serial/amba-pl011: Refactor and simplify TX FIFO handling

Commit 734745ca serial/amba-pl011: Activate TX IRQ passively
adds some complexity and overhead in the form of a softirq
mechanism for transmitting in the absence of interrupts.

This patch simplifies the code flow to reduce the reliance on
subtle behaviour and avoid fragility under future maintenance.

To this end, the TX softirq mechanism is removed and instead
pl011_start_tx() will now simply stuff the FIFO until full
(guaranteeing future TX IRQs), or until there are no more chars
to write (in which case we don't care whether an IRQ happens).
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl011.c

@@ -58,7 +58,6 @@
 #include <linux/pinctrl/consumer.h>
 #include <linux/sizes.h>
 #include <linux/io.h>
-#include <linux/workqueue.h>
 
 #define UART_NR			14
 
@@ -157,9 +156,7 @@ struct uart_amba_port {
 	unsigned int		lcrh_tx;	/* vendor-specific */
 	unsigned int		lcrh_rx;	/* vendor-specific */
 	unsigned int		old_cr;		/* state during shutdown */
-	struct delayed_work	tx_softirq_work;
 	bool			autorts;
-	unsigned int		tx_irq_seen;	/* 0=none, 1=1, 2=2 or more */
 	char			type[12];
 #ifdef CONFIG_DMA_ENGINE
 	/* DMA stuff */
@@ -1172,15 +1169,14 @@ static void pl011_stop_tx(struct uart_port *port)
 	pl011_dma_tx_stop(uap);
 }
 
-static bool pl011_tx_chars(struct uart_amba_port *uap);
+static void pl011_tx_chars(struct uart_amba_port *uap, bool from_irq);
 
 /* Start TX with programmed I/O only (no DMA) */
 static void pl011_start_tx_pio(struct uart_amba_port *uap)
 {
 	uap->im |= UART011_TXIM;
 	writew(uap->im, uap->port.membase + UART011_IMSC);
-	if (!uap->tx_irq_seen)
-		pl011_tx_chars(uap);
+	pl011_tx_chars(uap, false);
 }
 
 static void pl011_start_tx(struct uart_port *port)
@@ -1247,87 +1243,54 @@ __acquires(&uap->port.lock)
 	spin_lock(&uap->port.lock);
 }
 
-/*
- * Transmit a character
- * There must be at least one free entry in the TX FIFO to accept the char.
- *
- * Returns true if the FIFO might have space in it afterwards;
- * returns false if the FIFO definitely became full.
- */
-static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c)
+static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c,
+			  bool from_irq)
 {
+	if (unlikely(!from_irq) &&
+	    readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF)
+		return false; /* unable to transmit character */
+
 	writew(c, uap->port.membase + UART01x_DR);
 	uap->port.icount.tx++;
 
-	if (likely(uap->tx_irq_seen > 1))
-		return true;
-
-	return !(readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF);
+	return true;
 }
 
-static bool pl011_tx_chars(struct uart_amba_port *uap)
+static void pl011_tx_chars(struct uart_amba_port *uap, bool from_irq)
 {
 	struct circ_buf *xmit = &uap->port.state->xmit;
-	int count;
-
-	if (unlikely(uap->tx_irq_seen < 2))
-		/*
-		 * Initial FIFO fill level unknown: we must check TXFF
-		 * after each write, so just try to fill up the FIFO.
-		 */
-		count = uap->fifosize;
-	else /* tx_irq_seen >= 2 */
-		/*
-		 * FIFO initially at least half-empty, so we can simply
-		 * write half the FIFO without polling TXFF.
-
-		 * Note: the *first* TX IRQ can still race with
-		 * pl011_start_tx_pio(), which can result in the FIFO
-		 * being fuller than expected in that case.
-		 */
-		count = uap->fifosize >> 1;
-
-	/*
-	 * If the FIFO is full we're guaranteed a TX IRQ at some later point,
-	 * and can't transmit immediately in any case:
-	 */
-	if (unlikely(uap->tx_irq_seen < 2 &&
-		     readw(uap->port.membase + UART01x_FR) & UART01x_FR_TXFF))
-		return false;
+	int count = uap->fifosize >> 1;
 
 	if (uap->port.x_char) {
-		pl011_tx_char(uap, uap->port.x_char);
+		if (!pl011_tx_char(uap, uap->port.x_char, from_irq))
+			return;
 		uap->port.x_char = 0;
 		--count;
 	}
 	if (uart_circ_empty(xmit) || uart_tx_stopped(&uap->port)) {
 		pl011_stop_tx(&uap->port);
-		goto done;
+		return;
 	}
 
