linux-kernel - Re: [PATCH v2 4/7] serial: qcom-geni: fix receiver enable

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Message-ID: <CAD=FV=Vwmb8Miyca4kE1sdjMCx7LVCYqaXhLmPPqsojUHdEk-g@mail.gmail.com>
Date: Thu, 3 Oct 2024 13:10:47 -0700
From: Doug Anderson <dianders@...omium.org>
To: Johan Hovold <johan+linaro@...nel.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org, stable@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 4/7] serial: qcom-geni: fix receiver enable

Hi,

On Tue, Oct 1, 2024 at 5:51 AM Johan Hovold <johan+linaro@...nel.org> wrote:
>
> The receiver should be enabled in the startup() callback and there is no
> need to stop it on every termios update.
>
> Since commit 6f3c3cafb115 ("serial: qcom-geni: disable interrupts during
> console writes") the calls to manipulate the secondary interrupts, which
> were done without holding the port lock, can lead to the receiver being
> left disabled when set_termios() races with the console code (e.g. when
> init opens the tty during boot).
>
> The calls to stop and start rx in set_termios() can similarly race with
> DMA completion and, for example, cause the DMA buffer to be unmapped
> twice or the mapping to be leaked.
>
> Fixes: 6f3c3cafb115 ("serial: qcom-geni: disable interrupts during console writes")
> Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
> Fixes: c4f528795d1a ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
> Cc: stable@...r.kernel.org      # 6.3
> Cc: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> Signed-off-by: Johan Hovold <johan+linaro@...nel.org>
> ---
>  drivers/tty/serial/qcom_geni_serial.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index dea688db0d7c..5b6c5388efee 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -1179,6 +1179,11 @@ static int qcom_geni_serial_startup(struct uart_port *uport)
>                 if (ret)
>                         return ret;
>         }
> +
> +       uart_port_lock_irq(uport);
> +       qcom_geni_serial_start_rx(uport);
> +       uart_port_unlock_irq(uport);

I _think_ you don't need the locking here. The documentation for the
"startup" callback say:

 * Interrupts: globally disabled.

Other than that, this looks reasonable to me. I seem to recall
previous discussions where _someone_ was relying on the
qcom_geni_serial_start_rx() at the end of termios for some reason
(which always felt like a bad design), but I can't find those old
discussions. I suspect that the fact that you've added the start_rx in
startup() is what we needed.