Add hardware.ini assembly config, build identity in the header, and ;prev calibration history

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018GsFaaLUTCYy3ZjaNTJzMv

CMakeLists.txt

@@ -35,6 +35,22 @@ find_package(Boost REQUIRED CONFIG COMPONENTS program_options)
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(JXL REQUIRED IMPORTED_TARGET libjxl libjxl_threads)
 
+# ---------------------------------------------------------------------------
+# Build identity. deploy.sh computes `describe --always --dirty` and passes it in
+# the environment (GIT_REV=... cmake ...), so a deployed binary can name the
+# revision it came from in the TUI header. A plain local build has no GIT_REV and
+# falls back to "unknown", exactly as firmware/src/main.cpp does.
+#
+# Read at CONFIGURE time: `cmake --build` alone keeps whatever value the last
+# configure saw. deploy.sh reconfigures on every deploy, so the deployed path is
+# always fresh.
+# ---------------------------------------------------------------------------
+if(DEFINED ENV{GIT_REV} AND NOT "$ENV{GIT_REV}" STREQUAL "")
+  set(FGC_GIT_REV "$ENV{GIT_REV}")
+else()
+  set(FGC_GIT_REV "unknown")
+endif()
+
 # ---------------------------------------------------------------------------
 # fgc_core: SDK-independent core (config, path resolution, and - as the
 # refactor proceeds - logging, the capture scheduler and the I/O interfaces).
@@ -78,6 +94,8 @@ target_include_directories(fgc_core PUBLIC
   ${CMAKE_SOURCE_DIR}        # for the bundled ini.h
 )
 target_link_libraries(fgc_core PUBLIC Threads::Threads)
+# PUBLIC so the application and the tests both see the same build identity.
+target_compile_definitions(fgc_core PUBLIC FGC_GIT_REV="${FGC_GIT_REV}")
 
 # ---------------------------------------------------------------------------
 # Application executable. Camera image pipeline + serial controller always
@@ -170,4 +188,5 @@ message(STATUS "  WITH_VIMBA : ${WITH_VIMBA}")
 message(STATUS "  WITH_MQTT  : ${WITH_MQTT}")
 message(STATUS "  WITH_TUI   : ${WITH_TUI}")
 message(STATUS "  tests      : ${BUILD_TESTING}")
+message(STATUS "  build id   : ${FGC_GIT_REV}")
 message(STATUS "------------------------------------------------------------")

config/hardware.example.ini

@@ -0,0 +1,79 @@
+; ===========================================================================
+; hardware.ini - what this machine IS.
+;
+; Written once when the tower is assembled. Changing anything here means the
+; hardware itself changed: a different board, a sensor fitted or removed, a
+; cable moved to another port. Operational settings you tune to get better
+; images - capture interval, camera profiles, scan grid, output paths - live in
+; config.ini instead, and so does calibration, which is redone from time to time.
+;
+; WHERE THIS FILE GOES
+;   /etc/fire_gimbal_control/hardware.ini
+;
+; That is beside credentials.env, and it is deliberately OUTSIDE the directory
+; deploy.sh rsyncs: a deploy cannot overwrite or delete what it never touches.
+; The program looks, in order, at:
+;   --hardware <path>  |  $FGC_HARDWARE  |  /etc/...  |  beside config.ini
+;
+; Keys here override the same keys in config.ini, and any that are still set in
+; both are named in a warning at startup so the stale copy can be deleted.
+; ===========================================================================
+
+; --- Which computer -------------------------------------------------------
+[Board]
+; Free-form, but keep it matching a directory under hardware/ so the setup kit
+; for this board is easy to find.
+model        = lattepanda-3-delta
+; PlatformIO env used to build the motor firmware for this board's MCU.
+;   lattepanda-3-delta / sigma -> leonardo      (ATmega32U4, 5V logic)
+;   lattepanda-iota            -> iota_rp2040   (RP2040, 3.3V logic)
+firmware_env = leonardo
+
+; --- What is fitted -------------------------------------------------------
+; This is a DECLARATION, and its whole purpose is that startup can compare it
+; with what it actually finds. The program already copes gracefully when a part
+; is missing - the pitch axis, the IMU, the ambient sensor - which is correct for
+; a rig built without them, and indistinguishable from a fault on a rig that has
+; them. Saying what was assembled is what tells those two apart.
+[Hardware]
+; Axes this gimbal has. A yaw-only rig says: axes = yaw
+axes    = yaw, pitch
+; Orientation sensor: a model name, or 'none'. Most of the fleet has none.
+imu     = none
+; Ambient temperature/humidity sensor: a model name, or 'none'.
+env     = sht41
+; The [Camera.<name>] blocks expected to be present and found.
+cameras = rgb
+
+; --- Where to find it -----------------------------------------------------
+; Device bindings. These follow the hardware, not the tower, so they belong here
+; rather than in config.ini.
+
+[Serial]
+; The motor controller MCU. Prefer the stable by-id symlink over /dev/ttyACM0,
+; which is assigned in enumeration order and moves when USB devices change.
+;   Leonardo : /dev/serial/by-id/usb-Arduino_LLC_Arduino_Leonardo-if00
+device = /dev/serial/by-id/usb-Arduino_LLC_Arduino_Leonardo-if00
+baud   = 115200
+
+[IMU]
+; Only read when [Hardware] imu is not 'none'. On the LattePanda 3 Delta the MTi
+; is on the RS-232 header, which is a hardware UART: a stable /dev/ttyS* node,
+; NOT a USB device and NOT /dev/ttyUSB0-3 (those are the modem).
+device = /dev/ttyS0
+
+[Env]
+; Only read when [Hardware] env is not 'none'. This is the board's OWN native
+; I2C bus, not the motor controller's serial link. The bus number varies by
+; board - it is /dev/i2c-2 on the 3 Delta, not the /dev/i2c-1 default.
+;
+; Two traps when finding it: i2cdetect false-negatives on the SHT4x (probe with
+; a real measurement instead), and an SMBus-only adapter can never work at all
+; (i2cdetect -F N reports 'I2C = no').
+i2c_device = /dev/i2c-2
+i2c_addr   = 0x44
+
+[Camera.rgb]
+; The serial of the physical camera fitted to this tower. A USB DEV_ id, or a
+; GigE IP address.
+id = DEV_1AB22C049CD1

include/fgc/Config.h

@@ -19,6 +19,43 @@ struct GeneralConfig {
     bool        debug = false;        // print motor telemetry each loop tick
 };
 
