serial: 8250: lock port for rx_dma() callback

The rx_dma() callback (omap_8250_rx_dma) accesses UART_IER. This
register is modified twice by each console write
(serial8250_console_write()) under the port lock. However, not
all calls to the rx_dma() callback are under the port lock.

Add the missing port locking around rx_dma() callback calls. Add
lockdep notation to the omap_8250_rx_dma().

Note that this is not fixing a real problem because:

1. Currently DMA is forced off for 8250_omap consoles.

2. The serial devices are resumed before console printing is enabled.

However, adding this locking allows for clean locking semantics
for the rx_dma() callback so that lockdep can be used to identify
possible problems in the future. It also simplifies synchronization
of UART_IER in general by not needing to rely on implementation
details such as 1 and 2.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20230525093159.223817-6-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index c17d98161d5ef1b3ee9a48db8a44521ac0098f4a..3cb9cfa62331c6f55ed9e767e06cb106225ff44f 100644 (file)
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -728,8 +728,11 @@ static int omap_8250_startup(struct uart_port *port)
                priv->wer |= OMAP_UART_TX_WAKEUP_EN;
        serial_out(up, UART_OMAP_WER, priv->wer);
 
-       if (up->dma && !(priv->habit & UART_HAS_EFR2))
+       if (up->dma && !(priv->habit & UART_HAS_EFR2)) {
+               spin_lock_irq(&port->lock);
                up->dma->rx_dma(up);
+               spin_unlock_irq(&port->lock);
+       }
 
        enable_irq(up->port.irq);
 
@@ -1003,6 +1006,9 @@ static int omap_8250_rx_dma(struct uart_8250_port *p)
        unsigned long                   flags;
        u32                             reg;
 
+       /* Port locked to synchronize UART_IER access against the console. */
+       lockdep_assert_held_once(&p->port.lock);
+
        if (priv->rx_dma_broken)
                return -EINVAL;
 
@@ -1736,8 +1742,11 @@ static int omap8250_runtime_resume(struct device *dev)
        if (up && omap8250_lost_context(up))
                omap8250_restore_regs(up);
 
-       if (up && up->dma && up->dma->rxchan && !(priv->habit & UART_HAS_EFR2))
+       if (up && up->dma && up->dma->rxchan && !(priv->habit & UART_HAS_EFR2)) {
+               spin_lock_irq(&up->port.lock);
                omap_8250_rx_dma(up);
+               spin_unlock_irq(&up->port.lock);
+       }
 
        priv->latency = priv->calc_latency;
        schedule_work(&priv->qos_work);