• Alan Jenkins's avatar
    [SCSI] extend the last_sector_bug flag to cover more sectors · 2b142900
    Alan Jenkins authored
    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>
    2b142900
sd.h 3.19 KB