Re: [PATCH v4 2/4] serial: 8250: introduce serial8250_discard_data()

[Jiri Slaby <jirislaby@kernel.org>]

On 25. 04. 25, 8:24, Yunhui Cui wrote:
> To prevent triggering PSLVERR, it is necessary to check whether the
> UART_LSR_DR bit of UART_LSR is set before reading UART_RX.
> Ensure atomicity of UART_LSR and UART_RX, put serial8250_discard_data()
> under port->lock.
> 
> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
> ---
>   drivers/tty/serial/8250/8250.h      | 15 +++++++++++
>   drivers/tty/serial/8250/8250_port.c | 42 ++++++++++++++---------------
>   2 files changed, 36 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index b861585ca02a..5a106cf88207 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -162,6 +162,21 @@ static inline u16 serial_lsr_in(struct uart_8250_port *up)
>   	return lsr;
>   }
>   
> +/*
> + * To avoid PSLVERR, check UART_LSR_DR in UART_LSR before
> + * reading UART_RX.
> + */
> +static inline unsigned int serial8250_discard_data(struct uart_8250_port *up)
> +{
> +	u16 lsr;

Is lsr really 16-bit?

> +
> +	lsr = serial_in(up, UART_LSR);
> +	if (lsr & UART_LSR_DR)
> +		return serial_in(up, UART_RX);

Why does a discard function return a value at all?

> +
> +	return 0;
> +}
> +
>   /*
>    * For the 16C950
>    */
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index a913135d5217..802ac50357c0 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
...
> @@ -2137,25 +2136,21 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
>   static int serial8250_get_poll_char(struct uart_port *port)
>   {
>   	struct uart_8250_port *up = up_to_u8250p(port);
> -	int status;
> +	int status = NO_POLL_CHAR;
>   	u16 lsr;
>   
>   	serial8250_rpm_get(up);
>   
> +	uart_port_lock_irqsave(port, &flags);
>   	lsr = serial_port_in(port, UART_LSR);
> -
> -	if (!(lsr & UART_LSR_DR)) {
> -		status = NO_POLL_CHAR;
> -		goto out;
> -	}
> -
> -	status = serial_port_in(port, UART_RX);
> +	if ((lsr & UART_LSR_DR))

Too many parentheses.

> +		status = serial_port_in(port, UART_RX);
> +	uart_port_unlock_irqrestore(port, flags);

thanks,
-- 
js
suse labs