[libata] More robust parsing for IDENTIFY DEVICE multi_count field

Make libata more robust when parsing the multi_count
field from a drive's identify data.  This prevents us from
attempting to use dubious multi_count values ad infinitum.

Reset dev->multi_count to zero and reprobe it each time
through this routine, as it can change on device reset.

Also ensure that the reported "maximum" value is valid
and is a power of two, and that the reported "count" value
is valid and also a power of two.  And that the "count"
value is not greater than the "maximum" value.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-core.c

@@ -57,6 +57,7 @@
 #include <linux/scatterlist.h>
 #include <linux/io.h>
 #include <linux/async.h>
+#include <linux/log2.h>
 #include <scsi/scsi.h>
 #include <scsi/scsi_cmnd.h>
 #include <scsi/scsi_host.h>
@@ -2389,6 +2390,7 @@ int ata_dev_configure(struct ata_device *dev)
 	dev->cylinders = 0;
 	dev->heads = 0;
 	dev->sectors = 0;
+	dev->multi_count = 0;
 
 	/*
 	 * common ATA, ATAPI feature tests
@@ -2426,8 +2428,15 @@ int ata_dev_configure(struct ata_device *dev)
 
 		dev->n_sectors = ata_id_n_sectors(id);
 
-		if (dev->id[59] & 0x100)
-			dev->multi_count = dev->id[59] & 0xff;
+		/* get current R/W Multiple count setting */
+		if ((dev->id[47] >> 8) == 0x80 && (dev->id[59] & 0x100)) {
+			unsigned int max = dev->id[47] & 0xff;
+			unsigned int cnt = dev->id[59] & 0xff;
+			/* only recognize/allow powers of two here */
+			if (is_power_of_2(max) && is_power_of_2(cnt))
+				if (cnt <= max)
+					dev->multi_count = cnt;
+		}
 
 		if (ata_id_has_lba(id)) {
 			const char *lba_desc;