 	/* If we are using DMA mode, try to send some characters. */
 	if (pl011_dma_tx_irq(uap))
-		goto done;
+		return;
 
-	while (count-- > 0 && pl011_tx_char(uap, xmit->buf[xmit->tail])) {
-		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-		if (uart_circ_empty(xmit))
+	do {
+		if (likely(from_irq) && count-- == 0)
 			break;
-	}
+
+		if (!pl011_tx_char(uap, xmit->buf[xmit->tail], from_irq))
+			break;
+
+		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
+	} while (!uart_circ_empty(xmit));
 
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(&uap->port);
 
-	if (uart_circ_empty(xmit)) {
+	if (uart_circ_empty(xmit))
 		pl011_stop_tx(&uap->port);
-		goto done;
-	}
-
-	if (unlikely(!uap->tx_irq_seen))
-		schedule_delayed_work(&uap->tx_softirq_work, uap->port.timeout);
-
-done:
-	return false;
 }
 
 static void pl011_modem_status(struct uart_amba_port *uap)
@@ -1354,28 +1317,6 @@ static void pl011_modem_status(struct uart_amba_port *uap)
 	wake_up_interruptible(&uap->port.state->port.delta_msr_wait);
 }
 
-static void pl011_tx_softirq(struct work_struct *work)
-{
-	struct delayed_work *dwork = to_delayed_work(work);
-	struct uart_amba_port *uap =
-		container_of(dwork, struct uart_amba_port, tx_softirq_work);
-
-	spin_lock(&uap->port.lock);
-	while (pl011_tx_chars(uap)) ;
-	spin_unlock(&uap->port.lock);
-}
-
-static void pl011_tx_irq_seen(struct uart_amba_port *uap)
-{
-	if (likely(uap->tx_irq_seen > 1))
-		return;
-
-	uap->tx_irq_seen++;
-	if (uap->tx_irq_seen < 2)
-		/* first TX IRQ */
-		cancel_delayed_work(&uap->tx_softirq_work);
-}
-
 static irqreturn_t pl011_int(int irq, void *dev_id)
 {
 	struct uart_amba_port *uap = dev_id;
@@ -1414,10 +1355,8 @@ static irqreturn_t pl011_int(int irq, void *dev_id)
 			if (status & (UART011_DSRMIS|UART011_DCDMIS|
 				      UART011_CTSMIS|UART011_RIMIS))
 				pl011_modem_status(uap);
-			if (status & UART011_TXIS) {
-				pl011_tx_irq_seen(uap);
-				pl011_tx_chars(uap);
-			}
+			if (status & UART011_TXIS)
+				pl011_tx_chars(uap, true);
 
 			if (pass_counter-- == 0)
 				break;
@@ -1639,9 +1578,6 @@ static int pl011_startup(struct uart_port *port)
 
 	writew(uap->vendor->ifls, uap->port.membase + UART011_IFLS);
 
-	/* Assume that TX IRQ doesn't work until we see one: */
-	uap->tx_irq_seen = 0;
-
 	spin_lock_irq(&uap->port.lock);
 
 	/* restore RTS and DTR */
@@ -1697,8 +1633,6 @@ static void pl011_shutdown(struct uart_port *port)
 	    container_of(port, struct uart_amba_port, port);
 	unsigned int cr;
 
-	cancel_delayed_work_sync(&uap->tx_softirq_work);
-
 	/*
 	 * disable all interrupts
 	 */
@@ -2245,7 +2179,6 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
 	uap->port.ops = &amba_pl011_pops;
 	uap->port.flags = UPF_BOOT_AUTOCONF;
 	uap->port.line = i;
-	INIT_DELAYED_WORK(&uap->tx_softirq_work, pl011_tx_softirq);
 
 	/* Ensure interrupts from this UART are masked and cleared */
 	writew(0, uap->port.membase + UART011_IMSC);