[PATCH 06/15] tty: serial: 8250_dma: handle error on TX submit

[Sebastian Andrzej Siewior]

Right now it is possible that serial8250_tx_dma() fails and returns
-EBUSY. The caller (serial8250_start_tx()) will then enable
UART_IER_THRI which will generate an interrupt once the TX FIFO is
empty.
In serial8250_handle_irq() nothing will happen because up->dma is set
and so serial8250_tx_chars() won't be invoked. We end up with plenty of
interrupts and some "too much work for irq" output.

This patch introduces dma_tx_err in struct uart_8250_port to signal that
the last invocation of serial8250_tx_dma() failed so we can fill the TX
FIFO manually. Should the next invocation of serial8250_start_tx()
succeed then the dma_tx_err flag along with the THRI bit is removed and
DMA only usage may continue.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
---
 drivers/tty/serial/8250/8250_core.c |  2 +-
 drivers/tty/serial/8250/8250_dma.c  | 30 +++++++++++++++++++++++++-----
 include/linux/serial_8250.h         |  1 +
 3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index ab003b6..7498ce1 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1600,7 +1600,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 			status = serial8250_rx_chars(up, status);
 	}
 	serial8250_modem_status(up);
-	if (!up->dma && (status & UART_LSR_THRE))
+	if ((!up->dma || up->dma_tx_err) && (status & UART_LSR_THRE))
 		serial8250_tx_chars(up);
 
 	spin_unlock_irqrestore(&port->lock, flags);
diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 148ffe4..5c59b15 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -36,8 +36,16 @@ static void __dma_tx_complete(void *param)
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(&p->port);
 
-	if (!uart_circ_empty(xmit) && !uart_tx_stopped(&p->port))
-		serial8250_tx_dma(p);
+	if (!uart_circ_empty(xmit) && !uart_tx_stopped(&p->port)) {
+		int ret;
+
+		ret = serial8250_tx_dma(p);
+		if (ret) {
+			p->dma_tx_err = 1;
+			p->ier |= UART_IER_THRI;
+			serial_port_out(&p->port, UART_IER, p->ier);
+		}
+	}
 
 	spin_unlock_irqrestore(&p->port.lock, flags);
 }
@@ -69,6 +77,7 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 	struct uart_8250_dma		*dma = p->dma;
 	struct circ_buf			*xmit = &p->port.state->xmit;
 	struct dma_async_tx_descriptor	*desc;
+	int ret;
 
 	if (uart_tx_stopped(&p->port) || dma->tx_running ||
 	    uart_circ_empty(xmit))
@@ -80,8 +89,10 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 					   dma->tx_addr + xmit->tail,
 					   dma->tx_size, DMA_MEM_TO_DEV,
 					   DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
-	if (!desc)
-		return -EBUSY;
+	if (!desc) {
+		ret = -EBUSY;
+		goto err;
+	}
 
 	dma->tx_running = 1;
 
@@ -94,8 +105,17 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 				   UART_XMIT_SIZE, DMA_TO_DEVICE);
 
 	dma_async_issue_pending(dma->txchan);
-
+	if (p->dma_tx_err) {
+		p->dma_tx_err = 0;
+		if (p->ier & UART_IER_THRI) {
+			p->ier &= ~UART_IER_THRI;
+			serial_out(p, UART_IER, p->ier);
+		}
+	}
 	return 0;
+err:
+	p->dma_tx_err = 1;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(serial8250_tx_dma);
 
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index b161eee..02e82dc 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -85,6 +85,7 @@ struct uart_8250_port {
 	unsigned char		mcr_force;	/* mask of forced bits */
 	unsigned char		cur_iotype;	/* Running I/O type */
 	unsigned char		rpm_tx_active;
+	unsigned char		dma_tx_err;
 
 	/*
 	 * Some bits in registers are cleared on a read, so they must
-- 
2.0.1