stepper: Add some comments on s->position tracking

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/stepper.c b/src/stepper.c
index dfab89df..2e93ccb9 100644
--- a/src/stepper.c
+++ b/src/stepper.c
@@ -62,6 +62,7 @@ stepper_load_next(struct stepper *s, uint32_t min_next_time)
 {
     struct stepper_move *m = s->first;
     if (!m) {
+        // There is no next move - the queue is empty
         if (s->interval - s->add < s->min_stop_interval
             && !(s->flags & SF_NO_NEXT_CHECK))
             shutdown("No next step");
@@ -69,6 +70,7 @@ stepper_load_next(struct stepper *s, uint32_t min_next_time)
         return SF_DONE;
     }
 
+    // Load next 'struct stepper_move' into 'struct stepper'
     s->next_step_time += m->interval;
     s->add = m->add;
     s->interval = m->interval + m->add;
@@ -91,6 +93,7 @@ stepper_load_next(struct stepper *s, uint32_t min_next_time)
         }
         s->count = (uint32_t)m->count * 2;
     }
+    // Add all steps to s->position (stepper_get_position() can calc mid-move)
     if (m->flags & MF_DIR) {
         s->position = -s->position + m->count;
         gpio_out_toggle_noirq(s->dir_pin);
@@ -274,10 +277,12 @@ static uint32_t
 stepper_get_position(struct stepper *s)
 {
     uint32_t position = s->position;
+    // If stepper is mid-move, subtract out steps not yet taken
     if (CONFIG_STEP_DELAY <= 0)
         position -= s->count;
     else
         position -= s->count / 2;
+    // The top bit of s->position is an optimized reverse direction flag
     if (position & 0x80000000)
         return -position;
     return position;