usb: device_next: class: cdc_acm: configurable buffer size

When a custom pool is used for CDC ACM, allow the buffer sizes to be
configured in Kconfig.

Signed-off-by: Jordan Yates <jordan@embeint.com>

Author: JordanYates

subsys/usb/device_next/class/Kconfig.cdc_acm

@@ -37,6 +37,16 @@ config USBD_CDC_ACM_BUF_POOL
 	  ACM instances and the size of the bulk endpoints. When disabled, the
 	  implementation uses the UDC driver's pool.
 
+config USBD_CDC_ACM_BUF_POOL_SIZE
+	int "Size of the buffers in the dedicated buffer pool"
+	depends on USBD_CDC_ACM_BUF_POOL
+	default 512 if USBD_MAX_SPEED_HIGH
+	default 64 if USBD_MAX_SPEED_FULL
+	help
+	  Increasing the size of buffers past the endpoint size can reduce the
+	  number of round trips through the CDC ACM class when the TX FIFO is larger,
+	  than the endpoint size, increasing throughput.
+
 module = USBD_CDC_ACM
 module-str = usbd cdc_acm
 default-count = 1

subsys/usb/device_next/class/usbd_cdc_acm.c

@@ -132,7 +132,8 @@ static void cdc_acm_irq_rx_enable(const struct device *dev);
 #if CONFIG_USBD_CDC_ACM_BUF_POOL
 UDC_BUF_POOL_DEFINE(cdc_acm_ep_pool,
 		    DT_NUM_INST_STATUS_OKAY(DT_DRV_COMPAT) * 2,
-		    USBD_MAX_BULK_MPS, sizeof(struct udc_buf_info), NULL);
+		    CONFIG_USBD_CDC_ACM_BUF_POOL_SIZE, 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, size_t size_hint)