Add dlogf printf formatting, ipv4::to_string helper, clean up string building

firmware/include/debug_log.h

@@ -1,4 +1,6 @@
 #pragma once
+#include <cstdarg>
+#include <cstdio>
 #include <functional>
 #include <string>
 #include <string_view>
@@ -17,12 +19,22 @@ inline void dlog(std::string_view msg) {
     g_debug_log.push_overwrite(log_entry{static_cast<uint32_t>(time_us_32()), std::string(msg)});
 }
 
+__attribute__((format(printf, 1, 2)))
+inline void dlogf(const char* fmt, ...) {
+    char buf[128];
+    va_list args;
+    va_start(args, fmt);
+    vsnprintf(buf, sizeof(buf), fmt, args);
+    va_end(args);
+    dlog(buf);
+}
+
 inline void dlog_if_slow(std::string_view label, uint32_t threshold_us, std::function<void()> fn) {
     uint32_t t0 = time_us_32();
     fn();
     uint32_t elapsed = time_us_32() - t0;
     if (elapsed > threshold_us)
-        dlog(std::string(label) + " " + std::to_string(elapsed) + "us");
+        dlogf("%.*s %luus", static_cast<int>(label.size()), label.data(), static_cast<unsigned long>(elapsed));
 }
 
 inline std::vector<log_entry> dlog_drain() {

firmware/include/dispatch.h

@@ -1,5 +1,6 @@
 #pragma once
 #include <cstdint>
+#include <cstdio>
 #include <functional>
 #include <optional>
 #include <span>
@@ -32,8 +33,10 @@ void typed_handler(responder resp, std::span<const uint8_t> payload) {
     auto tup = req.as_tuple();
     auto r = msgpack::unpack(p, tup);
     if (!r) {
-        resp.respond(DeviceError{1, "decode request ext_id=" +
-            std::to_string(Req::ext_id) + ": msgpack error " + std::to_string(static_cast<int>(r.error()))});
+        char err[64];
+        snprintf(err, sizeof(err), "decode request ext_id=%d: msgpack error %d",
+                 Req::ext_id, static_cast<int>(r.error()));
+        resp.respond(DeviceError{1, err});
         return;
     }
     auto result = Fn(resp, req);

firmware/include/ipv4.h

@@ -1,8 +1,10 @@
 #pragma once
 #include <array>
 #include <cstdint>
+#include <cstdio>
 #include <functional>
 #include <span>
+#include <string>
 #include "eth.h"
 #include "span_writer.h"
 
@@ -10,6 +12,12 @@ namespace ipv4 {
 
 using ip4_addr = std::array<uint8_t, 4>;
 
+inline std::string to_string(const ip4_addr& ip) {
+    char buf[16];
+    snprintf(buf, sizeof(buf), "%u.%u.%u.%u", ip[0], ip[1], ip[2], ip[3]);
+    return buf;
+}
+
 struct __attribute__((packed)) header {
     eth::header eth;
     uint8_t ver_ihl;