Re: [PATCH v5 2/4] serial: 8250: avoid potential PSLVERR issue

Ilpo Järvinen
2025-05-06T12:00:35Z

On Tue, 6 May 2025, Yunhui Cui wrote:

> Failure to check the UART_LSR_DR before reading UART_RX, or the
> non-atomic nature of clearing the FIFO and reading UART_RX, poses
> potential risks that could lead to PSLVERR.

Don't expect reader to know the condition how PSLVERR is triggered. I know
it's worded out in the other patch but also explain it here.

You're only explaining problem and missing what this patch does to solve
the problem.

> Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
> ---
> drivers/tty/serial/8250/8250.h | 13 +++++++++
> drivers/tty/serial/8250/8250_port.c | 43 +++++++++++++++--------------
> 2 files changed, 35 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index b861585ca02a..6f97ff3a197d 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -162,6 +162,19 @@ 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 void serial8250_discard_data(struct uart_8250_port *up)
> +{
> + u16 lsr;
> +
> + lsr = serial_in(up, UART_LSR);
> + if (lsr & UART_LSR_DR)
> + serial_in(up, UART_RX);
> +}
> +
> /*
> * For the 16C950
> */
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index a913135d5217..1666b965f6a0 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1357,9 +1357,8 @@ static void autoconfig_irq(struct uart_8250_port *up)
> /* Synchronize UART_IER access against the console. */
> uart_port_lock_irq(port);
> serial_out(up, UART_IER, UART_IER_ALL_INTR);
> + serial8250_discard_data(up);
> uart_port_unlock_irq(port);
> - serial_in(up, UART_LSR);
> - serial_in(up, UART_RX);
> serial_in(up, UART_IIR);
> serial_in(up, UART_MSR);
> serial_out(up, UART_TX, 0xFF);
> @@ -2137,25 +2136,22 @@ 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;
> + unsigned long flags;
>
> serial8250_rpm_get(up);
>
> + uart_port_lock_irqsave(port, &flags);
> lsr = serial_port_in(port, UART_LSR);
> + if (lsr & UART_LSR_DR)
> + status = serial_port_in(port, UART_RX);
> + uart_port_unlock_irqrestore(port, flags);
>
> - if (!(lsr & UART_LSR_DR)) {
> - status = NO_POLL_CHAR;
> - goto out;
> - }
> -
> - status = serial_port_in(port, UART_RX);
> -out:
> serial8250_rpm_put(up);
> return status;

Not a problem that originates from you, but IMO calling this variable
"status" is quite misleading when it is the character (or NO_POLL_CHAR
is no character is present).

> }
>
> -
> static void serial8250_put_poll_char(struct uart_port *port,
> unsigned char c)
> {
> @@ -2264,13 +2260,17 @@ int serial8250_do_startup(struct uart_port *port)
> * Clear the FIFO buffers and disable them.
> * (they will be reenabled in set_termios())
> */
> + uart_port_lock_irqsave(port, &flags);
> serial8250_clear_fifos(up);
>
> /*
> - * Clear the interrupt registers.
> + * Read UART_RX to clear interrupts (e.g., Character Timeout).
> + * No data on UART_IIR_RX_TIMEOUT, UART_LSR_DR won't set.
> + * FIFO disabled, read UART_RX without LSR check, no PSLVERR.

I don't understand what the last two lines mean and I don't see the
connection to the code that is below the comment either, could you try to
rephrase the comment.

> */
> serial_port_in(port, UART_LSR);
> serial_port_in(port, UART_RX);
> + uart_port_unlock_irqrestore(port, flags);
> serial_port_in(port, UART_IIR);
> serial_port_in(port, UART_MSR);
>
> @@ -2429,15 +2429,14 @@ int serial8250_do_startup(struct uart_port *port)
> }
>
> dont_test_tx_en:
> - uart_port_unlock_irqrestore(port, flags);
>
> /*
> * Clear the interrupt registers again for luck, and clear the
> * saved flags to avoid getting false values from polling
> * routines or the previous session.
> */
> - serial_port_in(port, UART_LSR);
> - serial_port_in(port, UART_RX);
> + serial8250_discard_data(up);
> + uart_port_unlock_irqrestore(port, flags);
> serial_port_in(port, UART_IIR);
> serial_port_in(port, UART_MSR);
> up->lsr_saved_flags = 0;
> @@ -2519,7 +2518,6 @@ void serial8250_do_shutdown(struct uart_port *port)
> port->mctrl &= ~TIOCM_OUT2;
>
> serial8250_set_mctrl(port, port->mctrl);
> - uart_port_unlock_irqrestore(port, flags);
>
> /*
> * Disable break condition and FIFOs
> @@ -2527,6 +2525,14 @@ void serial8250_do_shutdown(struct uart_port *port)
> serial_port_out(port, UART_LCR,
> serial_port_in(port, UART_LCR) & ~UART_LCR_SBC);
> serial8250_clear_fifos(up);
> + /*
> + * Read data port to reset things, and then unlink from
> + * the IRQ chain.
> + * Since reading UART_RX clears interrupts, doing so with
> + * FIFO disabled won't trigger PSLVERR.
> + */
> + serial_port_in(port, UART_RX);
> + uart_port_unlock_irqrestore(port, flags);
>
> #ifdef CONFIG_SERIAL_8250_RSA
> /*
> @@ -2535,11 +2541,6 @@ void serial8250_do_shutdown(struct uart_port *port)
> disable_rsa(up);
> #endif
>
> - /*
> - * Read data port to reset things, and then unlink from
> - * the IRQ chain.
> - */
> - serial_port_in(port, UART_RX);
> serial8250_rpm_put(up);
>
> up->ops->release_irq(up);
>

--
i.