synclink_gt: fix transmit DMA stall

Fix transmit DMA stall when write() called in window after previous
transmit DMA completes but before previous serial transmission completes.
Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/synclink_gt.c

 /*
- * $Id: synclink_gt.c,v 4.36 2006/08/28 20:47:14 paulkf Exp $
+ * $Id: synclink_gt.c,v 4.50 2007/07/25 19:29:25 paulkf Exp $
  *
  * Device driver for Microgate SyncLink GT serial adapters.
  *
@@ -93,7 +93,7 @@
  * module identification
  */
 static char *driver_name     = "SyncLink GT";
-static char *driver_version  = "$Revision: 4.36 $";
+static char *driver_version  = "$Revision: 4.50 $";
 static char *tty_driver_name = "synclink_gt";
 static char *tty_dev_prefix  = "ttySLG";
 MODULE_LICENSE("GPL");
@@ -477,6 +477,7 @@ static void tx_set_idle(struct slgt_info *info);
 static unsigned int free_tbuf_count(struct slgt_info *info);
 static void reset_tbufs(struct slgt_info *info);
 static void tdma_reset(struct slgt_info *info);
+static void tdma_start(struct slgt_info *info);
 static void tx_load(struct slgt_info *info, const char *buf, unsigned int count);
 
 static void get_signals(struct slgt_info *info);
@@ -904,6 +905,8 @@ static int write(struct tty_struct *tty,
 		spin_lock_irqsave(&info->lock,flags);
 		if (!info->tx_active)
 		 	tx_start(info);
+		else
+			tdma_start(info);
 		spin_unlock_irqrestore(&info->lock,flags);
  	}
 
@@ -3871,10 +3874,45 @@ static void tx_start(struct slgt_info *info)
 			slgt_irq_on(info, IRQ_TXUNDER + IRQ_TXIDLE);
 			/* clear tx idle and underrun status bits */
 			wr_reg16(info, SSR, (unsigned short)(IRQ_TXIDLE + IRQ_TXUNDER));
+			if (info->params.mode == MGSL_MODE_HDLC)
+				mod_timer(&info->tx_timer, jiffies +
+						msecs_to_jiffies(5000));
+		} else {
+			slgt_irq_off(info, IRQ_TXDATA);
+			slgt_irq_on(info, IRQ_TXIDLE);
+			/* clear tx idle status bit */
+			wr_reg16(info, SSR, IRQ_TXIDLE);
+		}
+		tdma_start(info);
+		info->tx_active = 1;
+	}
+}
+
+/*
+ * start transmit DMA if inactive and there are unsent buffers
+ */
+static void tdma_start(struct slgt_info *info)
+{
+	unsigned int i;
 
-			if (!(rd_reg32(info, TDCSR) & BIT0)) {
-				/* tx DMA stopped, restart tx DMA */
+	if (rd_reg32(info, TDCSR) & BIT0)
+		return;
+
+	/* transmit DMA inactive, check for unsent buffers */
+	i = info->tbuf_start;
+	while (!desc_count(info->tbufs[i])) {
+		if (++i == info->tbuf_count)
+			i = 0;
+		if (i == info->tbuf_current)
+			return;
+	}
+	info->tbuf_start = i;
+
+	/* there are unsent buffers, start transmit DMA */
+
+	/* reset needed if previous error condition */
 	tdma_reset(info);
+
 	/* set 1st descriptor address */
 	wr_reg32(info, TDDAR, info->tbufs[info->tbuf_start].pdesc);
 	switch(info->params.mode) {
@@ -3886,27 +3924,6 @@ static void tx_start(struct slgt_info *info)
 	default:
 		wr_reg32(info, TDCSR, BIT0); /* DMA enable */
 	}
-			}
-
-			if (info->params.mode == MGSL_MODE_HDLC)
-				mod_timer(&info->tx_timer, jiffies +
-						msecs_to_jiffies(5000));
-		} else {
-			tdma_reset(info);
-			/* set 1st descriptor address */
-			wr_reg32(info, TDDAR, info->tbufs[info->tbuf_start].pdesc);
-
-			slgt_irq_off(info, IRQ_TXDATA);
-			slgt_irq_on(info, IRQ_TXIDLE);
-			/* clear tx idle status bit */
-			wr_reg16(info, SSR, IRQ_TXIDLE);
-
-			/* enable tx DMA */
-			wr_reg32(info, TDCSR, BIT0);
-		}
-
-		info->tx_active = 1;
-	}
 }
 
 static void tx_stop(struct slgt_info *info)
@@ -4642,8 +4659,8 @@ static unsigned int free_tbuf_count(struct slgt_info *info)
 			i=0;
 	} while (i != info->tbuf_current);
 
-	/* last buffer with zero count may be in use, assume it is */
-	if (count)
+	/* if tx DMA active, last zero count buffer is in use */
+	if (count && (rd_reg32(info, TDCSR) & BIT0))
 		--count;
 
 	return count;