linux-kernel - Re: UART/TTY console deadlock

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Message-ID: <20200702051213.GB3450@jagdpanzerIV.localdomain>
Date:   Thu, 2 Jul 2020 14:12:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andy.shevchenko@...il.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>,
        kurt@...utronix.de, "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

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?

---

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f77bf820b7a3..61ad6f5f0f88 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2140,6 +2140,11 @@ static void serial8250_put_poll_char(struct uart_port *port,
 
 #endif /* CONFIG_CONSOLE_POLL */
 
+static int serial8250_dummy_handle_irq(struct uart_port *port)
+{
+	return 0;
+}
+
 int serial8250_do_startup(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
@@ -2271,6 +2276,9 @@ int serial8250_do_startup(struct uart_port *port)
 
 	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
 		unsigned char iir1;
+		int (*handler)(struct uart_port *);
+
+		handler = port->handle_irq;
 		/*
 		 * Test for UARTs that do not reassert THRE when the
 		 * transmitter is idle and the interrupt has already
@@ -2280,8 +2288,7 @@ 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);
+		port->handle_irq = serial8250_dummy_handle_irq;
 
 		wait_for_xmitr(up, UART_LSR_THRE);
 		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
@@ -2293,8 +2300,7 @@ 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);
+		port->handle_irq = handler;
 		spin_unlock_irqrestore(&port->lock, flags);
 
 		/*