serial: 8250_mtk: Enable baud clock and manage in runtime PM

Some MediaTek SoCs got a gated UART baud clock, which currently gets
disabled as the clk subsystem believes it would be unused. This results in
the uart freezing right after "clk: Disabling unused clocks" on those
platforms.

Request the baud clock to be prepared and enabled during probe, and to
restore run-time power management capabilities to what it was before commit
e32a83c ("serial: 8250-mtk: modify mtk uart power and clock
management") disable and unprepare the baud clock when suspending the UART,
prepare and enable it again when resuming it.

Fixes: e32a83c ("serial: 8250-mtk: modify mtk uart power and clock management")
Fixes: b6c7ff2 ("serial: 8250_mtk: Simplify clock sequencing and runtime PM")
Cc: stable <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno <[email protected]>
Signed-off-by: Daniel Golle <[email protected]>
Link: https://patch.msgid.link/de5197ccc31e1dab0965cabcc11ca92e67246cf6.1758058441.git.daniel@makrotopia.org
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: dangowrt

drivers/tty/serial/8250/8250_mtk.c

@@ -435,6 +435,7 @@ static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
 	while
 		(serial_in(up, MTK_UART_DEBUG0));
 
+	clk_disable_unprepare(data->uart_clk);
 	clk_disable_unprepare(data->bus_clk);
 
 	return 0;
@@ -445,6 +446,7 @@ static int __maybe_unused mtk8250_runtime_resume(struct device *dev)
 	struct mtk8250_data *data = dev_get_drvdata(dev);
 
 	clk_prepare_enable(data->bus_clk);
+	clk_prepare_enable(data->uart_clk);
 
 	return 0;
 }
@@ -475,13 +477,13 @@ static int mtk8250_probe_of(struct platform_device *pdev, struct uart_port *p,
 	int dmacnt;
 #endif
 
-	data->uart_clk = devm_clk_get(&pdev->dev, "baud");
+	data->uart_clk = devm_clk_get_enabled(&pdev->dev, "baud");
 	if (IS_ERR(data->uart_clk)) {
 		/*
 		 * For compatibility with older device trees try unnamed
 		 * clk when no baud clk can be found.
 		 */
-		data->uart_clk = devm_clk_get(&pdev->dev, NULL);
+		data->uart_clk = devm_clk_get_enabled(&pdev->dev, NULL);
 		if (IS_ERR(data->uart_clk)) {
 			dev_warn(&pdev->dev, "Can't get uart clock\n");
 			return PTR_ERR(data->uart_clk);