linux-kernel - Re: [PATCH tty v1 49/74] serial: qcom-geni: Use port lock wrappers

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Date:   Thu, 14 Sep 2023 12:16:52 -0700
From:   Bjorn Andersson <quic_bjorande@...cinc.com>
To:     John Ogness <john.ogness@...utronix.de>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        <linux-serial@...r.kernel.org>, Petr Mladek <pmladek@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH tty v1 49/74] serial: qcom-geni: Use port lock wrappers

On Thu, Sep 14, 2023 at 08:44:06PM +0206, John Ogness wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> When a serial port is used for kernel console output, then all
> modifications to the UART registers which are done from other contexts,
> e.g. getty, termios, are interference points for the kernel console.
> 
> So far this has been ignored and the printk output is based on the
> principle of hope. The rework of the console infrastructure which aims to
> support threaded and atomic consoles, requires to mark sections which
> modify the UART registers as unsafe. This allows the atomic write function
> to make informed decisions and eventually to restore operational state. It
> also allows to prevent the regular UART code from modifying UART registers
> while printk output is in progress.
> 
> All modifications of UART registers are guarded by the UART port lock,
> which provides an obvious synchronization point with the console
> infrastructure.
> 
> To avoid adding this functionality to all UART drivers, wrap the
> spin_[un]lock*() invocations for uart_port::lock into helper functions
> which just contain the spin_[un]lock*() invocations for now. In a
> subsequent step these helpers will gain the console synchronization
> mechanisms.
> 
> Converted with coccinelle. No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: Bjorn Andersson <quic_bjorande@...cinc.com>

> ---
>  drivers/tty/serial/qcom_geni_serial.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index b8aa4c1293ba..7e78f97e8f43 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -482,9 +482,9 @@ static void qcom_geni_serial_console_write(struct console *co, const char *s,
>  
>  	uport = &port->uport;
>  	if (oops_in_progress)
> -		locked = spin_trylock_irqsave(&uport->lock, flags);
> +		locked = uart_port_trylock_irqsave(uport, &flags);
>  	else
> -		spin_lock_irqsave(&uport->lock, flags);
> +		uart_port_lock_irqsave(uport, &flags);
>  
>  	geni_status = readl(uport->membase + SE_GENI_STATUS);
>  
> @@ -520,7 +520,7 @@ static void qcom_geni_serial_console_write(struct console *co, const char *s,
>  		qcom_geni_serial_setup_tx(uport, port->tx_remaining);
>  
>  	if (locked)
> -		spin_unlock_irqrestore(&uport->lock, flags);
> +		uart_port_unlock_irqrestore(uport, flags);
>  }
>  
>  static void handle_rx_console(struct uart_port *uport, u32 bytes, bool drop)
> @@ -970,7 +970,7 @@ static irqreturn_t qcom_geni_serial_isr(int isr, void *dev)
>  	if (uport->suspended)
>  		return IRQ_NONE;
>  
> -	spin_lock(&uport->lock);
> +	uart_port_lock(uport);
>  

Nice, now this no longer look unbalanced with the unlock in
uart_port_unlock_irqrestore().

Regards,
Bjorn

>  	m_irq_status = readl(uport->membase + SE_GENI_M_IRQ_STATUS);
>  	s_irq_status = readl(uport->membase + SE_GENI_S_IRQ_STATUS);
> -- 
> 2.39.2
>