Revert "Improved Core-compatible jerk code"

Reverting commit 3cd7659

Marlin/src/module/planner.cpp

     }
   #endif
 
-  // Calculate and limit speed in mm/sec for each axis, calculate minimum acceleration ratio
+  // Calculate and limit speed in mm/sec for each axis
   float current_speed[NUM_AXIS], speed_factor = 1.0; // factor <1 decreases speed
-  float max_stepper_speed = 0, min_axis_accel_ratio = 1; // ratio < 1 means acceleration ramp needed
   LOOP_XYZE(i) {
     const float cs = FABS((current_speed[i] = delta_mm[i] * inverse_secs));
-    if (cs > max_jerk[i])
-      NOMORE(min_axis_accel_ratio, max_jerk[i] / cs);
-    NOLESS(max_stepper_speed, cs);
     #if ENABLED(DISTINCT_E_FACTORS)
       if (i == E_AXIS) i += extruder;
     #endif
     }
   #endif // XY_FREQUENCY_LIMIT
 
-  block->nominal_speed = max_stepper_speed; // (mm/sec) Always > 0
-  block->nominal_rate = CEIL(block->step_event_count * inverse_secs); // (step/sec) Always > 0
-
   // Correct the speed
   if (speed_factor < 1.0) {
     LOOP_XYZE(i) current_speed[i] *= speed_factor;
     block->nominal_rate *= speed_factor;
   }
 
-  float safe_speed = block->nominal_speed * min_axis_accel_ratio;
-  static float previous_safe_speed;
-
   // Compute and limit the acceleration rate for the trapezoid generator.
   const float steps_per_mm = block->step_event_count * inverse_millimeters;
   uint32_t accel;
     }
   #endif
 
+  /**
+   * Adapted from Průša MKS firmware
+   * https://github.com/prusa3d/Prusa-Firmware
+   *
+   * Start with a safe speed (from which the machine may halt to stop immediately).
+   */
+
+  // Exit speed limited by a jerk to full halt of a previous last segment
+  static float previous_safe_speed;
+
+  float safe_speed = block->nominal_speed;
+  uint8_t limited = 0;
+  LOOP_XYZE(i) {
+    const float jerk = FABS(current_speed[i]), maxj = max_jerk[i];
+    if (jerk > maxj) {
+      if (limited) {
+        const float mjerk = maxj * block->nominal_speed;
+        if (jerk * safe_speed > mjerk) safe_speed = mjerk / jerk;
+      }
+      else {
+        ++limited;
+        safe_speed = maxj;
+      }
+    }
+  }
+
   if (moves_queued && !UNEAR_ZERO(previous_nominal_speed)) {
     // Estimate a maximum velocity allowed at a joint of two successive segments.
     // If this maximum velocity allowed is lower than the minimum of the entry / exit safe velocities,
 
     // Factor to multiply the previous / current nominal velocities to get componentwise limited velocities.
     float v_factor = 1;
-    uint8_t limited = 0;
+    limited = 0;
 
     // Now limit the jerk in all axes.
     const float smaller_speed_factor = vmax_junction / previous_nominal_speed;