Rename inverse_mm_s => inverse_secs

Marlin/src/module/planner.cpp

   NOLESS(initial_rate, MINIMAL_STEP_RATE);
   NOLESS(final_rate, MINIMAL_STEP_RATE);
 
-  int32_t accel = block->acceleration_steps_per_s2,
-          accelerate_steps = CEIL(estimate_acceleration_distance(initial_rate, block->nominal_rate, accel)),
+  const int32_t accel = block->acceleration_steps_per_s2;
+
+          // Steps required for acceleration, deceleration to/from nominal rate
+  int32_t accelerate_steps = CEIL(estimate_acceleration_distance(initial_rate, block->nominal_rate, accel)),
           decelerate_steps = FLOOR(estimate_acceleration_distance(block->nominal_rate, final_rate, -accel)),
+          // Steps between acceleration and deceleration, if any
           plateau_steps = block->step_event_count - accelerate_steps - decelerate_steps;
 
-  // Is the Plateau of Nominal Rate smaller than nothing? That means no cruising, and we will
-  // have to use intersection_distance() to calculate when to abort accel and start braking
-  // in order to reach the final_rate exactly at the end of this block.
+  // Does accelerate_steps + decelerate_steps exceed step_event_count?
+  // Then we can't possibly reach the nominal rate, there will be no cruising.
+  // Use intersection_distance() to calculate accel / braking time in order to
+  // reach the final_rate exactly at the end of this block.
   if (plateau_steps < 0) {
     accelerate_steps = CEIL(intersection_distance(initial_rate, final_rate, accel, block->step_event_count));
     NOLESS(accelerate_steps, 0); // Check limits due to numerical round-off
   }
   float inverse_millimeters = 1.0 / block->millimeters;  // Inverse millimeters to remove multiple divides
 
-  // Calculate moves/second for this move. No divide by zero due to previous checks.
-  float inverse_mm_s = fr_mm_s * inverse_millimeters;
+  // Calculate inverse time for this move. No divide by zero due to previous checks.
+  // Example: At 120mm/s a 60mm move takes 0.5s. So this will give 2.0.
+  float inverse_secs = fr_mm_s * inverse_millimeters;
 
   const uint8_t moves_queued = movesplanned();
 
   // Slow down when the buffer starts to empty, rather than wait at the corner for a buffer refill
   #if ENABLED(SLOWDOWN) || ENABLED(ULTRA_LCD) || defined(XY_FREQUENCY_LIMIT)
     // Segment time im micro seconds
-    uint32_t segment_time_us = LROUND(1000000.0 / inverse_mm_s);
+    uint32_t segment_time_us = LROUND(1000000.0 / inverse_secs);
   #endif
   #if ENABLED(SLOWDOWN)
     if (WITHIN(moves_queued, 2, (BLOCK_BUFFER_SIZE) / 2 - 1)) {
       if (segment_time_us < min_segment_time_us) {
         // buffer is draining, add extra time.  The amount of time added increases if the buffer is still emptied more.
         const uint32_t nst = segment_time_us + LROUND(2 * (min_segment_time_us - segment_time_us) / moves_queued);
-        inverse_mm_s = 1000000.0 / nst;
+        inverse_secs = 1000000.0 / nst;
         #if defined(XY_FREQUENCY_LIMIT) || ENABLED(ULTRA_LCD)
           segment_time_us = nst;
         #endif
     CRITICAL_SECTION_END
   #endif
 
-  block->nominal_speed = block->millimeters * inverse_mm_s; // (mm/sec) Always > 0
-  block->nominal_rate = CEIL(block->step_event_count * inverse_mm_s); // (step/sec) Always > 0
+  block->nominal_speed = block->millimeters * inverse_secs;           //   (mm/sec) Always > 0
+  block->nominal_rate = CEIL(block->step_event_count * inverse_secs); // (step/sec) Always > 0
 
   #if ENABLED(FILAMENT_WIDTH_SENSOR)
     static float filwidth_e_count = 0, filwidth_delay_dist = 0;
   // Calculate and limit speed in mm/sec for each axis
   float current_speed[NUM_AXIS], speed_factor = 1.0; // factor <1 decreases speed
   LOOP_XYZE(i) {
-    const float cs = FABS((current_speed[i] = delta_mm[i] * inverse_mm_s));
+    const float cs = FABS((current_speed[i] = delta_mm[i] * inverse_secs));
     #if ENABLED(DISTINCT_E_FACTORS)
       if (i == E_AXIS) i += extruder;
     #endif
   previous_safe_speed = safe_speed;
 
   #if ENABLED(LIN_ADVANCE)
-
     /**
      *
      * Use LIN_ADVANCE for blocks if all these are true:

Marlin/src/module/planner.h

     /**
      * The current block. NULL if the buffer is empty.
      * This also marks the block as busy.
+     * WARNING: Called from Stepper ISR context!
      */
     static block_t* get_current_block() {
       if (blocks_queued()) {
     }
 
     /**
-     * Return the point at which you must start braking (at the rate of -'acceleration') if
+     * Return the point at which you must start braking (at the rate of -'accel') if
      * you start at 'initial_rate', accelerate (until reaching the point), and want to end at
      * 'final_rate' after traveling 'distance'.
      *