usb: device_next: cdc_acm: Prevent polling for buffer in TX

In case the host doesn't pull the new data from the endpoint, the work task
would schedule itself again delayed (at the max. priority). When there is
no terminal program or active application reading the endpoint this
results in a constant polling of the endpoint burning up to 5% of the
CPU cycles.

By using a atomic flag for tx busy, the polling is solved and changed into
a postponed execution of the next work task which saves up to 5% of
CPU cycles and allows a better real-time behavior for other tasks.

Secondly, if the TX interrupt is disabled but there is still data in the TX
FIFO (ring buffer), the implementation will continue to trigger subsequent
TX work and attempt to flush the data to the host.

Signed-off-by: Vincent van der Locht <[email protected]>

Author: SynchronicIT

subsys/usb/device_next/class/usbd_cdc_acm.c

@@ -49,6 +49,7 @@ UDC_BUF_POOL_DEFINE(cdc_acm_ep_pool,
 #define CDC_ACM_IRQ_RX_ENABLED		2
 #define CDC_ACM_IRQ_TX_ENABLED		3
 #define CDC_ACM_RX_FIFO_BUSY		4
+#define CDC_ACM_TX_FIFO_BUSY		5
 
 static struct k_work_q cdc_acm_work_q;
 static K_KERNEL_STACK_DEFINE(cdc_acm_stack,
@@ -228,6 +229,10 @@ static int usbd_cdc_acm_request(struct usbd_class_data *const c_data,
 			atomic_clear_bit(&data->state, CDC_ACM_RX_FIFO_BUSY);
 		}
 
+		if (bi->ep == cdc_acm_get_bulk_in(c_data)) {
+			atomic_clear_bit(&data->state, CDC_ACM_TX_FIFO_BUSY);
+		}
+
 		goto ep_request_error;
 	}
 
@@ -250,6 +255,14 @@ static int usbd_cdc_acm_request(struct usbd_class_data *const c_data,
 		if (data->cb) {
 			cdc_acm_work_submit(&data->irq_cb_work);
 		}
+
+		atomic_clear_bit(&data->state, CDC_ACM_TX_FIFO_BUSY);
+
+		if (!ring_buf_is_empty(data->tx_fifo.rb)) {
+			/* Queue pending TX data on IN endpoint */
+			cdc_acm_work_schedule(&data->tx_fifo_work, K_NO_WAIT);
+		}
+
 	}
 
 	if (bi->ep == cdc_acm_get_int_in(c_data)) {
@@ -548,8 +561,14 @@ static void cdc_acm_tx_fifo_handler(struct k_work *work)
 		return;
 	}
 
+	if (atomic_test_and_set_bit(&data->state, CDC_ACM_TX_FIFO_BUSY)) {
+		LOG_DBG("TX transfer already in progress");
+		return;
+	}
+
 	buf = cdc_acm_buf_alloc(cdc_acm_get_bulk_in(c_data));
 	if (buf == NULL) {
+		atomic_clear_bit(&data->state, CDC_ACM_TX_FIFO_BUSY);
 		cdc_acm_work_schedule(&data->tx_fifo_work, K_MSEC(1));
 		return;
 	}
@@ -561,6 +580,7 @@ static void cdc_acm_tx_fifo_handler(struct k_work *work)
 	if (ret) {
 		LOG_ERR("Failed to enqueue");
 		net_buf_unref(buf);
+		atomic_clear_bit(&data->state, CDC_ACM_TX_FIFO_BUSY);
 	}
 }
 
@@ -828,7 +848,9 @@ static void cdc_acm_irq_cb_handler(struct k_work *work)
 
 	if (data->tx_fifo.altered) {
 		LOG_DBG("tx fifo altered, submit work");
-		cdc_acm_work_schedule(&data->tx_fifo_work, K_NO_WAIT);
+		if (!atomic_test_bit(&data->state, CDC_ACM_TX_FIFO_BUSY)) {
+			cdc_acm_work_schedule(&data->tx_fifo_work, K_NO_WAIT);
+		}
 	}
 
 	if (atomic_test_bit(&data->state, CDC_ACM_IRQ_RX_ENABLED) &&