linux-kernel - Re: [PATCH v3] serial: 8250_mtk: Enable baud clock and manage in runtime PM

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Message-ID: <939cea9e-d7bd-4a13-b462-17e483f61e4c@gmail.com>
Date: Wed, 17 Sep 2025 12:00:57 +0200
From: Matthias Brugger <matthias.bgg@...il.com>
To: Daniel Golle <daniel@...rotopia.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Jiri Slaby <jirislaby@...nel.org>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 Yu Jiaoliang <yujiaoliang@...o.com>,
 Uwe Kleine-König <u.kleine-koenig@...libre.com>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Changqi Hu <changqi.hu@...iatek.com>, Sam Shih <sam.shih@...iatek.com>,
 Steven Liu <steven.liu@...iatek.com>, John Crispin <john@...ozen.org>,
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v3] serial: 8250_mtk: Enable baud clock and manage in
 runtime PM



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@...labora.com>
> Signed-off-by: Daniel Golle <daniel@...rotopia.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);