tty: serial: 8250_omap: add custom DMA-TX callback

This patch provides mostly a copy of serial8250_tx_dma() +
__dma_tx_complete() with the following extensions:

- DMA bug
  At least on AM335x the following problem exists: Even if the TX FIFO is
  empty and a TX transfer is programmed (and started) the UART does not
  trigger the DMA transfer.
  After $TRESHOLD number of bytes have been written to the FIFO manually the
  UART reevaluates the whole situation and decides that now there is enough
  room in the FIFO and so the transfer begins.
  This problem has not been seen on DRA7 or beagle board xm (OMAP3). I am not
  sure if this is UART-IP core specific or DMA engine.

  The workaround is to use a threshold of one byte, program the DMA
  transfer minus one byte and then to put the first byte into the FIFO to
  kick start the transfer.

- support for runtime PM
  RPM is enabled on start_tx(). We can't disable RPM on DMA complete callback
  because there is still data in the FIFO which is being sent. We have to wait
  until the FIFO is empty before we disable it.
  For this to happen we fake a TX sent error and enable THRI. Once the
  FIFO is empty we receive an interrupt and since the TTY-buffer is still
  empty we "put RPM" via __stop_tx(). Should it been filed then in the
  start_tx() path we should program the DMA transfer and remove the error
  flag and the THRI bit.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -22,6 +22,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/console.h>
 #include <linux/pm_qos.h>
+#include <linux/dma-mapping.h>
 
 #include "8250.h"
 
@@ -29,6 +30,7 @@
 
 #define UART_ERRATA_i202_MDR1_ACCESS	(1 << 0)
 #define OMAP_UART_WER_HAS_TX_WAKEUP	(1 << 1)
+#define OMAP_DMA_TX_KICK		(1 << 2)
 
 #define OMAP_UART_FCR_RX_TRIG		6
 #define OMAP_UART_FCR_TX_TRIG		4
@@ -616,6 +618,148 @@ static void omap_8250_unthrottle(struct uart_port *port)
 	pm_runtime_put_autosuspend(port->dev);
 }
 
+#ifdef CONFIG_SERIAL_8250_DMA
+static int omap_8250_tx_dma(struct uart_8250_port *p);
+
+static void omap_8250_dma_tx_complete(void *param)
+{
+	struct uart_8250_port	*p = param;
+	struct uart_8250_dma	*dma = p->dma;
+	struct circ_buf		*xmit = &p->port.state->xmit;
+	unsigned long		flags;
+	bool			en_thri = false;
+
+	dma_sync_single_for_cpu(dma->txchan->device->dev, dma->tx_addr,
+				UART_XMIT_SIZE, DMA_TO_DEVICE);
+
+	spin_lock_irqsave(&p->port.lock, flags);
+
+	dma->tx_running = 0;
+
+	xmit->tail += dma->tx_size;
+	xmit->tail &= UART_XMIT_SIZE - 1;
+	p->port.icount.tx += dma->tx_size;
+
+	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
+		uart_write_wakeup(&p->port);
+
+	if (!uart_circ_empty(xmit) && !uart_tx_stopped(&p->port)) {
+		int ret;
+
+		ret = omap_8250_tx_dma(p);
+		if (ret)
+			en_thri = true;
+
+	} else if (p->capabilities & UART_CAP_RPM) {
+		en_thri = true;
+	}
+
+	if (en_thri) {
+		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);
+}
+
+static int omap_8250_tx_dma(struct uart_8250_port *p)
+{
+	struct uart_8250_dma		*dma = p->dma;
+	struct omap8250_priv		*priv = p->port.private_data;
+	struct circ_buf			*xmit = &p->port.state->xmit;
+	struct dma_async_tx_descriptor	*desc;
+	unsigned int	skip_byte = 0;
+	int ret;
+
+	if (dma->tx_running)
+		return 0;
+	if (uart_tx_stopped(&p->port) || uart_circ_empty(xmit)) {
+
+		/*
+		 * Even if no data, we need to return an error for the two cases
+		 * below so serial8250_tx_chars() is invoked and properly clears
+		 * THRI and/or runtime suspend.
+		 */
+		if (dma->tx_err || p->capabilities & UART_CAP_RPM) {
+			ret = -EBUSY;
+			goto err;
+		}
+		if (p->ier & UART_IER_THRI) {
+			p->ier &= ~UART_IER_THRI;
+			serial_out(p, UART_IER, p->ier);
+		}
+		return 0;
+	}
+
+	dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
+	if (priv->habit & OMAP_DMA_TX_KICK) {
+		u8 tx_lvl;
+
+		/*
+		 * We need to put the first byte into the FIFO in order to start
+		 * the DMA transfer. For transfers smaller than four bytes we
+		 * don't bother doing DMA at all. It seem not matter if there
+		 * are still bytes in the FIFO from the last transfer (in case
+		 * we got here directly from omap_8250_dma_tx_complete()). Bytes
+		 * leaving the FIFO seem not to trigger the DMA transfer. It is
+		 * really the byte that we put into the FIFO.
+		 * If the FIFO is already full then we most likely got here from
+		 * omap_8250_dma_tx_complete(). And this means the DMA engine
+		 * just completed its work. We don't have to wait the complete
+		 * 86us at 115200,8n1 but around 60us (not to mention lower
+		 * baudrates). So in that case we take the interrupt and try
+		 * again with an empty FIFO.
+		 */
+		tx_lvl = serial_in(p, UART_OMAP_TX_LVL);
+		if (tx_lvl == p->tx_loadsz) {
+			ret = -EBUSY;
+			goto err;
+		}
+		if (dma->tx_size < 4) {
+			ret = -EINVAL;
+			goto err;
+		}
+		skip_byte = 1;
+	}
+
+	desc = dmaengine_prep_slave_single(dma->txchan,
+			dma->tx_addr + xmit->tail + skip_byte,
+			dma->tx_size - skip_byte, DMA_MEM_TO_DEV,
+			DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
+	if (!desc) {
+		ret = -EBUSY;
+		goto err;
+	}
+
+	dma->tx_running = 1;
+
+	desc->callback = omap_8250_dma_tx_complete;
+	desc->callback_param = p;
+
+	dma->tx_cookie = dmaengine_submit(desc);
+
+	dma_sync_single_for_device(dma->txchan->device->dev, dma->tx_addr,
+				   UART_XMIT_SIZE, DMA_TO_DEVICE);
+
+	dma_async_issue_pending(dma->txchan);
+	if (dma->tx_err)
+		dma->tx_err = 0;
+
+	if (p->ier & UART_IER_THRI) {
+		p->ier &= ~UART_IER_THRI;
+		serial_out(p, UART_IER, p->ier);
+	}
+	if (skip_byte)
+		serial_out(p, UART_TX, xmit->buf[xmit->tail]);
+	return 0;
+err:
+	dma->tx_err = 1;
+	return ret;
+}
+
+#endif
+
 static int omap8250_probe(struct platform_device *pdev)
 {
 	struct resource *regs = platform_get_resource(pdev, IORESOURCE_MEM, 0);

include/uapi/linux/serial_reg.h

@@ -359,6 +359,7 @@
 #define UART_OMAP_SYSC		0x15	/* System configuration register */
 #define UART_OMAP_SYSS		0x16	/* System status register */
 #define UART_OMAP_WER		0x17	/* Wake-up enable register */
+#define UART_OMAP_TX_LVL	0x1a	/* TX FIFO level register */
 
 /*
  * These are the definitions for the MDR1 register