+// [Board]: which computer this is. Assembly data - it changes only when the
+// machine is physically replaced, so it belongs in hardware.ini, not config.ini.
+struct BoardConfig {
+    std::string model;         // e.g. "lattepanda-3-delta", "lattepanda-iota"
+    std::string firmware_env;  // PlatformIO env for the motor MCU ("leonardo", ...)
+};
+
+// [Hardware]: what is actually fitted to THIS rig.
+//
+// The point of declaring it is that the rest of the program degrades gracefully
+// when something is missing - TestRunner skips the pitch and IMU leaves, CamEvent
+// falls back to encoder pitch, CaptureScheduler sends the single-axis MOVE form -
+// which is right for a rig that genuinely lacks them, and indistinguishable from a
+// FAILURE on a rig that does not. Declaring what was assembled is what lets
+// startup tell those two apart.
+struct HardwareConfig {
+    // Empty means "not declared": nothing was stated, so nothing is reconciled.
+    // This is the state every existing tower is in until it gets a hardware.ini.
+    std::vector<std::string> axes;     // e.g. {"yaw", "pitch"}
+    std::string              imu;      // sensor model, or "none"
+    std::string              env;      // sensor model, or "none"
+    std::vector<std::string> cameras;  // the [Camera.<name>] blocks expected to exist
+
+    // Was a hardware.ini actually read? False on every tower that has not been
+    // migrated yet, and the switch that keeps this change invisible on rollout.
+    bool declared = false;
+    // Where it came from, for logging and for `test hardware` to cite.
+    std::string source_path;
+
+    // True when `name` is listed in axes. An UNDECLARED rig answers true for
+    // everything: with nothing stated there is nothing to contradict, and callers
+    // must keep behaving exactly as they did before.
+    bool hasAxis(const std::string& name) const;
+    // "yaw, pitch" - for log lines and test output.
+    std::string axesLabel() const;
+};
+
 struct NetworkConfig {
     std::string broker_ip = "127.0.0.1";  // MQTT broker host/IP, or a full tcp://... URI
     int         mqtt_port = 1883;          // ignored when broker_ip carries its own port
@@ -177,6 +214,8 @@ struct AppConfig {
     SerialConfig   serial;
     CameraConfig   camera;
     PathsConfig    paths;
+    BoardConfig    board;     // [Board]    from hardware.ini
+    HardwareConfig hardware;  // [Hardware] from hardware.ini
     FeaturesConfig features;
     LoggingConfig  logging;
     UiConfig       ui;        // [UI] terminal dashboard toggle
@@ -205,7 +244,14 @@ public:
     // Reads the INI file at `path` (via the bundled inih parser), maps it into
     // a typed AppConfig, applies environment overrides, and validates it.
     // Throws std::runtime_error with a clear message on failure.
-    static AppConfig loadFromFile(const std::string& path);
+    //
+    // `hardware_path`, when given, is read AFTER the config and its keys win. It
+    // carries the assembly-time description of the machine ([Board], [Hardware],
+    // and the device bindings), which outlives any amount of operational tuning.
+    // Passing "" keeps the historical behaviour exactly: everything comes from
+    // config.ini and `hardware.declared` stays false.
+    static AppConfig loadFromFile(const std::string& path,
+                                  const std::string& hardware_path = "");
 
     // Same mapping/validation logic, but from an already-parsed key->value map
     // ("Section.name" => value). Exposed for unit testing without file IO.
@@ -216,14 +262,29 @@ public:
 // existing keys in that section are replaced in place (comments/order preserved),
 // and any missing ones are appended in a `[section]` block at the end. Pure (no
 // I/O) so it is unit-testable.
+//
+// With a non-empty `provenance`, a key whose value actually CHANGES also gets its
+// previous value kept above it as a comment, so a calibration can be reverted by
+// hand:
+//
+//     ;prev yaw_heading_at_center = 0.0   ; 2026-09-11T14:32Z, gimbal heading setup
+//     yaw_heading_at_center = 127.4
+//
+// Exactly one generation is kept - an existing `;prev` for a changing key is
+// replaced, not appended to - and unchanged keys are left completely alone, so
+// the file grows once and then holds steady.
 std::string updateIniSectionKeys(const std::string& contents, const std::string& section,
-                                 const std::vector<std::pair<std::string, std::string>>& kv);
+                                 const std::vector<std::pair<std::string, std::string>>& kv,
+                                 const std::string& provenance = "");
 
 // Persist the live `[Motor]` geometry (counts_per_deg / center_count /
 // deg_at_center for both axes, plus the endstop margins) back into the INI file
 // at `path`. Written via a temp file and rename, so an interrupted save cannot
 // leave a truncated config behind. Returns true on success.
+//
+// `provenance` names what produced these values (e.g. "2026-09-11T14:32Z, gimbal
+// heading setup"); when given, superseded values are kept as `;prev` comments.
 bool saveMotorGeometry(const std::string& path, const Geometry& geo, long yaw_margin,
-                       long pitch_margin);
+                       long pitch_margin, const std::string& provenance = "");
 
 }  // namespace fgc

include/fgc/Paths.h

@@ -22,6 +22,28 @@ std::vector<std::string> configSearchPaths(const std::string& cliArg = "");
 // readable file wins. Returns nullopt if none is found.
 std::optional<std::string> resolveConfigPath(const std::string& cliArg = "");
 
