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

yunhui cui
2025-05-12T06:37:41Z

Hi Ilpo,

On Tue, May 6, 2025 at 8:00 PM Ilpo Järvinen
<ilpo.jarvinen@linux.intel.com> wrote:
>
> 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.

The original intention was to check UART_LSR_DR first when reading
UART_RX. However, the purpose of serial_port_in(port, UART_RX) here is
to clear the interrupt, such as the interrupt caused by RX_TIMEOUT.

The logic for clearing the interrupt in the interrupt handling
function of RX_TIMEOUT is !UART_LSR_DR. And to avoid PSLVERR, we need
to check UART_LSR_DR first. To meet the requirements of both, the FIFO
needs to be disabled.

Therefore, we should put serial8250_clear_fifos() and the execution of
serial_port_in(port, UART_RX) without checking UART_LSR_DR under the
port->lock.

>
> > */
> > 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.
>

Thanks,
Yunhui