Fix broken reverse planner (#9943)

From #9914 by @Sebastianv650

Marlin/src/module/planner.cpp

@@ -289,18 +289,26 @@ void Planner::reverse_pass_kernel(block_t* const current, const block_t * const
  * Once in reverse and once forward. This implements the reverse pass.
  */
 void Planner::reverse_pass() {
-  if (movesplanned() > 3) {
-    const uint8_t endnr = BLOCK_MOD(block_buffer_tail + 2); // tail is running. tail+1 shouldn't be altered because it's connected to the running block.
-                                                            // tail+2 because the index is not yet advanced when checked
+  if (movesplanned() > 2) {
+    const uint8_t endnr = BLOCK_MOD(block_buffer_tail + 1); // tail is running. tail+1 shouldn't be altered because it's connected to the running block.
     uint8_t blocknr = prev_block_index(block_buffer_head);
     block_t* current = &block_buffer[blocknr];
 
+    // Last/newest block in buffer:
+    const float max_entry_speed = current->max_entry_speed;
+    if (current->entry_speed != max_entry_speed) {
+      // If nominal length true, max junction speed is guaranteed to be reached. Only compute
+      // for max allowable speed if block is decelerating and nominal length is false.
+      current->entry_speed = TEST(current->flag, BLOCK_BIT_NOMINAL_LENGTH)
+        ? max_entry_speed
+        : min(max_entry_speed, max_allowable_speed(-current->acceleration, MINIMUM_PLANNER_SPEED, current->millimeters));
+      SBI(current->flag, BLOCK_BIT_RECALCULATE);
+    }
+
     do {
       const block_t * const next = current;
       blocknr = prev_block_index(blocknr);
       current = &block_buffer[blocknr];
-      if (TEST(current->flag, BLOCK_BIT_START_FROM_FULL_HALT)) // Up to this every block is already optimized.
-        break;
       reverse_pass_kernel(current, next);
     } while (blocknr != endnr);
   }
@@ -920,7 +928,6 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE]
 
   // Enable extruder(s)
   if (esteps) {
-
     #if ENABLED(AUTO_POWER_CONTROL)
       powerManager.power_on();
     #endif
@@ -1425,17 +1432,11 @@ void Planner::_buffer_steps(const int32_t (&target)[XYZE]
     // Now the transition velocity is known, which maximizes the shared exit / entry velocity while
     // respecting the jerk factors, it may be possible, that applying separate safe exit / entry velocities will achieve faster prints.
     const float vmax_junction_threshold = vmax_junction * 0.99f;
-    if (previous_safe_speed > vmax_junction_threshold && safe_speed > vmax_junction_threshold) {
-      // Not coasting. The machine will stop and start the movements anyway,
-      // better to start the segment from start.
-      SBI(block->flag, BLOCK_BIT_START_FROM_FULL_HALT);
+    if (previous_safe_speed > vmax_junction_threshold && safe_speed > vmax_junction_threshold)
       vmax_junction = safe_speed;
-    }
   }
-  else {
-    SBI(block->flag, BLOCK_BIT_START_FROM_FULL_HALT);
+  else
     vmax_junction = safe_speed;
-  }
 
   // Max entry speed of this block equals the max exit speed of the previous block.
   block->max_entry_speed = vmax_junction;

Marlin/src/module/planner.h

@@ -53,9 +53,6 @@ enum BlockFlagBit {
   // from a safe speed (in consideration of jerking from zero speed).
   BLOCK_BIT_NOMINAL_LENGTH,
 
-  // Start from a halt at the start of this block, respecting the maximum allowed jerk.
-  BLOCK_BIT_START_FROM_FULL_HALT,
-
   // The block is busy
   BLOCK_BIT_BUSY,
 
@@ -66,7 +63,6 @@ enum BlockFlagBit {
 enum BlockFlag {
   BLOCK_FLAG_RECALCULATE          = _BV(BLOCK_BIT_RECALCULATE),
   BLOCK_FLAG_NOMINAL_LENGTH       = _BV(BLOCK_BIT_NOMINAL_LENGTH),
-  BLOCK_FLAG_START_FROM_FULL_HALT = _BV(BLOCK_BIT_START_FROM_FULL_HALT),
   BLOCK_FLAG_BUSY                 = _BV(BLOCK_BIT_BUSY),
   BLOCK_FLAG_CONTINUED            = _BV(BLOCK_BIT_CONTINUED)
 };