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Date:   Tue, 10 May 2022 11:37:23 +0200
From:   Petr Mladek <pmladek@suse.com>
To:     Neil Armstrong <narmstrong@baylibre.com>
Cc:     John Ogness <john.ogness@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kevin Hilman <khilman@baylibre.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-kernel@vger.kernel.org, Jiri Slaby <jirislaby@kernel.org>,
        Jerome Brunet <jbrunet@baylibre.com>,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
        linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-amlogic@lists.infradead.org,
        Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: [PATCH v1] serial: meson: acquire port->lock in startup()
Message-ID: <YnoyU3fydh46e2Sc@alley>
References: <20220508103547.626355-1-john.ogness@linutronix.de>
 <cf593ff3-bf57-ccd3-9a25-b28cc604d6f4@baylibre.com>
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On Mon 2022-05-09 09:36:40, Neil Armstrong wrote:
> On 08/05/2022 12:35, John Ogness wrote:
> > The uart_ops startup() callback is called without interrupts
> > disabled and without port->lock locked, relatively late during the
> > boot process (from the call path of console_on_rootfs()). If the
> > device is a console, it was already previously registered and could
> > be actively printing messages.
> > 
> > Since the startup() callback is reading/writing registers used by
> > the console write() callback (AML_UART_CONTROL), its access must
> > be synchronized using the port->lock. Currently it is not.
> > 
> > The startup() callback is the only function that explicitly enables
> > interrupts. Without the synchronization, it is possible that
> > interrupts become accidentally permanently disabled.
> > 
> > CPU0                           CPU1
> > meson_serial_console_write     meson_uart_startup
> > --------------------------     ------------------
> > spin_lock(port->lock)
> > val = readl(AML_UART_CONTROL)
> > uart_console_write()
> >                                 writel(INT_EN, AML_UART_CONTROL)
> > writel(val, AML_UART_CONTROL)
> > spin_unlock(port->lock)
> > 
> > Add port->lock synchronization to meson_uart_startup() to avoid
> > racing with meson_serial_console_write().
> > 
> > Also add detailed comments to meson_uart_reset() explaining why it
> > is *not* using port->lock synchronization.
> > 
> > Link: https://lore.kernel.org/lkml/2a82eae7-a256-f70c-fd82-4e510750906e@samsung.com
> > Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > Signed-off-by: John Ogness <john.ogness@linutronix.de>
> > Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > ---
> >   drivers/tty/serial/meson_uart.c | 13 +++++++++++++
> >   1 file changed, 13 insertions(+)
> > 
> > diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
> > index 2bf1c57e0981..39021dac09cc 100644
> > --- a/drivers/tty/serial/meson_uart.c
> > +++ b/drivers/tty/serial/meson_uart.c
> > @@ -253,6 +253,14 @@ static const char *meson_uart_type(struct uart_port *port)
> >   	return (port->type == PORT_MESON) ? "meson_uart" : NULL;
> >   }
> > +/*
> > + * This function is called only from probe() using a temporary io mapping
> > + * in order to perform a reset before setting up the device. Since the
> > + * temporarily mapped region was successfully requested, there can be no
> > + * console on this port at this time. Hence it is not necessary for this
> > + * function to acquire the port->lock. (Since there is no console on this
> > + * port at this time, the port->lock is not initialized yet.)
> > + */
> >   static void meson_uart_reset(struct uart_port *port)
> >   {
> >   	u32 val;
> > @@ -267,9 +275,12 @@ static void meson_uart_reset(struct uart_port *port)
> >   static int meson_uart_startup(struct uart_port *port)
> >   {
> > +	unsigned long flags;
> >   	u32 val;
> >   	int ret = 0;
> > +	spin_lock_irqsave(&port->lock, flags);
> > +
> >   	val = readl(port->membase + AML_UART_CONTROL);
> >   	val |= AML_UART_CLEAR_ERR;
> >   	writel(val, port->membase + AML_UART_CONTROL);
> > @@ -285,6 +296,8 @@ static int meson_uart_startup(struct uart_port *port)
> >   	val = (AML_UART_RECV_IRQ(1) | AML_UART_XMIT_IRQ(port->fifosize / 2));
> >   	writel(val, port->membase + AML_UART_MISC);
> > +	spin_unlock_irqrestore(&port->lock, flags);
> > +
> >   	ret = request_irq(port->irq, meson_uart_interrupt, 0,
> >   			  port->name, port);
> > 
> > base-commit: 672c0c5173427e6b3e2a9bbb7be51ceeec78093a
> 
> Thanks for fixing this, it may also fix an uart apparent lockup I encountered
> several time while developing on the platform, but the target was still alive
> so it matches.
> 
> So I'll add:
> Fixes: ff7693d079e5 ("ARM: meson: serial: add MesonX SoC on-chip uart driver")
> 
> and
> 
> Acked-by: Neil Armstrong <narmstrong@baylibre.com>

Neil, may I assume that you are going to queue this fix for 5.19, please?

Feel free to add:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr