[PATCH] Fix a DMA underrun problem with pmac IDE

This fixes the behaviour of the pmac "macio" IDE driver when a DMA
transfer happen to get less data out of the device than expected when
setting up the DMA commands (the device underruns).  This is very common
with recent ATAPI stuffs and used to cause problem & disable DMA.  This
patch fixes the way we handle that condition, thus also fixing DVD
burning on a bunch of recent pmacs. 

drivers/ide/ppc/pmac.c

@@ -55,7 +55,7 @@ extern void ide_do_request(ide_hwgroup_t *hwgroup, int masked_irq);
 
 #define IDE_PMAC_DEBUG
 
-#define DMA_WAIT_TIMEOUT	100
+#define DMA_WAIT_TIMEOUT	50
 
 typedef struct pmac_ide_hwif {
 	unsigned long			regbase;
@@ -2026,8 +2026,11 @@ pmac_ide_dma_end (ide_drive_t *drive)
 	dstat = readl(&dma->status);
 	writel(((RUN|WAKE|DEAD) << 16), &dma->control);
 	pmac_ide_destroy_dmatable(drive);
-	/* verify good dma status */
-	return (dstat & (RUN|DEAD|ACTIVE)) != RUN;
+	/* verify good dma status. we don't check for ACTIVE beeing 0. We should...
+	 * in theory, but with ATAPI decices doing buffer underruns, that would
+	 * cause us to disable DMA, which isn't what we want
+	 */
+	return (dstat & (RUN|DEAD)) != RUN;
 }
 
 /*
@@ -2041,7 +2044,7 @@ pmac_ide_dma_test_irq (ide_drive_t *drive)
 {
 	pmac_ide_hwif_t* pmif = (pmac_ide_hwif_t *)HWIF(drive)->hwif_data;
 	volatile struct dbdma_regs *dma;
-	unsigned long status;
+	unsigned long status, timeout;
 
 	if (pmif == NULL)
 		return 0;
@@ -2057,17 +2060,8 @@ pmac_ide_dma_test_irq (ide_drive_t *drive)
 	 * - The dbdma fifo hasn't yet finished flushing to
 	 * to system memory when the disk interrupt occurs.
 	 * 
-	 * The trick here is to increment drive->waiting_for_dma,
-	 * and return as if no interrupt occurred. If the counter
-	 * reach a certain timeout value, we then return 1. If
-	 * we really got the interrupt, it will happen right away
-	 * again.
-	 * Apple's solution here may be more elegant. They issue
-	 * a DMA channel interrupt (a separate irq line) via a DBDMA
-	 * NOP command just before the STOP, and wait for both the
-	 * disk and DBDMA interrupts to have completed.
 	 */
- 
+
 	/* If ACTIVE is cleared, the STOP command have passed and
 	 * transfer is complete.
 	 */
@@ -2079,15 +2073,26 @@ pmac_ide_dma_test_irq (ide_drive_t *drive)
 			called while not waiting\n", HWIF(drive)->index);
 
 	/* If dbdma didn't execute the STOP command yet, the
-	 * active bit is still set */
-	drive->waiting_for_dma++;
-	if (drive->waiting_for_dma >= DMA_WAIT_TIMEOUT) {
-		printk(KERN_WARNING "ide%d, timeout waiting \
-			for dbdma command stop\n", HWIF(drive)->index);
-		return 1;
-	}
-	udelay(5);
-	return 0;
+	 * active bit is still set. We consider that we aren't
+	 * sharing interrupts (which is hopefully the case with
+	 * those controllers) and so we just try to flush the
+	 * channel for pending data in the fifo
+	 */
+	udelay(1);
+	writel((FLUSH << 16) | FLUSH, &dma->control);
+	timeout = 0;
+	for (;;) {
+		udelay(1);
+		status = readl(&dma->status);
+		if ((status & FLUSH) == 0)
+			break;
+		if (++timeout > 100) {
+			printk(KERN_WARNING "ide%d, ide_dma_test_irq \
+			timeout flushing channel\n", HWIF(drive)->index);
+			break;
+		}
+	}	
+	return 1;
 }
 
 static int __pmac