[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 8e08d51a0f0531d8bf661dc0d3bf8f822d35c357..e5e7d78564545543925fc6136c6e7c7fc994c028 100644 (file)
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -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));

diff --git a/drivers/scsi/sd.h b/drivers/scsi/sd.h
index 550b2f70a1f8330f93fea284c19f13554297ef86..95b9f06534d579bf72797c20ecd5546970f4b190 100644 (file)
--- a/drivers/scsi/sd.h
+++ b/drivers/scsi/sd.h
@@ -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;

diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index 9cecc409f0f8418cea3e547447e210d8304f4178..291d56a19167db9b8688dbbba446d051daae3b17 100644 (file)
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -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 */