Re: [External] Re: [PATCH v9 1/4] serial: 8250: fix panic due to PSLVERR

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Jul 17, 2025 at 08:19:48PM +0800, yunhui cui wrote:
> Hi John,
> 
> On Fri, Jun 20, 2025 at 7:20 PM John Ogness <john.ogness@linutronix.de> wrote:
> >
> > On 2025-06-10, Yunhui Cui <cuiyunhui@bytedance.com> wrote:
> > > When the PSLVERR_RESP_EN parameter is set to 1, the device generates
> > > an error response if an attempt is made to read an empty RBR (Receive
> > > Buffer Register) while the FIFO is enabled.
> > >
> > > In serial8250_do_startup(), calling serial_port_out(port, UART_LCR,
> > > UART_LCR_WLEN8) triggers dw8250_check_lcr(), which invokes
> > > dw8250_force_idle() and serial8250_clear_and_reinit_fifos(). The latter
> > > function enables the FIFO via serial_out(p, UART_FCR, p->fcr).
> > > Execution proceeds to the serial_port_in(port, UART_RX).
> > > This satisfies the PSLVERR trigger condition.
> > >
> > > When another CPU (e.g., using printk()) is accessing the UART (UART
> > > is busy), the current CPU fails the check (value & ~UART_LCR_SPAR) ==
> > > (lcr & ~UART_LCR_SPAR) in dw8250_check_lcr(), causing it to enter
> > > dw8250_force_idle().
> > >
> > > Put serial_port_out(port, UART_LCR, UART_LCR_WLEN8) under the port->lock
> > > to fix this issue.
> > >
> > > Panic backtrace:
> > > [    0.442336] Oops - unknown exception [#1]
> > > [    0.442343] epc : dw8250_serial_in32+0x1e/0x4a
> > > [    0.442351]  ra : serial8250_do_startup+0x2c8/0x88e
> > > ...
> > > [    0.442416] console_on_rootfs+0x26/0x70
> > >
> > > Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround")
> > > Link: https://lore.kernel.org/all/84cydt5peu.fsf@jogness.linutronix.de/T/
> > > Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
> > > Cc: stable@vger.kernel.org
> >
> > Reviewed-by: John Ogness <john.ogness@linutronix.de>
> 
> In this patchset, patch[4] has been dropped, and patch[2] may still
> need discussion. Could you please pick patch[1] and patch[3] first?

Please resend just the patches you want applied, picking out of a series
is hard with our current tools, and confusing.

thanks,

greg k-h