serial: imx: Add DMA buffer configuration via DT

In order to optimize serial communication (performance/throughput VS
latency), we may need to tweak DMA period number and size. This adds
DT properties to configure those values before initialising DMA.
The defaults will stay the same as before.

[update documentation and commit message, rebase to current master,
switch back to DT instead of sysfs]

Signed-off-by: Fabien Lahoudere <[e-mail]>
Signed-off-by: Sebastian Reichel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml

···
       received, and that the peripheral should invert its input using the
       INVR registers.
 
+  fsl,dma-info:
+    $ref: /schemas/types.yaml#/definitions/uint32-array
+    minItems: 2
+    maxItems: 2
+    description: |
+      First cell contains the size of DMA buffer chunks, second cell contains
+      the amount of chunks used for the device. Multiplying both numbers is
+      the total size of memory used for receiving data.
+      When not being configured the system will use default settings, which
+      are sensible for most use cases. If you need low latency processing on
+      slow connections this needs to be configured appropriately.
+
   uart-has-rtscts: true
 
   rs485-rts-delay: true

drivers/tty/serial/imx.c

···
 	struct scatterlist	rx_sgl, tx_sgl[2];
 	void			*rx_buf;
 	struct circ_buf		rx_ring;
+	unsigned int		rx_buf_size;
+	unsigned int		rx_period_length;
 	unsigned int		rx_periods;
 	dma_cookie_t		rx_cookie;
 	unsigned int		tx_bytes;
···
 	}
 }
 
-/* RX DMA buffer periods */
-#define RX_DMA_PERIODS	16
-#define RX_BUF_SIZE	(RX_DMA_PERIODS * PAGE_SIZE / 4)
-
 static int imx_uart_start_rx_dma(struct imx_port *sport)
 {
 	struct scatterlist *sgl = &sport->rx_sgl;
···
 
 	sport->rx_ring.head = 0;
 	sport->rx_ring.tail = 0;
-	sport->rx_periods = RX_DMA_PERIODS;
 
-	sg_init_one(sgl, sport->rx_buf, RX_BUF_SIZE);
+	sg_init_one(sgl, sport->rx_buf, sport->rx_buf_size);
 	ret = dma_map_sg(dev, sgl, 1, DMA_FROM_DEVICE);
 	if (ret == 0) {
 		dev_err(dev, "DMA mapping error for RX.\n");
···
 		goto err;
 	}
 
-	sport->rx_buf = kzalloc(RX_BUF_SIZE, GFP_KERNEL);
+	sport->rx_buf_size = sport->rx_period_length * sport->rx_periods;
+	sport->rx_buf = kzalloc(sport->rx_buf_size, GFP_KERNEL);
 	if (!sport->rx_buf) {
 		ret = -ENOMEM;
 		goto err;
···
 	return HRTIMER_NORESTART;
 }
 
+/* Default RX DMA buffer configuration */
+#define RX_DMA_PERIODS		16
+#define RX_DMA_PERIOD_LEN	(PAGE_SIZE / 4)
+
 static int imx_uart_probe(struct platform_device *pdev)
 {
 	struct device_node *np = pdev->dev.of_node;
 	struct imx_port *sport;
 	void __iomem *base;
+	u32 dma_buf_conf[2];
 	int ret = 0;
 	u32 ucr1;
 	struct resource *res;
···
 
 	if (of_get_property(np, "fsl,inverted-rx", NULL))
 		sport->inverted_rx = 1;
+
+	if (!of_property_read_u32_array(np, "fsl,dma-info", dma_buf_conf, 2)) {
+		sport->rx_period_length = dma_buf_conf[0];
+		sport->rx_periods = dma_buf_conf[1];
+	} else {
+		sport->rx_period_length = RX_DMA_PERIOD_LEN;
+		sport->rx_periods = RX_DMA_PERIODS;
+	}
 
 	if (sport->port.line >= ARRAY_SIZE(imx_uart_ports)) {
 		dev_err(&pdev->dev, "serial%d out of range\n",