usbd: device_next: cdc: add option to enable short packet terminate

For usbser.sys driver, which is the default USB host driver for CDC
devices in windows, it is expected that USB device always indicate
completion of transmission by short packet. In case where the last
packet length is multiple of max packet size of the BULK IN endpoint,
the USB device shall indicate completion of transmission by sending a
zero length packet. This commit adds an option to usbd_cdc_acm to
enable the enforcement of zero length packet mentioned above, ensuring
that condition of short packet terminate is fulfilled.

Signed-off-by: Chew Zeh Yang <[email protected]>

subsys/usb/device_next/class/Kconfig.cdc_acm

@@ -21,6 +21,16 @@ config USBD_CDC_ACM_STACK_SIZE
 	help
 	  USB CDC ACM workqueue stack size.
 
+config USBD_CDC_ACM_ENSURE_SHORT_PKT_FOR_LAST_TX_PKT
+	bool "USB CDC ACM transmission always end with short packet"
+	default n
+	help
+	  If the length of CDC ACM TX payload to be sent to USB Host
+	  is in multiple of max-packet-size of the BULK-IN endpoint,
+	  a Zero-Length Packet will be sent followig the last packet
+	  of the payload, to ensure short packet termination for the
+	  BULK-IN endpoint.
+
 module = USBD_CDC_ACM
 module-str = usbd cdc_acm
 default-count = 1

subsys/usb/device_next/class/usbd_cdc_acm.c

@@ -549,6 +549,12 @@ static void cdc_acm_tx_fifo_handler(struct k_work *work)
 	len = ring_buf_get(data->tx_fifo.rb, buf->data, buf->size);
 	net_buf_add(buf, len);
 
+	if (IS_ENABLED(CONFIG_USBD_CDC_ACM_ENSURE_SHORT_PKT_FOR_LAST_TX_PKT)) {
+		if (len != 0 && len % cdc_acm_get_bulk_mps(c_data) == 0) {
+			udc_ep_buf_set_zlp(buf);
+		}
+	}
+
 	ret = usbd_ep_enqueue(c_data, buf);
 	if (ret) {
 		LOG_ERR("Failed to enqueue");