[PATCH tty v1 5/8] serial: 8250: lock port for rx_dma() callback

John Ogness
2023-05-25T09:35:13Z

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>
---
drivers/tty/serial/8250/8250_omap.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index c17d98161d5e..3cb9cfa62331 100644
--- 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);
--
2.30.2