serial: tegra: add support to adjust baud rate

Add support to adjust baud rates to fall under supported tolerance
range through DT.

Tegra186 chip has a hardware issue resulting in frame errors when
tolerance level for baud rate is negative. Provided entries to adjust
baud rate to be within acceptable range and work with devices that
can send negative baud rate. Also report error when baud rate set is
out of tolerance range of controller updated in device tree.
Signed-off-by: Shardar Shariff Md <smohammed@nvidia.com>
Signed-off-by: Krishna Yarlagadda <kyarlagadda@nvidia.com>
Link: https://lore.kernel.org/r/1567572187-29820-11-git-send-email-kyarlagadda@nvidia.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial-tegra.c

@@ -91,6 +91,12 @@ struct tegra_uart_chip_data {
 	int	max_dma_burst_bytes;
 };
 
+struct tegra_baud_tolerance {
+	u32 lower_range_baud;
+	u32 upper_range_baud;
+	s32 tolerance;
+};
+
 struct tegra_uart_port {
 	struct uart_port			uport;
 	const struct tegra_uart_chip_data	*cdata;
@@ -127,6 +133,8 @@ struct tegra_uart_port {
 	dma_cookie_t				rx_cookie;
 	unsigned int				tx_bytes_requested;
 	unsigned int				rx_bytes_requested;
+	struct tegra_baud_tolerance		*baud_tolerance;
+	int					n_adjustable_baud_rates;
 };
 
 static void tegra_uart_start_next_tx(struct tegra_uart_port *tup);
@@ -327,6 +335,21 @@ static void tegra_uart_fifo_reset(struct tegra_uart_port *tup, u8 fcr_bits)
 		set_rts(tup, true);
 }
 
+static long tegra_get_tolerance_rate(struct tegra_uart_port *tup,
+				     unsigned int baud, long rate)
+{
+	int i;
+
+	for (i = 0; i < tup->n_adjustable_baud_rates; ++i) {
+		if (baud >= tup->baud_tolerance[i].lower_range_baud &&
+		    baud <= tup->baud_tolerance[i].upper_range_baud)
+			return (rate + (rate *
+				tup->baud_tolerance[i].tolerance) / 10000);
+	}
+
+	return rate;
+}
+
 static int tegra_set_baudrate(struct tegra_uart_port *tup, unsigned int baud)
 {
 	unsigned long rate;
@@ -340,6 +363,9 @@ static int tegra_set_baudrate(struct tegra_uart_port *tup, unsigned int baud)
 
 	if (tup->cdata->support_clk_src_div) {
 		rate = baud * 16;
+		if (tup->n_adjustable_baud_rates)
+			rate = tegra_get_tolerance_rate(tup, baud, rate);
+
 		ret = clk_set_rate(tup->uart_clk, rate);
 		if (ret < 0) {
 			dev_err(tup->uport.dev,
@@ -1311,6 +1337,12 @@ static int tegra_uart_parse_dt(struct platform_device *pdev,
 {
 	struct device_node *np = pdev->dev.of_node;
 	int port;
+	int ret;
+	int index;
+	u32 pval;
+	int count;
+	int n_entries;
+
 
 	port = of_alias_get_id(np, "serial");
 	if (port < 0) {
@@ -1321,6 +1353,42 @@ static int tegra_uart_parse_dt(struct platform_device *pdev,
 
 	tup->enable_modem_interrupt = of_property_read_bool(np,
 					"nvidia,enable-modem-interrupt");
+	n_entries = of_property_count_u32_elems(np, "nvidia,adjust-baud-rates");
+	if (n_entries > 0) {
+		tup->n_adjustable_baud_rates = n_entries / 3;
+		tup->baud_tolerance =
+		devm_kzalloc(&pdev->dev, (tup->n_adjustable_baud_rates) *
+			     sizeof(*tup->baud_tolerance), GFP_KERNEL);
+		if (!tup->baud_tolerance)
+			return -ENOMEM;
+		for (count = 0, index = 0; count < n_entries; count += 3,
+		     index++) {
+			ret =
+			of_property_read_u32_index(np,
+						   "nvidia,adjust-baud-rates",
+						   count, &pval);
+			if (!ret)
+				tup->baud_tolerance[index].lower_range_baud =
+				pval;
+			ret =
+			of_property_read_u32_index(np,
+						   "nvidia,adjust-baud-rates",
+						   count + 1, &pval);
+			if (!ret)
+				tup->baud_tolerance[index].upper_range_baud =
+				pval;
+			ret =
+			of_property_read_u32_index(np,
+						   "nvidia,adjust-baud-rates",
+						   count + 2, &pval);
+			if (!ret)
+				tup->baud_tolerance[index].tolerance =
+				(s32)pval;
+		}
+	} else {
+		tup->n_adjustable_baud_rates = 0;
+	}
+
 	return 0;
 }