linux-kernel - Re: [PATCH v2 5/7] serial: 8250_dw: Rework IIR_NO

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Message-ID: <aXoXSxsem4hOLlrO@smile.fi.intel.com>
Date: Wed, 28 Jan 2026 16:03:55 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>, linux-serial@...r.kernel.org,
	qianfan Zhao <qianfanguijin@....com>,
	Adriana Nicolae <adriana@...sta.com>, 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 5/7] serial: 8250_dw: Rework IIR_NO_INT handling to
 stop interrupt storm

On Wed, Jan 28, 2026 at 12:52:59PM +0200, Ilpo Järvinen wrote:
> INTC10EE UART can end up into an interrupt storm where it reports
> IIR_NO_INT (0x1). If the storm happens during active UART operation, it
> is promptly stopped by IIR value change due to Rx or Tx events.
> However, when there is no activity, either due to idle serial line or
> due to specific circumstances such as during shutdown that writes
> IER=0, there is nothing to stop the storm.
> 
> During shutdown the storm is particularly problematic because
> serial8250_do_shutdown() calls synchronize_irq() that will hang in
> waiting for the storm to finish which never happens.
> 
> This problem can also result in triggering a warning:
> 
>   irq 45: nobody cared (try booting with the "irqpoll" option)
>   [...snip...]
>   handlers:
>     serial8250_interrupt
>   Disabling IRQ #45
> 
> Normal means to reset interrupt status by reading LSR, MSR, USR, or RX
> register do not result in the UART deasserting the IRQ.
> 
> Add a quirk to INTC10EE UARTs to enable Tx interrupts if UART's Tx is
> currently empty and inactive. Rework IIR_NO_INT to keep track of the
> number of consecutive IIR_NO_INT, and on fourth one perform the quirk.
> Enabling Tx interrupts should change IIR value from IIR_NO_INT to
> IIR_THRI which has been observed to stop the storm.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

...

> +/*
> + * Number of consecutive IIR_NO_INT interrupts required to trigger interrupt
> + * storm prevention code.
> + */
> +#define DW_UART_QUIRK_IER_KICK_THRES	4

Actually if we...

>  struct dw8250_platform_data {

>  	u8 usr_reg;

>  	unsigned int		skip_autocfg:1;
>  	unsigned int		uart_16550_compatible:1;
> +
> +	u8			no_int_count;

...define this as

	unsigned int		no_int_count:2;

there will be no need to have a modulo at all. But I agree it might look a bit
weird.

>  };

-- 
With Best Regards,
Andy Shevchenko