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>
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