From 2b142900784c6e38c8d39fa57d5f95ef08e735d8 Mon Sep 17 00:00:00 2001 From: Alan Jenkins Date: Sun, 27 Jul 2008 09:38:42 +0100 Subject: [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 Tested-by: Alan Jenkins Signed-off-by: James Bottomley --- drivers/scsi/sd.c | 21 +++++++++++++++------ 1 file changed, 15 insertions(+), 6 deletions(-) (limited to 'drivers/scsi/sd.c') diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c index 8e08d51a0f0..e5e7d785645 100644 --- 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)); -- cgit v1.2.3