usb: device_next: class: cdc_acm: batch writes

Do not artificially limit the size of the USB request to the endpoint
size, as this introduces multiple round trips through the CDC ACM class
implementation. The USB stack handles packetizing to the maximum
endpoint size. The maximum allocation size is limited to prevent
starving other USB users.

Throughput testing on a nRF52840DK with `CONFIG_UDC_BUF_POOL_SIZE=1024`
shows a throughput increase from ~1.7 Mbps to ~2.7 Mbps.

Signed-off-by: Jordan Yates <[email protected]>

Author: JordanYates

subsys/usb/device_next/class/usbd_cdc_acm.c

@@ -135,9 +135,10 @@ UDC_BUF_POOL_DEFINE(cdc_acm_ep_pool,
 		    USBD_MAX_BULK_MPS, sizeof(struct udc_buf_info), NULL);
 
 static struct net_buf *cdc_acm_buf_alloc(struct usbd_class_data *const c_data,
-					 const uint8_t ep)
+					 const uint8_t ep, const size_t size_hint)
 {
 	ARG_UNUSED(c_data);
+	ARG_UNUSED(size_hint);
 	struct net_buf *buf = NULL;
 	struct udc_buf_info *bi;
 
@@ -157,9 +158,21 @@ static struct net_buf *cdc_acm_buf_alloc(struct usbd_class_data *const c_data,
  * common (UDC) buffer, as this results in a smaller footprint.
  */
 static struct net_buf *cdc_acm_buf_alloc(struct usbd_class_data *const c_data,
-					 const uint8_t ep)
+					 const uint8_t ep, const size_t size_hint)
 {
-	return usbd_ep_buf_alloc(c_data, ep, USBD_MAX_BULK_MPS);
+	/* The lower layers handle packetizing to the endpoint MPS, therefore
+	 * allocating buffers largers than the endpoint MPS reduces the number
+	 * of round trips through this class driver when the TX FIFO size is
+	 * larger than the endpoint MPS, improving throughput.
+	 *
+	 * Limit the maximum allocation size to 1/4 of the total pool size
+	 * to prevent starving other USB users. Ensure the maximum allocation
+	 * never drops below USBD_MAX_BULK_MPS.
+	 */
+	size_t max_alloc = max(USBD_MAX_BULK_MPS, CONFIG_UDC_BUF_POOL_SIZE / 4);
+	size_t alloc_size = min(max_alloc, size_hint);
+
+	return usbd_ep_buf_alloc(c_data, ep, alloc_size);
 }
 #endif /* CONFIG_USBD_CDC_ACM_BUF_POOL */
 
@@ -651,7 +664,8 @@ static void cdc_acm_tx_fifo_handler(struct k_work *work)
 		return;
 	}
 
-	buf = cdc_acm_buf_alloc(c_data, cdc_acm_get_bulk_in(c_data));
+	buf = cdc_acm_buf_alloc(c_data, cdc_acm_get_bulk_in(c_data),
+				ring_buf_size_get(data->tx_fifo.rb));
 	if (buf == NULL) {
 		atomic_clear_bit(&data->state, CDC_ACM_TX_FIFO_BUSY);
 		cdc_acm_work_schedule(&data->tx_fifo_work, K_MSEC(1));
@@ -707,7 +721,7 @@ static void cdc_acm_rx_fifo_handler(struct k_work *work)
 		return;
 	}
 
-	buf = cdc_acm_buf_alloc(c_data, cdc_acm_get_bulk_out(c_data));
+	buf = cdc_acm_buf_alloc(c_data, cdc_acm_get_bulk_out(c_data), USBD_MAX_BULK_MPS);
 	if (buf == NULL) {
 		return;
 	}