tty: serial: fsl_lpuart: optimize the timer based EOP logic

DMA transfer may end prematurely due to the DMA Rx timeout even during an
active transfer because a constant timeout does not accurately simulate an
EOP (End Of Package) event. This patch uses a timer to simulate the
hardware EOP event. The timer should only complete a DMA transfer once the
idle period satisfies a specified interval which is baud rate dependent.
The problem has been observed with low baud rates but could occur also
with high baud rates.

Make the DMA Rx timeout baud rate dependent and check the DMA residue count
before copying data to the TTY buffer. If the residue count remains
unchanged since the last interrupt, that indicates no new data was
received. In this case, the DMA should complete as an EOP event. Otherwise,
new data was received during the interval and the EOP condition is not met
so restart the DMA Rx timeout
Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20230510203359.1353469-1-shenwei.wang@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -238,6 +238,7 @@
 
 /* Rx DMA timeout in ms, which is used to calculate Rx ring buffer size */
 #define DMA_RX_TIMEOUT		(10)
+#define DMA_RX_IDLE_CHARS	8
 #define UART_AUTOSUSPEND_TIMEOUT	3000
 
 #define DRIVER_NAME	"fsl-lpuart"
@@ -282,6 +283,7 @@ struct lpuart_port {
 	struct scatterlist	rx_sgl, tx_sgl[2];
 	struct circ_buf		rx_ring;
 	int			rx_dma_rng_buf_len;
+	int                     last_residue;
 	unsigned int		dma_tx_nents;
 	wait_queue_head_t	dma_wait;
 	bool			is_cs7; /* Set to true when character size is 7 */
@@ -331,7 +333,7 @@ static struct lpuart_soc_data imx8qxp_data = {
 	.devtype = IMX8QXP_LPUART,
 	.iotype = UPIO_MEM32,
 	.reg_off = IMX_REG_OFF,
-	.rx_watermark = 31,
+	.rx_watermark = 7, /* A lower watermark is ideal for low baud rates. */
 };
 static struct lpuart_soc_data imxrt1050_data = {
 	.devtype = IMXRT1050_LPUART,
@@ -1255,6 +1257,8 @@ static void lpuart_copy_rx_to_tty(struct lpuart_port *sport)
 		sport->port.icount.rx += copied;
 	}
 
+	sport->last_residue = state.residue;
+
 exit:
 	dma_sync_sg_for_device(chan->device->dev, &sport->rx_sgl, 1,
 			       DMA_FROM_DEVICE);
@@ -1272,11 +1276,37 @@ static void lpuart_dma_rx_complete(void *arg)
 	lpuart_copy_rx_to_tty(sport);
 }
 
+/*
+ * Timer function to simulate the hardware EOP (End Of Package) event.
+ * The timer callback is to check for new RX data and copy to TTY buffer.
+ * If no new data are received since last interval, the EOP condition is
+ * met, complete the DMA transfer by copying the data. Otherwise, just
+ * restart timer.
+ */
 static void lpuart_timer_func(struct timer_list *t)
 {
 	struct lpuart_port *sport = from_timer(sport, t, lpuart_timer);
+	struct dma_chan *chan = sport->dma_rx_chan;
+	struct circ_buf *ring = &sport->rx_ring;
+	struct dma_tx_state state;
+	unsigned long flags;
+	int count;
 
-	lpuart_copy_rx_to_tty(sport);
+	dmaengine_tx_status(chan, sport->dma_rx_cookie, &state);
+	ring->head = sport->rx_sgl.length - state.residue;
+	count = CIRC_CNT(ring->head, ring->tail, sport->rx_sgl.length);
+
+	/* Check if new data received before copying */
+	if ((count != 0) && (sport->last_residue == state.residue))
+		lpuart_copy_rx_to_tty(sport);
+	else
+		mod_timer(&sport->lpuart_timer,
+			  jiffies + sport->dma_rx_timeout);
+
+	if (spin_trylock_irqsave(&sport->port.lock, flags)) {
+		sport->last_residue = state.residue;
+		spin_unlock_irqrestore(&sport->port.lock, flags);
+	}
 }
 
 static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
@@ -1297,9 +1327,20 @@ static inline int lpuart_start_rx_dma(struct lpuart_port *sport)
 	 */
 	sport->rx_dma_rng_buf_len = (DMA_RX_TIMEOUT * baud /  bits / 1000) * 2;
 	sport->rx_dma_rng_buf_len = (1 << fls(sport->rx_dma_rng_buf_len));
+	sport->rx_dma_rng_buf_len = max_t(int,
+					  sport->rxfifo_size * 2,
+					  sport->rx_dma_rng_buf_len);
+	/*
+	 * Keep this condition check in case rxfifo_size is unavailable
+	 * for some SoCs.
+	 */
 	if (sport->rx_dma_rng_buf_len < 16)
 		sport->rx_dma_rng_buf_len = 16;
 
+	sport->last_residue = 0;
+	sport->dma_rx_timeout = max(nsecs_to_jiffies(
+		sport->port.frame_time * DMA_RX_IDLE_CHARS), 1UL);
+
 	ring->buf = kzalloc(sport->rx_dma_rng_buf_len, GFP_ATOMIC);
 	if (!ring->buf)
 		return -ENOMEM;
@@ -1687,12 +1728,13 @@ static void lpuart_rx_dma_startup(struct lpuart_port *sport)
 	if (!sport->dma_rx_chan)
 		goto err;
 
+	/* set default Rx DMA timeout */
+	sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
+
 	ret = lpuart_start_rx_dma(sport);
 	if (ret)
 		goto err;
 
-	/* set Rx DMA timeout */
-	sport->dma_rx_timeout = msecs_to_jiffies(DMA_RX_TIMEOUT);
 	if (!sport->dma_rx_timeout)
 		sport->dma_rx_timeout = 1;