+// The ordered list of locations searched for hardware.ini - the assembly-time
+// description of THIS machine (which board, which sensors are fitted, which
+// device nodes they are on). Deliberately NOT derived from the config.ini path:
+//
+//   --hardware <path>                        explicit, for dev and testing
+//   $FGC_HARDWARE                            the service environment
+//   /etc/fire_gimbal_control/hardware.ini    the normal answer on a tower
+//   <dir of config.ini>/hardware.ini         so a dev checkout just works
+//
+// /etc is where credentials.env already lives: root-owned, per-device, and
+// OUTSIDE the deploy tree, so an rsync cannot overwrite or delete it. Keeping the
+// assembly description there makes "a deploy never touches it" a property of
+// where the file is, not of an exclude rule someone has to maintain.
+std::vector<std::string> hardwareSearchPaths(const std::string& cliArg      = "",
+                                             const std::string& configPath = "");
+
+// Resolve hardware.ini using hardwareSearchPaths(); first existing file wins.
+// Returns nullopt when there is none - which is not an error: the program then
+// falls back to reading the assembly keys from config.ini as it always has.
+std::optional<std::string> resolveHardwarePath(const std::string& cliArg      = "",
+                                               const std::string& configPath = "");
+
 // Default image output directory when none is configured
 // ($XDG_DATA_HOME/fire_gimbal_control/images, else ~/.local/share/...).
 std::string defaultOutputDir();

main.cpp

@@ -16,6 +16,9 @@ int main(int argc, char* argv[]) {
     desc.add_options()
         ("help,h", "show this help")
         ("config,c", po::value<std::string>(), "path to config.ini")
+        ("hardware", po::value<std::string>(),
+         "path to hardware.ini (default: $FGC_HARDWARE, /etc/fire_gimbal_control/hardware.ini, "
+         "or one beside config.ini)")
         ("init,i", po::bool_switch(), "home the gimbal on startup (required: nothing is homed across a power cycle)")
         ("start,s", po::bool_switch(), "start capture automatically")
         ("demo,d", po::bool_switch(), "demo mode (copy placeholder image instead of encoding)")
@@ -56,9 +59,20 @@ int main(int argc, char* argv[]) {
         return 1;
     }
 
+    // hardware.ini describes the machine itself and is optional: a tower that has
+    // not been migrated yet simply has none, and everything comes from config.ini
+    // exactly as before. An explicit --hardware that does not exist IS an error,
+    // though - it means someone expected an assembly description to be in force.
+    const std::string cli_hw = vm.count("hardware") ? vm["hardware"].as<std::string>() : "";
+    auto hw_path = paths::resolveHardwarePath(cli_hw, *cfg_path);
+    if (!cli_hw.empty() && !hw_path) {
+        std::cerr << "No hardware.ini at --hardware " << cli_hw << "\n";
+        return 1;
+    }
+
     AppConfig cfg;
     try {
-        cfg = ConfigLoader::loadFromFile(*cfg_path);
+        cfg = ConfigLoader::loadFromFile(*cfg_path, hw_path.value_or(""));
     } catch (const std::exception& e) {
         std::cerr << "Config error (" << *cfg_path << "): " << e.what() << "\n";
         return 1;

src/core/Application.cpp

@@ -40,6 +40,7 @@
 #include <csignal>
 #include <cstdio>
 #include <cstdlib>
+#include <ctime>
 #include <filesystem>
 #include <fstream>
 #include <memory>
@@ -81,6 +82,24 @@ long long nowEpochMs() {
         .count();
 }
 
+// "2026-09-11T14:32Z" - UTC, minute resolution. Stamped into the `;prev` comments
+// saveMotorGeometry leaves behind, where the point is telling two calibrations
+// apart later, not sub-second precision. UTC because towers are read remotely and
+// a local-time stamp says nothing without also knowing the host's zone.
+std::string utcStampNow() {
+    const std::time_t t = std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
+    std::tm           tm{};
+#if defined(_WIN32)
+    gmtime_s(&tm, &t);
+#else
+    gmtime_r(&t, &tm);
+#endif
+    char b[32];
+    std::snprintf(b, sizeof(b), "%04d-%02d-%02dT%02d:%02dZ", tm.tm_year + 1900, tm.tm_mon + 1,
+                  tm.tm_mday, tm.tm_hour, tm.tm_min);
+    return std::string(b);
+}
+
 // True if `dir` can be created and a probe file written there. A successful
 // create_directories alone isn't enough (the dir may exist read-only), so we
 // actually write+remove a probe file.
@@ -219,6 +238,9 @@ struct Application::Impl {
     std::vector<std::string> last_calib_summary;
     long long                last_calib_ts = 0;   // epoch ms
     bool                     calib_save_pending_ = false;  // awaiting y/n to persist to config
+    // Which command produced the pending values, recorded into the `;prev`
+    // comment when they are saved so the file says what superseded what.
+    std::string              calib_save_source_;
 
     // Hardware self-test (`test` command) results, for the activity strip.
     TestResultView           last_test_view;
@@ -607,7 +629,9 @@ struct Application::Impl {
 
         const bool mock_serial = opts.mock_serial.value_or(cfg.features.mock_serial);
         s.header.tower     = cfg.general.tower_name;
-        s.header.build     = "dev";
+        // Set by CMake from $GIT_REV, which deploy.sh passes in; "unknown" for a
+        // plain local build. This is the TUI header's build id.
+        s.header.build     = FGC_GIT_REV;
         s.header.uptime_ms = std::chrono::duration_cast<std::chrono::milliseconds>(
                                  std::chrono::steady_clock::now() - start_time).count();
         s.header.live      = !mock_serial;
@@ -1254,6 +1278,10 @@ struct Application::Impl {
         y.deg_at_center  = r.deg_at_center;
         applyYawMapChange();
 
