feat: add low-speed tier to cruise warning, gate param writes on change

Speed limit ≤25 mph now allows +8 over before warning (e.g. limit 25 →
33 is ok, warn at 34). Existing tiers unchanged: ≥50 → +9 ok warn at +10,
26-49 → +6 ok warn at +7.

Also gate all 7 speed_logic param writes on change (same pattern as the
earlier carstate/controlsd perf fix). Called at 2Hz so not as hot, but
unit/is_metric never change mid-drive and the cruise warning rarely
flips — no reason to thrash /dev/shm on every update.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/clearpilot/speed_logic.py

@@ -24,6 +24,21 @@ class SpeedState:
     self.prev_warning = ""
     self.prev_warning_speed_limit = 0
 
+    # Cache last-written param values — each put() is mkstemp+fsync+flock+rename.
+    # Sentinel None so the first call always writes.
+    self._w_has_speed = None
+    self._w_speed_display = None
+    self._w_speed_limit_display = None
+    self._w_speed_unit = None
+    self._w_is_metric = None
+    self._w_cruise_warning = None
+    self._w_cruise_warning_speed = None
+
+  def _put_if_changed(self, key, value, attr):
+    if getattr(self, attr) != value:
+      self.params_memory.put(key, value)
+      setattr(self, attr, value)
+
   def update(self, speed_ms: float, has_speed: bool, speed_limit_ms: float, is_metric: bool,
              cruise_speed_ms: float = 0.0, cruise_active: bool = False, cruise_standstill: bool = False):
     """
@@ -48,12 +63,12 @@ class SpeedState:
 
     self.prev_speed_limit = speed_limit_int
 
-    # Write display-ready values to params_memory
+    # Write display-ready values to params_memory (gated on change)
-    self.params_memory.put("ClearpilotHasSpeed", "1" if has_speed and speed_int > 0 else "0")
+    self._put_if_changed("ClearpilotHasSpeed", "1" if has_speed and speed_int > 0 else "0", "_w_has_speed")
-    self.params_memory.put("ClearpilotSpeedDisplay", str(speed_int) if has_speed and speed_int > 0 else "")
+    self._put_if_changed("ClearpilotSpeedDisplay", str(speed_int) if has_speed and speed_int > 0 else "", "_w_speed_display")
-    self.params_memory.put("ClearpilotSpeedLimitDisplay", str(speed_limit_int) if speed_limit_int > 0 else "0")
+    self._put_if_changed("ClearpilotSpeedLimitDisplay", str(speed_limit_int) if speed_limit_int > 0 else "0", "_w_speed_limit_display")
-    self.params_memory.put("ClearpilotSpeedUnit", unit)
+    self._put_if_changed("ClearpilotSpeedUnit", unit, "_w_speed_unit")
-    self.params_memory.put("ClearpilotIsMetric", "1" if is_metric else "0")
+    self._put_if_changed("ClearpilotIsMetric", "1" if is_metric else "0", "_w_is_metric")
 
     # Cruise warning logic
     warning = ""
@@ -61,7 +76,16 @@ class SpeedState:
     cruise_engaged = cruise_active and not cruise_standstill
 
     if speed_limit_int >= 20 and cruise_engaged and cruise_int > 0:
-      over_threshold = 10 if speed_limit_int >= 50 else 7
+      # Tiers (warning fires at >= limit + threshold):
+      #   limit >= 50:  +9 over ok, warn at +10   (e.g. 60 → warn at 70)
+      #   limit 26-49:  +6 over ok, warn at +7    (e.g. 35 → warn at 42)
+      #   limit <= 25:  +8 over ok, warn at +9    (e.g. 25 → warn at 34, so 33 is ok)
+      if speed_limit_int >= 50:
+        over_threshold = 10
+      elif speed_limit_int <= 25:
+        over_threshold = 9
+      else:
+        over_threshold = 7
       if cruise_int >= speed_limit_int + over_threshold:
         warning = "over"
         warning_speed = str(cruise_int)
@@ -69,8 +93,8 @@ class SpeedState:
         warning = "under"
         warning_speed = str(cruise_int)
 
-    self.params_memory.put("ClearpilotCruiseWarning", warning)
+    self._put_if_changed("ClearpilotCruiseWarning", warning, "_w_cruise_warning")
-    self.params_memory.put("ClearpilotCruiseWarningSpeed", warning_speed)
+    self._put_if_changed("ClearpilotCruiseWarningSpeed", warning_speed, "_w_cruise_warning_speed")
 
     # Ding logic: play when warning sign appears or speed limit changes while visible
     should_ding = False