usb_canbus: Send echo frame before processing the frame

The Linux kernel reports a canbus message as transmitted when it gets
the echo frame back.  Processing the message prior to sending the echo
frame can lead to odd looking debugging logs (as the response messages
may appear to predate the request messages).  This doesn't impact the
Klipper code, but it does make analyzing logs harder.  Fix by sending
the echo frame prior to processing the frame.

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

src/generic/usb_canbus.c

@@ -240,52 +240,55 @@ static void
 drain_host_queue(void)
 {
     uint32_t pull_pos = UsbCan.host_pull_pos, push_pos = UsbCan.host_push_pos;
-    uint32_t pullp = pull_pos % ARRAY_SIZE(UsbCan.host_frames);
-    struct gs_host_frame *gs = &UsbCan.host_frames[pullp];
     for (;;) {
-        // See if previous host frame needs to be transmitted
+        uint32_t pullp = pull_pos % ARRAY_SIZE(UsbCan.host_frames);
+        struct gs_host_frame *gs = &UsbCan.host_frames[pullp];
         uint_fast8_t host_status = UsbCan.host_status;
-        if (host_status & (HS_TX_HW | HS_TX_LOCAL)) {
-            struct canbus_msg msg;
-            msg.id = gs->can_id;
-            msg.dlc = gs->can_dlc;
-            msg.data32[0] = gs->data32[0];
-            msg.data32[1] = gs->data32[1];
-            if (host_status & HS_TX_LOCAL) {
-                canserial_process_data(&msg);
-                UsbCan.host_status = host_status = host_status & ~HS_TX_LOCAL;
-            }
-            if (host_status & HS_TX_HW) {
-                int ret = try_canmsg_send(&msg);
-                if (ret < 0)
-                    break;
-                UsbCan.host_status = host_status = host_status & ~HS_TX_HW;
-            }
+
+        // Extract next frame from host
+        if (! host_status) {
+            if (pull_pos == push_pos)
+                // No frame available - no more work to be done
+                break;
+            uint32_t id = gs->can_id;
+            host_status = HS_TX_ECHO | HS_TX_HW;
+            if (id == CANBUS_ID_ADMIN)
+                host_status = HS_TX_ECHO | HS_TX_HW | HS_TX_LOCAL;
+            else if (UsbCan.assigned_id && UsbCan.assigned_id == id)
+                host_status = HS_TX_ECHO | HS_TX_LOCAL;
+            UsbCan.host_status = host_status;
         }
 
-        // Send any previous echo frames
-        if (host_status) {
+        // Send echo frames back to host
+        if (host_status & HS_TX_ECHO) {
             if (UsbCan.notify_local || UsbCan.usb_send_busy)
                 // Don't send echo frame until other traffic is sent
                 break;
             int ret = usb_send_bulk_in(gs, sizeof(*gs));
             if (ret < 0)
                 break;
-            UsbCan.host_status = 0;
-            UsbCan.host_pull_pos = pull_pos = pull_pos + 1;
+            UsbCan.host_status = host_status = host_status & ~HS_TX_ECHO;
         }
 
-        // Process next frame from host
-        if (pull_pos == push_pos)
-            // No frame available - no more work to be done
-            break;
-        gs = &UsbCan.host_frames[pull_pos % ARRAY_SIZE(UsbCan.host_frames)];
-        uint32_t id = gs->can_id;
-        UsbCan.host_status = HS_TX_ECHO | HS_TX_HW;
-        if (id == CANBUS_ID_ADMIN)
-            UsbCan.host_status = HS_TX_ECHO | HS_TX_HW | HS_TX_LOCAL;
-        else if (UsbCan.assigned_id && UsbCan.assigned_id == id)
-            UsbCan.host_status = HS_TX_ECHO | HS_TX_LOCAL;
+        // See if host frame needs to be transmitted
+        struct canbus_msg msg;
+        msg.id = gs->can_id;
+        msg.dlc = gs->can_dlc;
+        msg.data32[0] = gs->data32[0];
+        msg.data32[1] = gs->data32[1];
+        if (host_status & HS_TX_LOCAL) {
+            canserial_process_data(&msg);
+            UsbCan.host_status = host_status = host_status & ~HS_TX_LOCAL;
+        }
+        if (host_status & HS_TX_HW) {
+            int ret = try_canmsg_send(&msg);
+            if (ret < 0)
+                break;
+            UsbCan.host_status = host_status = host_status & ~HS_TX_HW;
+        }
+
+        // Note message fully processed
+        UsbCan.host_pull_pos = pull_pos = pull_pos + 1;
     }
 }