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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Sat, 4 Jul 2020 14:37:14 +0300
Message-ID: <CAHp75Vc42k2G5VJDECBQ=HkkMoQq_VX+rpxVrjZHwMyAQ6O7gw@mail.gmail.com>
Subject: Re: UART/TTY console deadlock
To:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc:     Tony Lindgren <tony@atomide.com>, Petr Mladek <pmladek@suse.com>,
        Raul Rangel <rrangel@google.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        kurt@linutronix.de, "S, Shirish" <Shirish.S@amd.com>,
        Peter Zijlstra <peterz@infradead.org>,
        John Ogness <john.ogness@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>
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On Fri, Jul 3, 2020 at 1:53 PM Sergey Senozhatsky
<sergey.senozhatsky@gmail.com> wrote:
>
> On (20/07/02 11:20), Andy Shevchenko wrote:
> >
> > I didn't look into this deeply, but my understanding that this is something for
> > special case when you have several UART ports sharing the IRQ (multi-port card)
> > and IRQ even maybe undesirable b/c it will confuse real IRQ handler. I don't
> > remember details, but AFAIR IRQ handler does a busyloop to service as much as
> > possible and in between it may release the lock (again, multi-port UART cards),
> > that's why we better avoid IRQ event in the first place.
> > But it's my pure speculation.
>
> Hmm. Would different IRQ handlers use different UART port structures? ->irq
> might be the only thing they share. Even if they use the same port, we
> keep IRQs disabled on local CPU, and UART port locked. To unlock the UART
> port during IRQ, the handler (on another CPU) first needs to acquire the
> port->lock, which is locked by serial8250_do_startup().

It appears to be a RT kernel related. Don't know the state of affairs
in RT nowadays, but maybe still relevant.
See the commit 768aec0b5bcc ("serial: 8250: fix shared interrupts
issues with SMP and RT kernels").


-- 
With Best Regards,
Andy Shevchenko