+        // The landmark wizard and a bare `gimbal heading fit` land here alike; name
+        // whichever it was before the flag is cleared.
+        calib_save_source_    = heading_setup_active_ ? "gimbal heading setup"
+                                                      : "gimbal heading fit";
         heading_setup_active_ = false;
         // Offer to persist, through the same activity-strip prompt the pitch setup
         // uses - the operator answers y/n without leaving the dashboard.
@@ -1490,12 +1518,19 @@ struct Application::Impl {
     }
 
     // Persist the live (just-calibrated) [Motor] geometry back into config.ini.
+    // Superseded values are kept above their key as `;prev` comments, stamped with
+    // when and what replaced them, so a calibration that turns out worse than the
+    // one it replaced can be reverted by uncommenting.
     void saveCalibration() {
         if (!calib_save_pending_) return;
         calib_save_pending_ = false;
+        const std::string provenance =
+            utcStampNow() + ", " +
+            (calib_save_source_.empty() ? std::string("calibration") : calib_save_source_);
         if (saveMotorGeometry(opts.config_path, cfg.geometry, cfg.yaw_soft_margin_counts,
-                              cfg.pitch_soft_margin_counts))
+                              cfg.pitch_soft_margin_counts, provenance))
-            LOG_INFO << "[Motor] calibration saved as the new default in " << opts.config_path;
+            LOG_INFO << "[Motor] calibration saved as the new default in " << opts.config_path
+                     << " (previous values kept as ';prev' comments)";
         else
             LOG_WARN << "could not write [Motor] calibration to " << opts.config_path;
     }
@@ -1723,6 +1758,7 @@ struct Application::Impl {
                 last_calib_view = v;
                 last_calib_ts   = nowEpochMs();
                 // Offer to persist it as the new default (answered via the activity strip).
+                calib_save_source_ = "gimbal pitch setup";
                 if (rep.valid && !opts.config_path.empty()) calib_save_pending_ = true;
             }
         }

src/core/Config.cpp

@@ -98,6 +98,33 @@ bool getBool(const std::map<std::string, std::string>& kv, const std::string& ke
     throw std::runtime_error("Config key '" + key + "' must be a boolean, got '" + v + "'");
 }
 
+// "yaw, pitch" -> {"yaw","pitch"}. Lowercased and trimmed; empty entries dropped,
+// so trailing commas and stray spacing are not errors an operator has to hunt.
+std::vector<std::string> splitList(const std::string& s) {
+    std::vector<std::string> out;
+    std::string              cur;
+    auto flush = [&] {
+        size_t a = cur.find_first_not_of(" \t");
+        if (a == std::string::npos) { cur.clear(); return; }
+        size_t b = cur.find_last_not_of(" \t");
+        std::string t = cur.substr(a, b - a + 1);
+        for (char& c : t) c = static_cast<char>(std::tolower((unsigned char)c));
+        if (!t.empty()) out.push_back(t);
+        cur.clear();
+    };
+    for (char c : s) {
+        if (c == ',') flush();
+        else cur += c;
+    }
+    flush();
+    return out;
+}
+
+std::string lowered(std::string s) {
+    for (char& c : s) c = static_cast<char>(std::tolower((unsigned char)c));
+    return s;
+}
+
 std::string envOr(const char* name, const std::string& fallback) {
     const char* v = std::getenv(name);
     return (v && *v) ? std::string(v) : fallback;
@@ -210,6 +237,20 @@ AppConfig ConfigLoader::fromMap(const std::map<std::string, std::string>& kv) {
     cfg.general.image_interval = getInt(kv, "General.image_interval", cfg.general.image_interval);
     cfg.general.debug          = getBool(kv, "General.debug", cfg.general.debug);
 
+    // [Board] / [Hardware]: normally from hardware.ini, but read from the same
+    // merged map, so a config.ini carrying them still works during migration.
+    cfg.board.model        = get(kv, "Board.model", cfg.board.model);
+    cfg.board.firmware_env = get(kv, "Board.firmware_env", cfg.board.firmware_env);
+
+    cfg.hardware.axes    = splitList(get(kv, "Hardware.axes"));
+    cfg.hardware.cameras = splitList(get(kv, "Hardware.cameras"));
+    cfg.hardware.imu     = lowered(get(kv, "Hardware.imu"));
+    cfg.hardware.env     = lowered(get(kv, "Hardware.env"));
+    for (const auto& a : cfg.hardware.axes)
+        if (a != "yaw" && a != "pitch")
+            throw std::runtime_error("Hardware.axes: unknown axis '" + a +
+                                     "' (expected yaw and/or pitch)");
+
     cfg.network.broker_ip = get(kv, "Network.zkms_server_ip", cfg.network.broker_ip);
     cfg.network.mqtt_port = getInt(kv, "Network.mqtt_port", cfg.network.mqtt_port);
     if (cfg.network.mqtt_port < 1 || cfg.network.mqtt_port > 65535)
@@ -430,14 +471,56 @@ AppConfig ConfigLoader::fromMap(const std::map<std::string, std::string>& kv) {
     return cfg;
 }
 
