linux-kernel - Re: [PATCH] serial: exar: Fix stuck MSIs

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Date:   Mon, 24 Apr 2017 11:06:17 +0200
From:   Jan Kiszka <jan.kiszka@....de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-serial@...r.kernel.org,
        Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
Subject: Re: [PATCH] serial: exar: Fix stuck MSIs

On 2017-04-24 10:59, Andy Shevchenko wrote:
> On Sat, 2017-04-22 at 11:36 +0200, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@...mens.com>
>>
>> After migrating 8250_exar to MSI in 172c33cb61da, we can get stuck
>> without further interrupts because of the special wake-up event these
>> chips send. They are only cleared by reading INT0. As we fail to do so
>> during startup and shutdown, we can leave the interrupt line asserted,
>> which is fatal with edge-triggered MSIs.
>>
>> Add the required reading of INT0 to startup and shutdown. Also account
>> for the fact that a pending wake-up interrupt means we have to return
>> 1
>> from exar_handle_irq.
>>
>> An alternative approach would have been disabling the wake-up
>> interrupt.
>> Unfortunately, this feature (REGB[17] = 1) is not available on the
>> XR17D15X.
>>
>> Fixes: 172c33cb61da ("serial: exar: Enable MSI support")
>> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
>> ---
>>
>> Regression of upcoming 4.11.
>>
>>  drivers/tty/serial/8250/8250_port.c | 19 ++++++++++---------
>>  1 file changed, 10 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/tty/serial/8250/8250_port.c
>> b/drivers/tty/serial/8250/8250_port.c
>> index 6119516ef5fc..3a3667880fcf 100644
>> --- a/drivers/tty/serial/8250/8250_port.c
>> +++ b/drivers/tty/serial/8250/8250_port.c
>> @@ -47,6 +47,7 @@
>>  /*
>>   * These are definitions for the Exar XR17V35X and XR17(C|D)15X
>>   */
>> +#define UART_EXAR_INT0		0x80
>>  #define UART_EXAR_SLEEP		0x8b	/* Sleep mode */
>>  #define UART_EXAR_DVID		0x8d	/* Device
>> identification */
>>  
>> @@ -1869,17 +1870,13 @@ static int
>> serial8250_default_handle_irq(struct uart_port *port)
>>  static int exar_handle_irq(struct uart_port *port)
>>  {
>>  	unsigned int iir = serial_port_in(port, UART_IIR);
>> -	int ret;
>> +	int ret = 0;
>>  
>> -	ret = serial8250_handle_irq(port, iir);
>> +	if (((port->type == PORT_XR17V35X) || (port->type ==
>> PORT_XR17D15X)) &&
>> +	    serial_port_in(port, UART_EXAR_INT0) != 0)
>> +		ret = 1;
>>  
>> -	if ((port->type == PORT_XR17V35X) ||
>> -	   (port->type == PORT_XR17D15X)) {
> 
>> -		serial_port_in(port, 0x80);
>> -		serial_port_in(port, 0x81);
>> -		serial_port_in(port, 0x82);
>> -		serial_port_in(port, 0x83);
> 
> You replaced 4 reads by one. I'm suspecting that on multi-port cards you
> still need to read all of them (I assume they are called INT0, INT1,
> ...). Perhaps you need a helper where you do that and call it from all
> necessary places.

Nope, we never had to read them all: "Wake-up Indicator is cleared by
reading the INT0 register." (Exar manual) INT0 contains the interrupt
status for all channels.

Jan

> 
>> -	}
>> +	ret |= serial8250_handle_irq(port, iir);
>>  
>>  	return ret;
>>  }
>> @@ -2177,6 +2174,8 @@ int serial8250_do_startup(struct uart_port
>> *port)
>>  	serial_port_in(port, UART_RX);
>>  	serial_port_in(port, UART_IIR);
>>  	serial_port_in(port, UART_MSR);
>> +	if ((port->type == PORT_XR17V35X) || (port->type ==
>> PORT_XR17D15X))
>> +		serial_port_in(port, UART_EXAR_INT0);
>>  
>>  	/*
>>  	 * At this point, there's no way the LSR could still be 0xff;
>> @@ -2335,6 +2334,8 @@ int serial8250_do_startup(struct uart_port
>> *port)
>>  	serial_port_in(port, UART_RX);
>>  	serial_port_in(port, UART_IIR);
>>  	serial_port_in(port, UART_MSR);
>> +	if ((port->type == PORT_XR17V35X) || (port->type ==
>> PORT_XR17D15X))
>> +		serial_port_in(port, UART_EXAR_INT0);
>>  	up->lsr_saved_flags = 0;
>>  	up->msr_saved_flags = 0;
>>  
> 




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