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
e32a83c70cf9 ("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: e32a83c70cf9 ("serial: 8250-mtk: modify mtk uart power and clock management")
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
---
v3: squash into single patch
v2: add managing run-time PM in dedicated patch

 drivers/tty/serial/8250/8250_mtk.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
index b44de2ed7413..5875a7b9b4b1 100644
--- a/drivers/tty/serial/8250/8250_mtk.c
+++ b/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);
-- 
2.51.0

On 16/09/2025 23:37, Daniel Golle wrote:
> 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
> e32a83c70cf9 ("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: e32a83c70cf9 ("serial: 8250-mtk: modify mtk uart power and clock management")

Most of the issues from this commit are already fixed in
b6c7ff2693ddc ("serial: 8250_mtk: Simplify clock sequencing and runtime PM")
which is more or less a revert of e32a83c70cf9.

I think we should add
Fixes: b6c7ff2693ddc ("serial: 8250_mtk: Simplify clock sequencing and runtime PM")
here as well. It's a fix of a fix :)

Regards,
Matthias


> Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> ---
> v3: squash into single patch
> v2: add managing run-time PM in dedicated patch
> 
>   drivers/tty/serial/8250/8250_mtk.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
> index b44de2ed7413..5875a7b9b4b1 100644
> --- a/drivers/tty/serial/8250/8250_mtk.c
> +++ b/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);