pbdrv/usb: Transmit response message.

Transmit the response to a command. This is used to communicate any
errors in handling the command back to the user and provide backpressure
to ensure the next command isn't sent until the previous has been
handled.

Signed-off-by: Nate Karstens <[email protected]>

Author: nkarstens

lib/pbio/drv/usb/usb_stm32.c

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2022 The Pybricks Authors
+// Copyright (c) 2022-2025 The Pybricks Authors
 
 // Main file for STM32F4 USB driver.
 
@@ -18,6 +18,7 @@
 #include <usbd_pybricks.h>
 
 #include <pbdrv/usb.h>
+#include <pbio/protocol.h>
 #include <pbio/util.h>
 #include <pbsys/command.h>
 #include <pbsys/status.h>
@@ -30,7 +31,10 @@ PROCESS(pbdrv_usb_process, "USB");
 // These buffers need to be 32-bit aligned because the USB driver moves data
 // to/from FIFOs in 32-bit chunks.
 static uint8_t usb_in_buf[USBD_PYBRICKS_MAX_PACKET_SIZE] __aligned(4);
+static uint8_t usb_response_buf[1 + sizeof(uint32_t)] __aligned(4) __aligned(4);
 static volatile uint32_t usb_in_sz;
+static volatile uint32_t usb_response_sz;
+static volatile bool transmitting;
 
 static USBD_HandleTypeDef husbd;
 static PCD_HandleTypeDef hpcd;
@@ -141,6 +145,8 @@ void pbdrv_usb_stm32_handle_vbus_irq(bool active) {
 static USBD_StatusTypeDef Pybricks_Itf_Init(void) {
     USBD_Pybricks_SetRxBuffer(&husbd, usb_in_buf);
     usb_in_sz = 0;
+    usb_response_sz = 0;
+    transmitting = false;
 
     return USBD_OK;
 }
@@ -183,8 +189,17 @@ static USBD_StatusTypeDef Pybricks_Itf_Receive(uint8_t *Buf, uint32_t Len) {
   * @retval Result of the operation: USBD_OK if all operations are OK else USBD_FAIL
   */
 static USBD_StatusTypeDef Pybricks_Itf_TransmitCplt(uint8_t *Buf, uint32_t Len, uint8_t epnum) {
+    USBD_StatusTypeDef ret = USBD_OK;
+
+    if (Buf == usb_response_buf) {
+        usb_response_sz = 0;
+    } else {
+        ret = USBD_FAIL;
+    }
+
+    transmitting = false;
     process_poll(&pbdrv_usb_process);
-    return USBD_OK;
+    return ret;
 }
 
 static USBD_Pybricks_ItfTypeDef USBD_Pybricks_fops = {
@@ -225,6 +240,7 @@ PROCESS_THREAD(pbdrv_usb_process, ev, data) {
     static PBIO_ONESHOT(bcd_oneshot);
     static PBIO_ONESHOT(pwrdn_oneshot);
     static bool bcd_busy;
+    static pbio_pybricks_error_t result;
 
     PROCESS_POLLHANDLER({
         if (!bcd_busy && pbio_oneshot(!vbus_active, &no_vbus_oneshot)) {
@@ -269,14 +285,28 @@ PROCESS_THREAD(pbdrv_usb_process, ev, data) {
         if (usb_in_sz) {
             switch (usb_in_buf[0]) {
             case USBD_PYBRICKS_OUT_EP_MSG_COMMAND:
-                pbsys_command(usb_in_buf + 1, usb_in_sz - 1);
+                if (usb_response_sz == 0) {
+                    result = pbsys_command(usb_in_buf + 1, usb_in_sz - 1);
+                    usb_response_buf[0] = USBD_PYBRICKS_IN_EP_MSG_RESPONSE;
+                    pbio_set_uint32_le(&usb_response_buf[1], result);
+                    usb_response_sz = sizeof(usb_response_buf);
+                }
                 break;
             }
 
             // Prepare to receive the next packet
             usb_in_sz = 0;
             USBD_Pybricks_ReceivePacket(&husbd);
         }
+
+        if (transmitting) {
+            continue;
+        }
+
+        if (usb_response_sz) {
+            transmitting = true;
+            USBD_Pybricks_TransmitPacket(&husbd, usb_response_buf, usb_response_sz);
+        }
     }
 
     PROCESS_END();