-AppConfig ConfigLoader::loadFromFile(const std::string& path) {
+bool HardwareConfig::hasAxis(const std::string& name) const {
+    // An undeclared rig answers true for everything - see the header.
+    if (axes.empty()) return true;
+    return std::find(axes.begin(), axes.end(), name) != axes.end();
+}
+
+std::string HardwareConfig::axesLabel() const {
+    std::string s;
+    for (const auto& a : axes) s += (s.empty() ? "" : ", ") + a;
+    return s;
+}
+
+AppConfig ConfigLoader::loadFromFile(const std::string& path, const std::string& hardware_path) {
     std::map<std::string, std::string> kv;
     int rc = ini_parse(path.c_str(), iniHandler, &kv);
     if (rc < 0) throw std::runtime_error("Cannot read config file: " + path);
     if (rc > 0)
         throw std::runtime_error("Parse error in config file " + path + " at line " +
                                  std::to_string(rc));
+
+    // hardware.ini is layered ON TOP, so the assembly description wins over a
+    // config.ini that still carries the same keys from before the split. Any such
+    // leftover is named once, because it is a migration leftover to delete rather
+    // than a setting that is quietly doing nothing.
+    std::vector<std::string> shadowed;
+    if (!hardware_path.empty()) {
+        std::map<std::string, std::string> hw;
+        int hrc = ini_parse(hardware_path.c_str(), iniHandler, &hw);
+        if (hrc < 0) throw std::runtime_error("Cannot read hardware file: " + hardware_path);
+        if (hrc > 0)
+            throw std::runtime_error("Parse error in hardware file " + hardware_path +
+                                     " at line " + std::to_string(hrc));
+        for (const auto& [k, v] : hw) {
+            if (auto it = kv.find(k); it != kv.end() && it->second != v) shadowed.push_back(k);
+            kv[k] = v;
+        }
+    }
+
     AppConfig cfg = fromMap(kv);
+
+    if (!hardware_path.empty()) {
+        cfg.hardware.declared    = true;
+        cfg.hardware.source_path = hardware_path;
+        LOG_INFO << "hardware: " << hardware_path
+                 << (cfg.board.model.empty() ? "" : "  board " + cfg.board.model)
+                 << (cfg.hardware.axes.empty() ? "" : "  axes " + cfg.hardware.axesLabel());
+        for (const auto& k : shadowed)
+            LOG_WARN << "config.ini still sets " << k
+                     << ", which hardware.ini overrides -- delete it from config.ini";
+    }
     // Anchor a relative scan grid_file to the config file's directory so it resolves
     // no matter which working directory the process is launched from (the config
     // itself is found via a search path that may be unrelated to the cwd).
@@ -463,10 +546,80 @@ std::string trimmed(const std::string& s) {
     size_t b = s.find_last_not_of(ws);
     return s.substr(a, b - a + 1);
 }
+
+// Marker for a superseded value, kept so a calibration can be reverted by hand.
+// It is a comment, so inih ignores it on the next read, but it is distinctive
+// enough for updateIniSectionKeys to find its own earlier markers and replace
+// them rather than pile them up.
+const std::string kPrevMarker = ";prev ";
+
+// The value inih would actually parse from the right-hand side of `key = value`:
+// everything up to an inline comment, which is a ';' PRECEDED BY WHITESPACE (see
+// ini_find_chars_or_comment in ini.c, with INI_ALLOW_INLINE_COMMENTS=1 and
+// INI_INLINE_COMMENT_PREFIXES=";"). Mirroring that rule matters here: "did this
+// value change?" has to compare what the program read, not the raw line, or a
+// key carrying a trailing comment would look changed on every save.
+std::string iniValueOf(const std::string& rhs) {
+    bool was_space = false;
+    for (size_t i = 0; i < rhs.size(); ++i) {
+        if (was_space && rhs[i] == ';') return trimmed(rhs.substr(0, i));
+        was_space = std::isspace(static_cast<unsigned char>(rhs[i])) != 0;
+    }
+    return trimmed(rhs);
+}
+
+// If `t` (already trimmed) is a ";prev <key> = ..." marker, return <key>, else "".
+std::string prevMarkerKey(const std::string& t) {
+    const size_t n = kPrevMarker.size();
+    if (t.compare(0, n, kPrevMarker) != 0) return "";
+    const size_t eq = t.find('=', n);
+    if (eq == std::string::npos) return "";
+    return trimmed(t.substr(n, eq - n));
+}
 }  // namespace
 
 std::string updateIniSectionKeys(const std::string& contents, const std::string& section,
-                                 const std::vector<std::pair<std::string, std::string>>& kv) {
+                                 const std::vector<std::pair<std::string, std::string>>& kv,
+                                 const std::string& provenance) {
+    // Which keys are actually changing, and what they hold now.
+    //
+    // Only a changing key gets a ";prev" line, and only its marker is replaced:
+    // a key whose value is unchanged keeps the history it has, because that still
+    // names the value before the current one. This also keeps the file from
+    // growing on every save - saveMotorGeometry rewrites all eight keys each time,
+    // and without this filter seven of them would be noise.
+    //
+    // Computed in a first pass because a key's ";prev" marker sits ABOVE its value
+    // line, so the writing pass meets the marker before it has seen whether the
+    // value differs.
+    std::vector<bool>        changed(kv.size(), false);
+    std::vector<std::string> old_value(kv.size());
+    {
+        std::istringstream scan(contents);
+        std::string        line, cur;
+        std::vector<bool>  seen(kv.size(), false);
+        while (std::getline(scan, line)) {
+            const std::string t = trimmed(line);
+            if (t.size() >= 2 && t.front() == '[' && t.back() == ']') {
+                cur = trimmed(t.substr(1, t.size() - 2));
+                continue;
+            }
+            if (cur != section || t.empty() || t[0] == ';' || t[0] == '#') continue;
+            const size_t eq = line.find('=');
+            if (eq == std::string::npos) continue;
+            const std::string key = trimmed(line.substr(0, eq));
+            for (size_t i = 0; i < kv.size(); ++i) {
+                if (!seen[i] && key == kv[i].first) {
+                    seen[i]      = true;
+                    old_value[i] = iniValueOf(line.substr(eq + 1));
+                    changed[i]   = old_value[i] != kv[i].second;
+                    break;
+                }
+            }
+        }
+    }
+    const bool keep_history = !provenance.empty();
+
     std::vector<bool> written(kv.size(), false);
     std::istringstream in(contents);
     std::ostringstream out;
@@ -475,15 +628,28 @@ std::string updateIniSectionKeys(const std::string& contents, const std::string&
         const std::string t = trimmed(line);
         if (t.size() >= 2 && t.front() == '[' && t.back() == ']') {
             cur = trimmed(t.substr(1, t.size() - 2));
-        } else if (cur == section && !t.empty() && t[0] != ';' && t[0] != '#') {
+        } else if (cur == section) {
-            const size_t eq = line.find('=');
+            // An earlier ";prev" for a key that is changing now is dropped here
-            if (eq != std::string::npos) {
+            // and re-emitted below, so exactly one generation is ever kept.
-                const std::string key = trimmed(line.substr(0, eq));
+            const std::string pk = prevMarkerKey(t);
-                for (size_t i = 0; i < kv.size(); ++i) {
+            if (!pk.empty()) {
-                    if (!written[i] && key == kv[i].first) {
+                bool supersede = false;
-                        line = kv[i].first + " = " + kv[i].second;
+                for (size_t i = 0; i < kv.size(); ++i)
-                        written[i] = true;
+                    if (pk == kv[i].first && changed[i]) { supersede = true; break; }
-                        break;
+                if (supersede) continue;
+            } else if (!t.empty() && t[0] != ';' && t[0] != '#') {
+                const size_t eq = line.find('=');
+                if (eq != std::string::npos) {
+                    const std::string key = trimmed(line.substr(0, eq));
+                    for (size_t i = 0; i < kv.size(); ++i) {
+                        if (!written[i] && key == kv[i].first) {
+                            if (keep_history && changed[i])
+                                out << kPrevMarker << kv[i].first << " = " << old_value[i]
+                                    << "   ; " << provenance << "\n";
+                            line = kv[i].first + " = " + kv[i].second;
+                            written[i] = true;
+                            break;
+                        }
                     }
                 }
             }
@@ -503,7 +669,7 @@ std::string updateIniSectionKeys(const std::string& contents, const std::string&
 }
 
