I noticed that the serial8250_tx_dma() is invoked sometimes while
uart_circ_empty() says that the buffer is empty.
I tracked one occuring down to:
n_tty_write()
=> O_OPOST(tty))
=> the while loop did something but neither tty's ->write()
nor its ->uart_put_char() callback was invoked().
=> tty->ops->flush_chars() is invoked with an empty buffer.
For the 8250 uart driver we end up with:
- DMA enabled
nothing, just return (except there is DMA_TX bug or runtime-PM then we
behave like in the no DMA case)
- no DMA
enable THRI interrupt, wait for it, disable THRI interrupt again
because there is nothing to be done.
While I don't know if it safe to drop that flush in n_tty if the buffer
is empty, it should not do any harm in serial's core part to not invoke
->start_tx() if the buffer is empty.
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
---
drivers/tty/serial/serial_core.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 5a78f6940760..e55724a911d5 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -90,7 +90,8 @@ static void __uart_start(struct tty_struct *tty)
struct uart_state *state = tty->driver_data;
struct uart_port *port = state->uart_port;
- if (!tty->stopped && !tty->hw_stopped)
+ if (!tty->stopped && !tty->hw_stopped &&
+ !uart_circ_empty(&port->state->xmit))
port->ops->start_tx(port);
}
--
2.1.0
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