Implement write combining for crashdumps. This is useful when

write caching is disabled on both SCSI and IDE disks where large
memory dumps could take up to an hour to complete.

Taking an i386 scsi based system with 512MB of ram and timing (in
seconds) how long it took to complete a dump, the following results
were obtained:

Before:				After:
	WCE           TIME		WCE           TIME
	------------------		------------------
	1	141.820972		1	 15.600111
	0	797.265072		0	 65.480465

Obtained from:	Yahoo!
Reviewed by:	peter

1 files changed, 20 insertions, 8 deletions

diff --git a/sys/dev/ata/ata-disk.c b/sys/dev/ata/ata-disk.c
index dba5fbe..85421c2 100644
--- a/sys/dev/ata/ata-disk.c
+++ b/sys/dev/ata/ata-disk.c
@@ -257,7 +257,10 @@ addump(dev_t dev)
     struct ad_request request;
     u_int count, blkno, secsize;
     vm_offset_t addr = 0;
+    long blkcnt;
+    int dumppages = MAXDUMPPGS;
     int error;
+    int i;
 
     if ((error = disk_dumpcheck(dev, &count, &blkno, &secsize)))
 	return error;
@@ -269,18 +272,27 @@ addump(dev_t dev)
     adp->controller->mode[ATA_DEV(adp->unit)] = ATA_PIO;
     ata_reinit(adp->controller);
 
+    blkcnt = howmany(PAGE_SIZE, secsize);
+
     while (count > 0) {
 	caddr_t va;
 	DELAY(1000);
-	if (is_physical_memory(addr))
-	    va = pmap_kenter_temporary(trunc_page(addr));
-	else
-	    va = pmap_kenter_temporary(trunc_page(0));
+
+	if ((count / blkcnt) < dumppages)
+	    dumppages = count / blkcnt;
+
+	for (i = 0; i < dumppages; ++i) {
+	    vm_offset_t a = addr + (i * PAGE_SIZE);
+	    if (is_physical_memory(a))
+		va = pmap_kenter_temporary(trunc_page(a), i);
+	    else
+		va = pmap_kenter_temporary(trunc_page(0), i);
+	}
 
 	bzero(&request, sizeof(struct ad_request));
 	request.device = adp;
 	request.blockaddr = blkno;
-	request.bytecount = PAGE_SIZE;
+	request.bytecount = PAGE_SIZE * dumppages;
 	request.data = va;
 
 	while (request.bytecount > 0) {
@@ -300,9 +312,9 @@ addump(dev_t dev)
 	    printf("%ld ", (long)(count * DEV_BSIZE) / (1024 * 1024));
 	}
 
-	blkno += howmany(PAGE_SIZE, secsize);
-	count -= howmany(PAGE_SIZE, secsize);
-	addr += PAGE_SIZE;
+	blkno += blkcnt * dumppages;
+	count -= blkcnt * dumppages;
+	addr += PAGE_SIZE * dumppages;
 	if (cncheckc() != -1)
 	    return EINTR;
     }