 bool saveMotorGeometry(const std::string& path, const Geometry& geo, long yaw_margin,
-                       long pitch_margin) {
+                       long pitch_margin, const std::string& provenance) {
     if (path.empty()) return false;
     std::ifstream f(path);
     if (!f) return false;
@@ -529,7 +695,7 @@ bool saveMotorGeometry(const std::string& path, const Geometry& geo, long yaw_ma
         {"pitch_angle_at_center", num(geo.pitch.deg_at_center)},
         {"pitch_soft_margin_counts", std::to_string(pitch_margin)},
     };
-    const std::string updated = updateIniSectionKeys(buf.str(), "Motor", kv);
+    const std::string updated = updateIniSectionKeys(buf.str(), "Motor", kv, provenance);
 
     // Write a sibling temp file and rename over the original: a truncating write
     // that dies partway leaves config.ini destroyed, and three commands rewrite

src/core/Paths.cpp

@@ -92,6 +92,34 @@ std::optional<std::string> resolveConfigPath(const std::string& cliArg) {
     return std::nullopt;
 }
 
+std::vector<std::string> hardwareSearchPaths(const std::string& cliArg,
+                                             const std::string& configPath) {
+    std::vector<std::string> paths;
+    if (!cliArg.empty()) paths.push_back(expandUser(cliArg));
+
+    if (const char* env = std::getenv("FGC_HARDWARE"); env && *env)
+        paths.push_back(expandUser(env));
+
+    paths.push_back("/etc/fire_gimbal_control/hardware.ini");
+
+    // Beside config.ini, so a checkout with a local config.ini picks up a local
+    // hardware.ini without needing root or an env var.
+    if (!configPath.empty()) {
+        const fs::path dir = fs::path(expandUser(configPath)).parent_path();
+        paths.push_back((dir.empty() ? fs::path(".") : dir) / "hardware.ini");
+    }
+    return paths;
+}
+
+std::optional<std::string> resolveHardwarePath(const std::string& cliArg,
+                                               const std::string& configPath) {
+    std::error_code ec;
+    for (const auto& p : hardwareSearchPaths(cliArg, configPath)) {
+        if (fs::is_regular_file(p, ec)) return p;
+    }
+    return std::nullopt;
+}
+
 std::string defaultOutputDir() {
     std::string base = envOr("XDG_DATA_HOME", expandUser("~/.local/share"));
     return (fs::path(base) / "fire_gimbal_control" / "images").string();

tests/test_config.cpp

@@ -12,6 +12,20 @@
 
 using namespace fgc;
 
+namespace {
+// How many times `needle` appears in `hay`. Used to assert that calibration
+// history replaces itself rather than accumulating.
+int countOccurrences(const std::string& hay, const std::string& needle) {
+    int    n = 0;
+    size_t i = hay.find(needle);
+    while (i != std::string::npos) {
+        ++n;
+        i = hay.find(needle, i + needle.size());
+    }
+    return n;
+}
+}  // namespace
+
 TEST_CASE("ConfigLoader maps and defaults typed values") {
     unsetenv("FGC_MQTT_USER");
     unsetenv("FGC_MQTT_PW");
@@ -260,6 +274,90 @@ TEST_CASE("updateIniSectionKeys appends keys missing from the section") {
     CHECK(out.find("pitch_center_count = 7") != std::string::npos);
 }
 
