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

yunhui cui
2025-05-12T14:00:42Z

Hi Ilpo,

On Mon, May 12, 2025 at 8:03 PM Ilpo Järvinen
<ilpo.jarvinen@linux.intel.com> wrote:
>
> On Mon, 12 May 2025, yunhui cui wrote:
>
> > Hi Ilpo,
> >
> > On Mon, May 12, 2025 at 6:27 PM Ilpo Järvinen
> > <ilpo.jarvinen@linux.intel.com> wrote:
> > >
> > > On Mon, 12 May 2025, yunhui cui wrote:
> > >
> > > > 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.
> > >
> > > I understood the first sentence in the comment but the rest of it is very
> > > cryptic and has many grammar issues too. Also, the extent of passive voice
> > > there makes it hard to know who does what (UART / kernel).
> > >
> > > > 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.
> > >
> > > The grammar is so broken, it failed to convey that message.
> >
> > The purpose of serial_port_in(port, UART_RX) is to clear interrupts
> > such as rx_timeout. In dw8250_handle_irq(), serial_port_in(p, UART_RX)
> > is called when the LSR does not have the UART_LSR_DR bit set.
> >
> > To avoid PSLVERR when the FIFO is enabled, serial_in(up, UART_RX)
> > should be called only when the LSR has the UART_LSR_DR bit set.
> >
> > These two logics are clearly contradictory. Therefore, both
> > serial8250_clear_fifos() and serial_port_in(port, UART_RX) are placed
> > under the protection of port->lock.
> >
> > If you believe this is not a potential issue, that's fine. I can
> > remove this patch in the next patchset version.
>
> No, my goal is not to get this removed from the patch series.
>
> I meant that the comment wording needs to be fixed for the next version
> such that it is understandable for those that are not deeply familiar with
> what is related to PSLVERR. Currently even I struggle to follow what's
> written into that comment (unless I read heavily between lines and base
> guesses on the extra knowledge I've about how this entire patchset relates
> to PSLVERR).
>

I plan to change the commit message as follows:
When the PSLVERR_RESP_EN parameter is set to 1, reading UART_RX while
the FIFO is enabled and UART_LSR_DR is not set will generate a PSLVERR
error.
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.
PSLVERR is addressed through two methods. One is to introduce
serial8250_discard_data() to check whether UART_LSR_DR is set before
reading UART_RX, thus solving the PSLVERR issue when the FIFO is
enabled. The other is to place FIFO clearing and reading of UART_RX
under port->lock.

The comment here will be changed as follows:
To prevent PSLVERR, we can either disable the FIFO before reading
UART_RX or read UART_RX only when UART_LSR_DR is set while the FIFO
remains enabled. If using the latter approach to avoid PSLVERR, it
creates a contradiction with the interrupt - clearing (see the
rx_timeout handling in dw8250_handle_irq()).

What do you think?

> > > > 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
> > > >
> > >
> > > --
> > > i.
> >
> > Thanks,
> > Yunhui
> >
>
> --
> i.

Thanks,
Yunhui