[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: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Tested-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 8e08d51..e5e7d78 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -375,6 +375,7 @@
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 @@
}
/*
- * 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));