[PATCH 13/13] tty: serial: 8250: omap: add dma support

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

This patch adds the required pieces to 8250-OMAP UART driver for DMA
support. The TX burst size is set to 1 so we can send an arbitrary
amount of bytes.

The RX burst is currently set to 48 which means we receive an DMA
interrupt every 48 bytes and have to reprogram everything. Less bytes in
the RX-FIFO mean that no DMA transfer will happen and the UART will send a
RX-timeout _or_ RDI event at which point the FIFO will be manually purged.
There is a workaround for TX-DMA on AM33xx where we put the first byte
into the FIFO to kick start the DMA process. Haven't seen this problem on
OMAP36xx (beagle board xm) or DRA7xx.

On AM375x there is "Usage Note 2.7: UART: Cannot Acknowledge Idle
Requests in Smartidle Mode When Configured for DMA Operations" in the
errata document. This problem persists even after disabling DMA in the
UART and will be addressed in the HWMOD.

v10:
	- delay update_registers() from set_termios() until TX-DMA is
	  done. It has been reported / proved that invoking
	  update_registers() while TX-DMA is in progress may stall the
	  DMA operation and it won't finish.
	- use the new omap DMA-TX-RX hooks and DMA only interrupt
	  routine.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/tty/serial/8250/8250_omap.c | 71 +++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 6500547f8fda..57a8b1241b85 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -88,6 +88,7 @@ struct omap8250_priv {
 	u8 wer;
 	u8 xon;
 	u8 xoff;
+	u8 delayed_restore;
 	u16 quot;
 
 	bool is_suspending;
@@ -211,6 +212,18 @@ static void omap8250_update_scr(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);
@@ -375,6 +388,10 @@ static void omap_8250_set_termios(struct uart_port *port,
 	priv->scr = OMAP_UART_SCR_RX_TRIG_GRANU1_MASK | OMAP_UART_SCR_TX_EMPTY |
 		OMAP_UART_SCR_TX_TRIG_GRANU1_MASK;
 
+	if (up->dma)
+		priv->scr |= OMAP_UART_SCR_DMAMODE_1 |
+			OMAP_UART_SCR_DMAMODE_CTL;
+
 	priv->xon = termios->c_cc[VSTART];
 	priv->xoff = termios->c_cc[VSTOP];
 
@@ -554,6 +571,9 @@ 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)
+		up->dma->rx_dma(up, 0);
+
 	pm_runtime_mark_last_busy(port->dev);
 	pm_runtime_put_autosuspend(port->dev);
 	return 0;
@@ -572,6 +592,8 @@ static void omap_8250_shutdown(struct uart_port *port)
 	struct omap8250_priv *priv = port->private_data;
 
 	flush_work(&priv->qos_work);
+	if (up->dma)
+		up->dma->rx_dma(up, UART_IIR_RX_TIMEOUT);
 
 	pm_runtime_get_sync(port->dev);
 
@@ -725,6 +747,7 @@ static void omap_8250_dma_tx_complete(void *param)
 	struct circ_buf		*xmit = &p->port.state->xmit;
 	unsigned long		flags;
 	bool			en_thri = false;
+	struct omap8250_priv	*priv = p->port.private_data;
 
 	dma_sync_single_for_cpu(dma->txchan->device->dev, dma->tx_addr,
 				UART_XMIT_SIZE, DMA_TO_DEVICE);
@@ -737,6 +760,11 @@ static void omap_8250_dma_tx_complete(void *param)
 	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);
 
@@ -909,6 +937,18 @@ static int omap_8250_dma_handle_irq(struct uart_port *port)
 	serial8250_rpm_put(up);
 	return 1;
 }
+
+static bool the_no_dma_filter_fn(struct dma_chan *chan, void *param)
+{
+	return false;
+}
+
+#else
+
+static inline int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
+{
+	return -EINVAL;
+}
 #endif
 
 static int omap8250_probe(struct platform_device *pdev)
@@ -1010,6 +1050,32 @@ static int omap8250_probe(struct platform_device *pdev)
 	pm_runtime_get_sync(&pdev->dev);
 
 	omap_serial_fill_features_erratas(&up, priv);
+#ifdef CONFIG_SERIAL_8250_DMA
+	if (pdev->dev.of_node) {
+		/*
+		 * Oh DMA support. If there are no DMA properties in the DT then
+		 * we will fall back to a generic DMA channel which does not
+		 * really work here. To ensure that we do not get a generic DMA
+		 * channel assigned, we have the the_no_dma_filter_fn() here.
+		 * To avoid "failed to request DMA" messages we check for DMA
+		 * properties in DT.
+		 */
+		ret = of_property_count_strings(pdev->dev.of_node, "dma-names");
+		if (ret == 2) {
+			up.dma = &priv->omap8250_dma;
+			up.port.handle_irq = omap_8250_dma_handle_irq;
+			priv->omap8250_dma.fn = the_no_dma_filter_fn;
+			priv->omap8250_dma.tx_dma = omap_8250_tx_dma;
+			priv->omap8250_dma.rx_dma = omap_8250_rx_dma;
+			priv->omap8250_dma.rx_size = RX_TRIGGER;
+			priv->omap8250_dma.rxconf.src_maxburst = RX_TRIGGER;
+			priv->omap8250_dma.txconf.dst_maxburst = TX_TRIGGER;
+
+			if (of_machine_is_compatible("ti,am33xx"))
+				priv->habit |= OMAP_DMA_TX_KICK;
+		}
+	}
+#endif
 	ret = serial8250_register_8250_port(&up);
 	if (ret < 0) {
 		dev_err(&pdev->dev, "unable to register 8250 port\n");
@@ -1146,6 +1212,8 @@ static int omap8250_runtime_suspend(struct device *dev)
 	}
 
 	omap8250_enable_wakeup(priv, true);
+	if (up->dma)
+		omap_8250_rx_dma(up, UART_IIR_RX_TIMEOUT);
 
 	priv->latency = PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE;
 	schedule_work(&priv->qos_work);
@@ -1170,6 +1238,9 @@ static int omap8250_runtime_resume(struct device *dev)
 	if (loss_cntx)
 		omap8250_restore_regs(up);
 
+	if (up->dma)
+		omap_8250_rx_dma(up, 0);
+
 	priv->latency = priv->calc_latency;
 	schedule_work(&priv->qos_work);
 	return 0;
-- 
2.1.0