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    Subject[PATCH 5.18 063/339] serial: meson: acquire port->lock in startup()
    Date
    From: John Ogness <john.ogness@linutronix.de>

    [ Upstream commit 589f892ac8ef244e47c5a00ffd8605daa1eaef8e ]

    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
    Fixes: ff7693d079e5 ("ARM: meson: serial: add MesonX SoC on-chip uart driver")
    Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
    Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
    Reviewed-by: Petr Mladek <pmladek@suse.com>
    Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
    Acked-by: Neil Armstrong <narmstrong@baylibre.com>
    Signed-off-by: John Ogness <john.ogness@linutronix.de>
    Link: https://lore.kernel.org/r/20220508103547.626355-1-john.ogness@linutronix.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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);

    --
    2.35.1


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