Commit 2b142900 authored by Alan Jenkins's avatar Alan Jenkins Committed by James Bottomley
Browse files

[SCSI] extend the last_sector_bug flag to cover more sectors 



The last_sector_bug flag was added to work around a bug in certain usb
cardreaders, where they would crash if a multiple sector read included the
last sector. The original implementation avoids this by e.g. splitting an 8
sector read which includes the last sector into a 7 sector read, and a single
sector read for the last sector.  The flag is enabled for all USB devices.

This revealed a second bug in other usb cardreaders, which crash when they
get a multiple sector read which stops 1 sector short of the last sector.
Affected hardware includes the Kingston "MobileLite" external USB cardreader
and the internal USB cardreader on the Asus EeePC.

Extend the last_sector_bug workaround to ensure that any access which touches
the last 8 hardware sectors of the device is a single sector long.  Requests
are shrunk as necessary to meet this constraint.

This gives us a safety margin against potential unknown or future bugs
affecting multi-sector access to the end of the device.  The two known bugs
only affect the last 2 sectors.  However, they suggest that these devices
are prone to fencepost errors and that multi-sector access to the end of the
device is not well tested.  Popular OS's use multi-sector accesses, but they
rarely read the last few sectors.  Linux (with udev & vol_id) automatically
reads sectors from the end of the device on insertion.  It is assumed that
single sector accesses are more thoroughly tested during development.

Signed-off-by: default avatarAlan Jenkins <alan-jenkins@tuffmail.co.uk>
Tested-by: default avatarAlan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: default avatarJames Bottomley <James.Bottomley@HansenPartnership.com>
parent 4cfc5101

drivers/scsi/sd.c

+15 −6
Original line number Diff line number Diff line
@@ -375,6 +375,7 @@ static int sd_prep_fn(struct request_queue *q, struct request *rq) 
	struct gendisk *disk = rq->rq_disk;

	struct scsi_disk *sdkp;

	sector_t block = rq->sector;

	sector_t threshold;

	unsigned int this_count = rq->nr_sectors;

	unsigned int timeout = sdp->timeout;

	int ret;
@@ -422,13 +423,21 @@ static int sd_prep_fn(struct request_queue *q, struct request *rq) 
	}



	/*

	 * Some devices (some sdcards for one) don't like it if the

	 * last sector gets read in a larger then 1 sector read.

	 * Some SD card readers can't handle multi-sector accesses which touch

	 * the last one or two hardware sectors.  Split accesses as needed.

	 */

	if (unlikely(sdp->last_sector_bug &&

	    rq->nr_sectors > sdp->sector_size / 512 &&

	    block + this_count == get_capacity(disk)))

		this_count -= sdp->sector_size / 512;

	threshold = get_capacity(disk) - SD_LAST_BUGGY_SECTORS *

		(sdp->sector_size / 512);



	if (unlikely(sdp->last_sector_bug && block + this_count > threshold)) {

		if (block < threshold) {

			/* Access up to the threshold but not beyond */

			this_count = threshold - block;

		} else {

			/* Access only a single hardware sector */

			this_count = sdp->sector_size / 512;

		}

	}



	SCSI_LOG_HLQUEUE(2, scmd_printk(KERN_INFO, SCpnt, "block=%llu\n",

					(unsigned long long)block));

drivers/scsi/sd.h

+6 −0
Original line number Diff line number Diff line
@@ -31,6 +31,12 @@ 
 */

#define SD_BUF_SIZE		512



/*

 * Number of sectors at the end of the device to avoid multi-sector

 * accesses to in the case of last_sector_bug

 */

#define SD_LAST_BUGGY_SECTORS	8



struct scsi_disk {

	struct scsi_driver *driver;	/* always &sd_template */

	struct scsi_device *device;

include/scsi/scsi_device.h

+2 −1
Original line number Diff line number Diff line
@@ -140,7 +140,8 @@ struct scsi_device { 
	unsigned fix_capacity:1;	/* READ_CAPACITY is too high by 1 */

	unsigned guess_capacity:1;	/* READ_CAPACITY might be too high by 1 */

	unsigned retry_hwerror:1;	/* Retry HARDWARE_ERROR */

	unsigned last_sector_bug:1;	/* Always read last sector in a 1 sector read */

	unsigned last_sector_bug:1;	/* do not use multisector accesses on

					   SD_LAST_BUGGY_SECTORS */



	DECLARE_BITMAP(supported_events, SDEV_EVT_MAXBITS); /* supported events */

	struct list_head event_list;	/* asserted events */

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