send_raw(std::span<const uint8_t> data);
+void register_ethertype(uint16_t ethertype_be, ethertype_handler fn);
+void register_mac_filter(mac_filter fn);
+
+} // namespace eth