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

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Message-ID: <2025071729-exemplary-smelting-08fe@gregkh>
Date: Thu, 17 Jul 2025 14:32:54 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: yunhui cui <cuiyunhui@...edance.com>
Cc: John Ogness <john.ogness@...utronix.de>, arnd@...db.de,
	andriy.shevchenko@...ux.intel.com, benjamin.larsson@...exis.eu,
	heikki.krogerus@...ux.intel.com, ilpo.jarvinen@...ux.intel.com,
	jirislaby@...nel.org, jkeeping@...usicbrands.com,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	markus.mayer@...aro.org, matt.porter@...aro.org,
	namcao@...utronix.de, paulmck@...nel.org, pmladek@...e.com,
	schnelle@...ux.ibm.com, sunilvl@...tanamicro.com,
	tim.kryger@...aro.org, stable@...r.kernel.org
Subject: Re: [External] Re: [PATCH v9 1/4] serial: 8250: fix panic due to
 PSLVERR

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@...utronix.de> wrote:
> >
> > On 2025-06-10, Yunhui Cui <cuiyunhui@...edance.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@...edance.com>
> > > Cc: stable@...r.kernel.org
> >
> > Reviewed-by: John Ogness <john.ogness@...utronix.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