+TEST_CASE("updateIniSectionKeys keeps the superseded value as a ';prev' comment") {
+    const std::string in =
+        "[Motor]\n"
+        "; the heading the gimbal points at when parked\n"
+        "yaw_heading_at_center = 0.0\n"
+        "yaw_center_count = 500000\n";
+
+    auto out = updateIniSectionKeys(in, "Motor",
+                                    {{"yaw_heading_at_center", "127.4"},
+                                     {"yaw_center_count", "500000"}},  // unchanged
+                                    "2026-09-11T14:32Z, gimbal heading setup");
+
+    SUBCASE("the changed key keeps its old value, stamped") {
+        CHECK(out.find(";prev yaw_heading_at_center = 0.0") != std::string::npos);
+        CHECK(out.find("2026-09-11T14:32Z, gimbal heading setup") != std::string::npos);
+        CHECK(out.find("yaw_heading_at_center = 127.4") != std::string::npos);
+    }
+    SUBCASE("the history sits directly above the value it superseded") {
+        CHECK(out.find(";prev yaw_heading_at_center = 0.0   ; 2026-09-11T14:32Z, gimbal "
+                       "heading setup\nyaw_heading_at_center = 127.4") != std::string::npos);
+    }
+    SUBCASE("an UNCHANGED key gets no history - otherwise every save adds noise") {
+        CHECK(out.find(";prev yaw_center_count") == std::string::npos);
+    }
+    SUBCASE("hand-written comments survive") {
+        CHECK(out.find("; the heading the gimbal points at when parked") != std::string::npos);
+    }
+}
+
+TEST_CASE("updateIniSectionKeys keeps exactly one generation of history") {
+    // Saving twice must REPLACE the marker, not stack markers up: the file has to
+    // grow once and then hold steady however often the gimbal is recalibrated.
+    std::string s = "[Motor]\nyaw_heading_at_center = 0.0\n";
+    s = updateIniSectionKeys(s, "Motor", {{"yaw_heading_at_center", "10"}}, "t1, first");
+    s = updateIniSectionKeys(s, "Motor", {{"yaw_heading_at_center", "20"}}, "t2, second");
+    s = updateIniSectionKeys(s, "Motor", {{"yaw_heading_at_center", "30"}}, "t3, third");
+
+    CHECK(countOccurrences(s, ";prev yaw_heading_at_center") == 1);
+    CHECK(s.find(";prev yaw_heading_at_center = 20") != std::string::npos);  // the one before
+    CHECK(s.find("yaw_heading_at_center = 30") != std::string::npos);
+    CHECK(s.find("= 0.0") == std::string::npos);  // the oldest is gone
+    CHECK(s.find("t3, third") != std::string::npos);
+    CHECK(s.find("t1, first") == std::string::npos);
+}
+
+TEST_CASE("updateIniSectionKeys history respects inline comments and unchanged values") {
+    // inih treats ' ;' as an inline comment, so the stored value is "983.33", not
+    // "983.33   ; measured". Comparing raw text instead would call this key changed
+    // on every save and bury the file in useless history.
+    const std::string in = "[Motor]\nyaw_counts_per_deg = 983.33   ; measured on the rig\n";
+
+    SUBCASE("same value behind an inline comment counts as unchanged") {
+        auto out = updateIniSectionKeys(in, "Motor", {{"yaw_counts_per_deg", "983.33"}}, "t, c");
+        CHECK(out.find(";prev") == std::string::npos);
+    }
+    SUBCASE("a real change records only the parsed value, not the comment") {
+        auto out = updateIniSectionKeys(in, "Motor", {{"yaw_counts_per_deg", "1000"}}, "t, c");
+        CHECK(out.find(";prev yaw_counts_per_deg = 983.33   ; t, c") != std::string::npos);
+    }
+}
+
+TEST_CASE("updateIniSectionKeys writes no history without a provenance") {
+    // The default keeps the old behaviour, so callers that have nothing useful to
+    // say about where the values came from do not litter the file.
+    const std::string in = "[Motor]\nyaw_center_count = 1\n";
+    auto out = updateIniSectionKeys(in, "Motor", {{"yaw_center_count", "2"}});
+    CHECK(out.find(";prev") == std::string::npos);
+    CHECK(out.find("yaw_center_count = 2") != std::string::npos);
+}
+
+TEST_CASE("a ';prev' comment does not survive a reload as a real key") {
+    // The marker has to be inert to inih, or reverting by uncommenting would be
+    // pointless: the stale value would already be in force.
+    std::string s = "[General]\ntower_name = T\n[Motor]\nyaw_center_count = 1\n";
+    s = updateIniSectionKeys(s, "Motor", {{"yaw_center_count", "2"}}, "t, c");
+    REQUIRE(s.find(";prev yaw_center_count = 1") != std::string::npos);
+
+    const std::string path = "test_prev_marker_reload.ini";
+    { std::ofstream(path) << s; }
+    auto cfg = ConfigLoader::loadFromFile(path);
+    CHECK(cfg.geometry.yaw.center_count == 2);
+    std::filesystem::remove(path);
+}
+
 TEST_CASE("saveMotorGeometry round-trips the full per-axis map") {
     // Write a starting config, save a calibrated Geometry into it, reload, and
     // confirm every [Motor] parameter survives, while another section is left
@@ -401,6 +499,89 @@ TEST_CASE("[IMU] x/y/z describe the mount and default to no rotation") {
     CHECK_FALSE(c.imu.mount.identity());
 }
 
