linux-kernel - Re: [PATCH v2 6/7] serial: 8250: Add late synchronize

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Message-ID: <7d6747b0-5cdc-6936-d52f-414edb063d3e@linux.intel.com>
Date: Wed, 28 Jan 2026 17:07:33 +0200 (EET)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    Jiri Slaby <jirislaby@...nel.org>, 
    linux-serial <linux-serial@...r.kernel.org>, 
    qianfan Zhao <qianfanguijin@....com>, Adriana Nicolae <adriana@...sta.com>, 
    Jamie Iles <jamie@...ieiles.com>, LKML <linux-kernel@...r.kernel.org>, 
    "Bandal, Shankar" <shankar.bandal@...el.com>, 
    "Murthy, Shanth" <shanth.murthy@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 6/7] serial: 8250: Add late synchronize_irq() to
 shutdown to handle DW UART BUSY

On Wed, 28 Jan 2026, Andy Shevchenko wrote:

> On Wed, Jan 28, 2026 at 12:53:00PM +0200, Ilpo Järvinen wrote:
> > When DW UART is !uart_16550_compatible, it can indicate BUSY at any
> > point (when under constant Rx pressure) unless a complex sequence of
> > steps is performed. Any LCR write can run a foul with the condition
> > that prevents writing LCR while the UART is BUSY, which triggers
> > BUSY_DETECT interrupt that seems unmaskable using IER bits.
> > 
> > Normal flow is that dw8250_handle_irq() handles BUSY_DETECT condition
> > by reading USR register. This BUSY feature, however, breaks the
> > assumptions made in serial8250_do_shutdown(), which runs
> > synchronize_irq() after clearing IER and assumes no interrupts can
> > occur after that point but then proceeds to update LCR, which on DW
> > UART can trigger an interrupt.
> > 
> > If serial8250_do_shutdown() releases the interrupt handler before the
> > handler has run and processed the BUSY_DETECT condition by read the USR
> > register, the IRQ is not deasserted resulting in interrupt storm that
> > triggers "irq x: nobody cared" warning leading to disabling the IRQ.
> > 
> > Add late synchronize_irq() into serial8250_do_shutdown() to ensure
> > BUSY_DETECT from DW UART is handled before port's interrupt handler is
> > released. Alternative would be to add DW UART specific shutdown
> > function but it would mostly duplicate the generic code and the extra
> > synchronize_irq() seems pretty harmless in serial8250_do_shutdown().
> 
> Dunno if the triggered interrupt may lead to a new DMA transfers (since
> this is generic 8520 code...) in some cases.

If BUSY_DETECT interrupt fires (like I'd expect), the handler won't start 
anything but exits early from dw8250_handle_irq().

> Anyway I've just sent a patch
> that is Cc'ed to you to prevent that from happening. Not sure if it needs
> to be incorporated into your series or should have a Fixes tag.
> 
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>


-- 
 i.