[patch] 2.3.40pre6: Take 3: avoid bounce buffers

[Tim Waugh]

Hi Linus,

Here's my third attempt to get this right. ;-)  It tries to avoid a DMA
bounce buffer when possible.

Tim.
*/

Index: linux/drivers/parport/parport_pc.c
diff -u linux/drivers/parport/parport_pc.c:1.1.1.12 linux/drivers/parport/parport_pc.c:1.20
--- linux/drivers/parport/parport_pc.c:1.1.1.12	Tue Jan 18 11:16:06 2000
+++ linux/drivers/parport/parport_pc.c	Thu Jan 20 18:03:59 2000
@@ -549,9 +549,23 @@
 {
 	int ret = 0;
 	unsigned long dmaflag;
-	size_t left  = length;
+	size_t left = length;
 	const struct parport_pc_private *priv = port->physport->private_data;
+	unsigned long dma_addr;
+	size_t maxlen = 0x10000; /* max 64k per DMA transfer */
 
+	/* above 16 MB we use a bounce buffer as ISA-DMA is not possible */
+	if (buf+length <= MAX_DMA_ADDRESS) {
+                /* If it would cross a 64k boundary, cap it at the end. */
+                if ((buf ^ (buf+length-1)) & ~0xffff)
+                        maxlen = (0x10000 - buf) & 0xffff;
+
+		dma_addr = virt_to_bus(buf);
+        } else {
+		dma_addr = virt_to_bus(priv->dma_buf);
+		maxlen   = PAGE_SIZE;          /* sizeof(priv->dma_buf) */
+	}
+
 	port = port->physport;
 
 	/* We don't want to be interrupted every character. */
@@ -566,16 +580,17 @@
 
 		size_t count = left;
 
-		if (count > PAGE_SIZE)
-			count = PAGE_SIZE;
+		if (count > maxlen)
+			count = maxlen;
 
-		memcpy(priv->dma_buf, buf, count);
+		if (maxlen == PAGE_SIZE)   /* bounce buffer ! */
+			memcpy(priv->dma_buf, buf, count);
 
 		dmaflag = claim_dma_lock();
 		disable_dma(port->dma);
 		clear_dma_ff(port->dma);
 		set_dma_mode(port->dma, DMA_MODE_WRITE);
-		set_dma_addr(port->dma, virt_to_bus((volatile char *) priv->dma_buf));
+		set_dma_addr(port->dma, dma_addr);
 		set_dma_count(port->dma, count);
 
 		/* Set DMA mode */


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