On 09/10/2014 03:33 PM, Sebastian Andrzej Siewior wrote: > 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. The serial core can't assume that start_tx() does not need invoking because hardware that can stop_tx() with data in the transmitter won't restart if the ring buffer is empty but data is still in the transmitter. [Note that the 16C950 port type does this in the 8250 driver.] So this has to be handled in the 8250 driver. What is the actual issue? Are you trying not to unnecessarily wake the omap hardware if runtime-PM is on? Regards, Peter Hurley > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > --- > 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); > } > > -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
