Re: [PATCH v3 1/1] serial: 8250: Toggle IER bits on only after irq has been set up

[Ilpo Järvinen]

On Mon, 19 Sep 2022, Ilpo Järvinen wrote:

> Invoking TIOCVHANGUP on 8250_mid port on Ice Lake-D and then reopening
> the port triggers these faults during serial8250_do_startup():
> 
>   DMAR: DRHD: handling fault status reg 3
>   DMAR: [DMA Write NO_PASID] Request device [00:1a.0] fault addr 0x0 [fault reason 0x05] PTE Write access is not set
> 
> If the IRQ hasn't been set up yet, the UART will have zeroes in its MSI
> address/data registers. Disabling the IRQ at the interrupt controller
> won't stop the UART from performing a DMA write to the address programmed
> in its MSI address register (zero) when it wants to signal an interrupt.
> 
> The UARTs (in Ice Lake-D) implement PCI 2.1 style MSI without masking
> capability, so there is no way to mask the interrupt at the source PCI
> function level, except disabling the MSI capability entirely, but that
> would cause it to fall back to INTx# assertion, and the PCI specification
> prohibits disabling the MSI capability as a way to mask a function's
> interrupt service request.
> 
> The MSI address register is zeroed during by the hangup as the irq is
> freed. The interrupt is signalled during serial8250_do_startup()
> performing a THRE test that temporarily toggles THRI in IER. The THRE
> test currently occurs before UART's irq (and MSI address) is properly
> set up.
> 
> Refactor serial8250_do_startup() such that irq is set up before the
> THRE test. The current irq setup code is intermixed with the timer
> setup code. As THRE test must be performed prior to the timer setup,
> extract it into own function and call it only after the THRE test.
> 
> Reported-by: Lennert Buytenhek <buytenh@xxxxxxxxxx>
> Tested-by: Lennert Buytenhek <buytenh@xxxxxxxxxx>
> Fixes: 40b36daad0ac ("[PATCH] 8250 UART backup timer")
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> ---
> 
> v3:
> - Improved the commit with Lennert's superior descriptions.
> - Added Andy's Rev-by
> 
>  drivers/tty/serial/8250/8250.h      |  2 ++
>  drivers/tty/serial/8250/8250_core.c | 16 +++++++++++-----
>  drivers/tty/serial/8250/8250_port.c |  8 +++++---
>  3 files changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index 287153d32536..dbf4c1204bf3 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -403,3 +403,5 @@ static inline int serial_index(struct uart_port *port)
>  {
>  	return port->minor - 64;
>  }
> +
> +void univ8250_setup_timer(struct uart_8250_port *up);
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 2e83e7367441..10d535640434 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -298,10 +298,9 @@ static void serial8250_backup_timeout(struct timer_list *t)
>  		jiffies + uart_poll_timeout(&up->port) + HZ / 5);
>  }
>  
> -static int univ8250_setup_irq(struct uart_8250_port *up)
> +void univ8250_setup_timer(struct uart_8250_port *up)
>  {
>  	struct uart_port *port = &up->port;
> -	int retval = 0;
>  
>  	/*
>  	 * The above check will only give an accurate result the first time
> @@ -322,10 +321,17 @@ static int univ8250_setup_irq(struct uart_8250_port *up)
>  	 */
>  	if (!port->irq)
>  		mod_timer(&up->timer, jiffies + uart_poll_timeout(port));
> -	else
> -		retval = serial_link_irq_chain(up);
> +}
> +EXPORT_SYMBOL_GPL(univ8250_setup_timer);
>  
> -	return retval;
> +static int univ8250_setup_irq(struct uart_8250_port *up)
> +{
> +	struct uart_port *port = &up->port;
> +
> +	if (port->irq)
> +		return serial_link_irq_chain(up);
> +
> +	return 0;
>  }
>  
>  static void univ8250_release_irq(struct uart_8250_port *up)
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 39b35a61958c..6e8e16227a3a 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2294,6 +2294,10 @@ int serial8250_do_startup(struct uart_port *port)
>  	if (port->irq && (up->port.flags & UPF_SHARE_IRQ))
>  		up->port.irqflags |= IRQF_SHARED;
>  
> +	retval = up->ops->setup_irq(up);
> +	if (retval)
> +		goto out;
> +
>  	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
>  		unsigned char iir1;
>  
> @@ -2336,9 +2340,7 @@ int serial8250_do_startup(struct uart_port *port)
>  		}
>  	}
>  
> -	retval = up->ops->setup_irq(up);
> -	if (retval)
> -		goto out;
> +	univ8250_setup_timer(up);
>  
>  	/*
>  	 * Now, initialize the UART

Hi Greg,

Please scratch this patch. It seems to create a circular dependency issue 
with allmodconfig. I'll send v4 once the problem is sorted out.

-- 
 i.