Re: [PATCH printk v8 11/35] serial: core: Introduce wrapper to set @uart_port->cons

[Ilpo Järvinen]

On Tue, 20 Aug 2024, John Ogness wrote:

> Introduce uart_port_set_cons() as a wrapper to set @cons of a
> uart_port. The wrapper sets @cons under the port lock in order
> to prevent @cons from disappearing while another context is
> holding the port lock. This is necessary for a follow-up
> commit relating to the port lock wrappers, which rely on @cons
> not changing between lock and unlock.
> 
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> Tested-by: Théo Lebrun <theo.lebrun@xxxxxxxxxxx> # EyeQ5, AMBA-PL011
> Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>

-- 
 i.

> ---
>  drivers/tty/serial/8250/8250_core.c |  6 +++---
>  drivers/tty/serial/amba-pl011.c     |  2 +-
>  drivers/tty/serial/serial_core.c    | 16 ++++++++--------
>  include/linux/serial_core.h         | 17 +++++++++++++++++
>  4 files changed, 29 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 29e4b83e0376..5f9f06911795 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -423,11 +423,11 @@ static int univ8250_console_setup(struct console *co, char *options)
>  
>  	port = &serial8250_ports[co->index].port;
>  	/* link port to console */
> -	port->cons = co;
> +	uart_port_set_cons(port, co);
>  
>  	retval = serial8250_console_setup(port, options, false);
>  	if (retval != 0)
> -		port->cons = NULL;
> +		uart_port_set_cons(port, NULL);
>  	return retval;
>  }
>  
> @@ -485,7 +485,7 @@ static int univ8250_console_match(struct console *co, char *name, int idx,
>  			continue;
>  
>  		co->index = i;
> -		port->cons = co;
> +		uart_port_set_cons(port, co);
>  		return serial8250_console_setup(port, options, true);
>  	}
>  
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 8b1644f5411e..7d0134ecd82f 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -2480,7 +2480,7 @@ static int pl011_console_match(struct console *co, char *name, int idx,
>  			continue;
>  
>  		co->index = i;
> -		port->cons = co;
> +		uart_port_set_cons(port, co);
>  		return pl011_console_setup(co, options);
>  	}
>  
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 9a18d0b95a41..61c7e1268957 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -3168,8 +3168,15 @@ static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *u
>  	state->uart_port = uport;
>  	uport->state = state;
>  
> +	/*
> +	 * If this port is in use as a console then the spinlock is already
> +	 * initialised.
> +	 */
> +	if (!uart_console_registered(uport))
> +		uart_port_spin_lock_init(uport);
> +
>  	state->pm_state = UART_PM_STATE_UNDEFINED;
> -	uport->cons = drv->cons;
> +	uart_port_set_cons(uport, drv->cons);
>  	uport->minor = drv->tty_driver->minor_start + uport->line;
>  	uport->name = kasprintf(GFP_KERNEL, "%s%d", drv->dev_name,
>  				drv->tty_driver->name_base + uport->line);
> @@ -3178,13 +3185,6 @@ static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *u
>  		goto out;
>  	}
>  
> -	/*
> -	 * If this port is in use as a console then the spinlock is already
> -	 * initialised.
> -	 */
> -	if (!uart_console_registered(uport))
> -		uart_port_spin_lock_init(uport);
> -
>  	if (uport->cons && uport->dev)
>  		of_console_check(uport->dev->of_node, uport->cons->name, uport->line);
>  
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 8872cd21e70a..2cf03ff2056a 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -608,6 +608,23 @@ static inline void __uart_port_unlock_irqrestore(struct uart_port *up, unsigned
>  	spin_unlock_irqrestore(&up->lock, flags);
>  }
>  
> +/**
> + * uart_port_set_cons - Safely set the @cons field for a uart
> + * @up:		The uart port to set
> + * @con:	The new console to set to
> + *
> + * This function must be used to set @up->cons. It uses the port lock to
> + * synchronize with the port lock wrappers in order to ensure that the console
> + * cannot change or disappear while another context is holding the port lock.
> + */
> +static inline void uart_port_set_cons(struct uart_port *up, struct console *con)
> +{
> +	unsigned long flags;
> +
> +	__uart_port_lock_irqsave(up, &flags);
> +	up->cons = con;
> +	__uart_port_unlock_irqrestore(up, flags);
> +}
>  /**
>   * uart_port_lock - Lock the UART port
>   * @up:		Pointer to UART port structure
>