linux-kernel - Re: UART/TTY console deadlock

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Date:   Mon, 06 Jul 2020 13:31:01 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tony Lindgren <tony@...mide.com>, Petr Mladek <pmladek@...e.com>,
        Raul Rangel <rrangel@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "S\, Shirish" <Shirish.S@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        John Ogness <john.ogness@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: UART/TTY console deadlock

Hi,

On Sat Jul 04 2020, Andy Shevchenko wrote:
> On Fri, Jul 3, 2020 at 1:32 PM Sergey Senozhatsky
> <sergey.senozhatsky@...il.com> wrote:
>>
>> On (20/07/02 09:05), Tony Lindgren wrote:
>> > * Sergey Senozhatsky <sergey.senozhatsky@...il.com> [200702 05:13]:
>> > > On (20/06/30 11:02), Tony Lindgren wrote:
>> > > > This conditional disable for irq_shared does not look nice to me
>> > > > from the other device point of view :)
>> > > >
>> > > > Would it be possible to just set up te dummy interrupt handler
>> > > > for the startup, then change it back afterwards? See for example
>> > > > omap8250_no_handle_irq().
>> > >
>> > > I think we can do it. serial8250_do_startup() and irq handler take
>> > > port->lock, so they should be synchronized.
>> > >
>> > > Something like this then?
>> >
>> > Yeah thanks this should work better for shared irq.
>>
>> This is, basically, an equivalent of
>>
>> ---
>>  drivers/tty/serial/8250/8250_port.c | 5 +----
>>  1 file changed, 1 insertion(+), 4 deletions(-)
>>
>> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
>> index d64ca77d9cfa..dba7747d2ddd 100644
>> --- a/drivers/tty/serial/8250/8250_port.c
>> +++ b/drivers/tty/serial/8250/8250_port.c
>> @@ -2275,6 +2275,7 @@ int serial8250_do_startup(struct uart_port *port)
>>
>>         if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
>>                 unsigned char iir1;
>> +
>>                 /*
>>                  * Test for UARTs that do not reassert THRE when the
>>                  * transmitter is idle and the interrupt has already
>> @@ -2284,8 +2285,6 @@ int serial8250_do_startup(struct uart_port *port)
>>                  * allow register changes to become visible.
>>                  */
>>                 spin_lock_irqsave(&port->lock, flags);
>> -               if (up->port.irqflags & IRQF_SHARED)
>> -                       disable_irq_nosync(port->irq);
>>
>>                 wait_for_xmitr(up, UART_LSR_THRE);
>>                 serial_port_out_sync(port, UART_IER, UART_IER_THRI);
>> @@ -2297,8 +2296,6 @@ int serial8250_do_startup(struct uart_port *port)
>>                 iir = serial_port_in(port, UART_IIR);
>>                 serial_port_out(port, UART_IER, 0);
>>
>> -               if (port->irqflags & IRQF_SHARED)
>> -                       enable_irq(port->irq);
>>                 spin_unlock_irqrestore(&port->lock, flags);
>>
>>                 /*
>
> ...which effectively is a revert of
>
> 768aec0b5bcc ("serial: 8250: fix shared interrupts issues with SMP and
> RT kernels")

Please, don't revert that commit. I've faced the same issue as described
in the commit log. There is hardware available with shared UART
interrupt lines.

Thanks,
Kurt

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