On 08/03/2015 06:34 PM, Peter Hurley wrote:

Hi Sebastian,

Hi Peter,

quoted

 struct old_serial_port {

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index d9a37191a1ae..12249125a218 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c

@@ -100,9 +100,9 @@ struct omap8250_priv {
 	u8 wer;
 	u8 xon;
 	u8 xoff;
-	u8 delayed_restore;
 	u16 quot;
 
+	wait_queue_head_t termios_wait;
 	bool is_suspending;
 	int wakeirq;
 	int wakeups_enabled;

@@ -256,18 +256,6 @@ static void omap8250_update_mdr1(struct uart_8250_port *up,
 static void omap8250_restore_regs(struct uart_8250_port *up)
 {
 	struct omap8250_priv *priv = up->port.private_data;
-	struct uart_8250_dma	*dma = up->dma;
-
-	if (dma && dma->tx_running) {
-		/*
-		 * TCSANOW requests the change to occur immediately however if
-		 * we have a TX-DMA operation in progress then it has been
-		 * observed that it might stall and never complete. Therefore we
-		 * delay DMA completes to prevent this hang from happen.
-		 */
-		priv->delayed_restore = 1;
-		return;
-	}
 
 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
 	serial_out(up, UART_EFR, UART_EFR_ECB);

@@ -309,6 +297,7 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
 	up->port.ops->set_mctrl(&up->port, up->port.mctrl);
 }
 
+static void omap_8250_dma_tx_complete(void *param);
 /*
  * OMAP can use "CLK / (16 or 13) / div" for baud rate. And then we have have
  * some differences in how we want to handle flow control.

@@ -322,6 +311,7 @@ static void omap_8250_set_termios(struct uart_port *port,
 	struct omap8250_priv *priv = up->port.private_data;
 	unsigned char cval = 0;
 	unsigned int baud;
+	unsigned int complete_dma = 0;
 
 	switch (termios->c_cflag & CSIZE) {
 	case CS5:

@@ -473,6 +463,25 @@ static void omap_8250_set_termios(struct uart_port *port,
 		if (termios->c_iflag & IXANY)
 			up->mcr |= UART_MCR_XONANY;
 	}
+
+	if (up->dma && up->dma->tx_running) {
+		struct uart_8250_dma	*dma = up->dma;
+
+		/*
+		 * TCSANOW requests the change to occur immediately however if
+		 * we have a TX-DMA operation in progress then it has been
+		 * observed that it might stall and never complete. Therefore we
+		 * wait until DMA completes to prevent this hang from happen.
+		 */
+
+		dma->tx_running = 2;
+
+		spin_unlock_irq(&up->port.lock);
+		wait_event(priv->termios_wait,
+			   dma->tx_running == 3);

Doesn't the dmaengine api offer a race-free way to wait for pending tx dma
to complete?

Not that I know of. You still need to ensure that once that DMA
completed, nobody triggers another TX transfer before you do what you
planned. This is ensures by the tx_running != 0 and the spin lock.

Maybe we could wrap that in the 8250 dma api?

You mean a function in 8250-dma API which does what I did just here
with the wait_event() and the wake_up in the callback? That way I could
move the termios_wait into the dma struct instead of keeping in the
omap specific part. I am also not sure if OMAP is the only one that may
hang here or the other people just didn't notice it yet.

Regards,
Peter Hurley

quoted

+		spin_lock_irq(&up->port.lock);
+		complete_dma = 1;
+	}
 	omap8250_restore_regs(up);
 
 	spin_unlock_irq(&up->port.lock);

@@ -488,6 +497,8 @@ static void omap_8250_set_termios(struct uart_port *port,
 	/* Don't rewrite B0 */
 	if (tty_termios_baud_rate(termios))
 		tty_termios_encode_baud_rate(termios, baud, baud);
+	if (complete_dma)
+		omap_8250_dma_tx_complete(up);
 }
 
 /* same as 8250 except that we may have extra flow bits set in EFR */

@@ -869,17 +880,18 @@ static void omap_8250_dma_tx_complete(void *param)
 
 	spin_lock_irqsave(&p->port.lock, flags);
 
+	if (dma->tx_running == 2) {
+		dma->tx_running = 3;
+		wake_up(&priv->termios_wait);
+		goto out;
+	}
+
 	dma->tx_running = 0;
 
 	xmit->tail += dma->tx_size;
 	xmit->tail &= UART_XMIT_SIZE - 1;
 	p->port.icount.tx += dma->tx_size;
 
-	if (priv->delayed_restore) {
-		priv->delayed_restore = 0;
-		omap8250_restore_regs(p);
-	}
-
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(&p->port);
 

@@ -899,7 +911,7 @@ static void omap_8250_dma_tx_complete(void *param)
 		p->ier |= UART_IER_THRI;
 		serial_port_out(&p->port, UART_IER, p->ier);
 	}
-
+out:
 	spin_unlock_irqrestore(&p->port.lock, flags);
 }
 

@@ -1216,6 +1228,7 @@ static int omap8250_probe(struct platform_device *pdev)
 			priv->omap8250_dma.rx_size = RX_TRIGGER;
 			priv->omap8250_dma.rxconf.src_maxburst = RX_TRIGGER;
 			priv->omap8250_dma.txconf.dst_maxburst = TX_TRIGGER;
+			init_waitqueue_head(&priv->termios_wait);
 
 			if (of_machine_is_compatible("ti,am33xx"))
 				priv->habit |= OMAP_DMA_TX_KICK;

Sebastian