linux-kernel - Re: [PATCH 1/3] tty: serial: qcom-geni-serial: Fix kdb/kgdb after port shutdown (again)

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Date:   Tue, 7 Mar 2023 17:01:16 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-serial@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        kgdb-bugreport@...ts.sourceforge.net,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Andy Gross <agross@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] tty: serial: qcom-geni-serial: Fix kdb/kgdb after
 port shutdown (again)

On Tue, Mar 07, 2023 at 07:32:11AM -0800, Douglas Anderson wrote:
> Commit d8aca2f96813 ("tty: serial: qcom-geni-serial: stop operations
> in progress at shutdown") was basically a straight revert of the
> commit it claims to fix without any explanation of why the problems
> talked about in the original patch were no longer relevant. Indeed,
> commit d8aca2f96813 ("tty: serial: qcom-geni-serial: stop operations
> in progress at shutdown") re-introduces the same problem that commit
> e83766334f96 ("tty: serial: qcom_geni_serial: No need to stop tx/rx on
> UART shutdown") fixed.
> 
> The problems are very easy to see by simply doing this on a
> sc7180-based Chromebook:
> 
> 1. Boot in developer mode with serial console enabled and kdb setup on
>    the serial console.
> 2. via ssh: stop console-ttyMSM0; echo g > /proc/sysrq-trigger
> 
> When you do the above you'll see the "kdb" prompt printed on the
> serial console but be unable to interact with it.
> 
> Let's fix the problem again by noting that the console port is never
> configured in DMA mode and thus we don't need to stop things for the
> console.
> 
> Fixes: d8aca2f96813 ("tty: serial: qcom-geni-serial: stop operations in progress at shutdown")

The offending commit broke serial console more generally by breaking TX
and thus hanging the system when stopping the getty on reboot.

The underlying bug has been there since this driver was first merged,
and as fixing it properly is going to be a bit involved, I was about to
post a patch equivalent to this one to fix the immediate regression and
get us back to status quo.

> Signed-off-by: Douglas Anderson <dianders@...omium.org>

Reviewed-by: Johan Hovold <johan+linaro@...nel.org>

> ---
> 
>  drivers/tty/serial/qcom_geni_serial.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index d69592e5e2ec..74a0e074c2de 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -1070,8 +1070,10 @@ static int setup_fifos(struct qcom_geni_serial_port *port)
>  static void qcom_geni_serial_shutdown(struct uart_port *uport)
>  {
>  	disable_irq(uport->irq);
> -	qcom_geni_serial_stop_tx(uport);
> -	qcom_geni_serial_stop_rx(uport);
> +	if (!uart_console(uport)) {
> +		qcom_geni_serial_stop_tx(uport);
> +		qcom_geni_serial_stop_rx(uport);
> +	}
>  }
>  
>  static int qcom_geni_serial_port_setup(struct uart_port *uport)

Johan