+TEST_CASE("hardware.ini layers over config.ini and wins") {
+    const std::string cfg_path = "test_hw_layer_config.ini";
+    const std::string hw_path  = "test_hw_layer_hardware.ini";
+    {
+        std::ofstream c(cfg_path);
+        c << "[General]\ntower_name = NorthRidge\nimage_interval = 3\n"
+             "[Camera.rgb]\nid = OLD_ID\nprofile_dir = /tmp/p\n"
+             "[Serial]\ndevice = /dev/ttyACM0\n";
+        std::ofstream h(hw_path);
+        h << "[Board]\nmodel = lattepanda-iota\nfirmware_env = iota_rp2040\n"
+             "[Hardware]\naxes = yaw, pitch\nimu = none\nenv = sht41\ncameras = rgb\n"
+             "[Serial]\ndevice = /dev/serial/by-id/usb-RP2040-if00\n"
+             "[Camera.rgb]\nid = NEW_ID\n";
+    }
+    auto c = ConfigLoader::loadFromFile(cfg_path, hw_path);
+
+    SUBCASE("assembly data arrives from hardware.ini") {
+        CHECK(c.board.model == "lattepanda-iota");
+        CHECK(c.board.firmware_env == "iota_rp2040");
+        CHECK(c.hardware.axes == std::vector<std::string>{"yaw", "pitch"});
+        CHECK(c.hardware.imu == "none");
+        CHECK(c.hardware.env == "sht41");
+        CHECK(c.hardware.cameras == std::vector<std::string>{"rgb"});
+        CHECK(c.hardware.declared);
+        CHECK(c.hardware.source_path == hw_path);
+    }
+    SUBCASE("bindings in hardware.ini override the ones left in config.ini") {
+        CHECK(c.serial.device == "/dev/serial/by-id/usb-RP2040-if00");
+        REQUIRE(c.camera.cameras.size() == 1);
+        CHECK(c.camera.cameras[0].id == "NEW_ID");
+    }
+    SUBCASE("operational settings stay with config.ini") {
+        CHECK(c.general.tower_name == "NorthRidge");
+        CHECK(c.general.image_interval == 3);
+    }
+    std::filesystem::remove(cfg_path);
+    std::filesystem::remove(hw_path);
+}
+
+TEST_CASE("without a hardware.ini nothing changes and nothing is declared") {
+    // The migration guarantee: an un-migrated tower must behave exactly as before.
+    const std::string path = "test_hw_absent.ini";
+    {
+        std::ofstream c(path);
+        c << "[General]\ntower_name = Old\n"
+             "[Camera.rgb]\nid = D\nprofile_dir = /tmp/p\n"
+             "[Serial]\ndevice = /dev/ttyACM0\n";
+    }
+    auto c = ConfigLoader::loadFromFile(path);
+    CHECK_FALSE(c.hardware.declared);
+    CHECK(c.hardware.axes.empty());
+    CHECK(c.serial.device == "/dev/ttyACM0");
+    CHECK(c.general.tower_name == "Old");
+
+    // An undeclared rig must not answer "no" to any axis, or every caller that
+    // asks would start refusing work on towers that have not been migrated.
+    CHECK(c.hardware.hasAxis("yaw"));
+    CHECK(c.hardware.hasAxis("pitch"));
+    std::filesystem::remove(path);
+}
+
+TEST_CASE("Hardware.axes parsing and membership") {
+    auto c = ConfigLoader::fromMap({{"Camera.rgb.id", "D"},
+                                    {"Camera.rgb.profile_dir", "/tmp/p"},
+                                    {"Hardware.axes", " YAW ,, pitch ,"}});
+    SUBCASE("case, spacing and stray commas are tolerated") {
+        CHECK(c.hardware.axes == std::vector<std::string>{"yaw", "pitch"});
+        CHECK(c.hardware.axesLabel() == "yaw, pitch");
+    }
+    SUBCASE("a declared yaw-only rig answers no to pitch") {
+        auto y = ConfigLoader::fromMap({{"Camera.rgb.id", "D"},
+                                        {"Camera.rgb.profile_dir", "/tmp/p"},
+                                        {"Hardware.axes", "yaw"}});
+        CHECK(y.hardware.hasAxis("yaw"));
+        CHECK_FALSE(y.hardware.hasAxis("pitch"));
+    }
+    SUBCASE("an unknown axis is a typo worth refusing, not ignoring") {
+        CHECK_THROWS(ConfigLoader::fromMap({{"Camera.rgb.id", "D"},
+                                            {"Camera.rgb.profile_dir", "/tmp/p"},
+                                            {"Hardware.axes", "yaw, roll"}}));
+    }
+}
+
 TEST_CASE("A mount angle that cannot be a rotation is rejected") {
     auto with = [](const std::string& k, const std::string& v) {
         return ConfigLoader::fromMap({{"Camera.rgb.id", "D"},

tests/test_paths.cpp

@@ -23,3 +23,43 @@ TEST_CASE("configSearchPaths honours the CLI argument first") {
     REQUIRE(!p.empty());
     CHECK(p.front() == "/explicit/config.ini");
 }
+
+TEST_CASE("hardwareSearchPaths looks in the documented order") {
+    unsetenv("FGC_HARDWARE");
+    auto p = paths::hardwareSearchPaths("", "/opt/fgc/config.ini");
+
+    auto at = [&](const std::string& s) {
+        for (size_t i = 0; i < p.size(); ++i)
+            if (p[i] == s) return static_cast<int>(i);
+        return -1;
+    };
+    // /etc is the normal answer on a tower - root-owned, per-device, and OUTSIDE
+    // the deploy tree - so it is searched before a checkout-local file.
+    REQUIRE(at("/etc/fire_gimbal_control/hardware.ini") >= 0);
+    REQUIRE(at("/opt/fgc/hardware.ini") >= 0);
+    CHECK(at("/etc/fire_gimbal_control/hardware.ini") < at("/opt/fgc/hardware.ini"));
+
+    SUBCASE("an explicit argument outranks everything") {
+        auto q = paths::hardwareSearchPaths("/tmp/mine.ini", "/opt/fgc/config.ini");
+        CHECK(q.front() == "/tmp/mine.ini");
+    }
+    SUBCASE("the environment outranks /etc but not the argument") {
+        setenv("FGC_HARDWARE", "/tmp/env.ini", 1);
+        auto q = paths::hardwareSearchPaths("/tmp/cli.ini", "/opt/fgc/config.ini");
+        REQUIRE(q.size() >= 2);
+        CHECK(q[0] == "/tmp/cli.ini");
+        CHECK(q[1] == "/tmp/env.ini");
+        unsetenv("FGC_HARDWARE");
+    }
+    SUBCASE("with no config path there is simply no sibling candidate") {
+        auto q = paths::hardwareSearchPaths("", "");
+        for (const auto& s : q) CHECK(s.find("/opt/fgc/") == std::string::npos);
+    }
+}
+
+TEST_CASE("resolveHardwarePath returns nothing when there is no hardware.ini") {
+    // Not an error: every existing tower is in this state until it is migrated,
+    // and the program must go on reading everything from config.ini.
+    unsetenv("FGC_HARDWARE");
+    CHECK_FALSE(paths::resolveHardwarePath("/nonexistent/nope.ini", "").has_value());
+}