floppy: support arbitrary first-sector numbers
The current floppy_struct allows floppies to number sectors starting from 0 or 1. This patch allows arbitrary first-sector numbers - for example, 0xC1 for Amstrad CPC disks. This extends the existing 1-bit field (FD_ZEROBASED, bit 2 of stretch) to 8 bits (FD_SECTMASK, bits 2 to 9). Currently 0x00 denotes a first sector number of 1, and 0x01 denotes a first sector number of 0. We extend this by interpreting FD_SECTMASK as the first sector number with the LSB flipped. Signed-off-by: NKeith Wansbrough <keith@lochan.org> Cc: Alain Knaff <alain@linux.lu> Cc: Michael Kerrisk <mtk.manpages@googlemail.com> Cc: Karel Zak <kzak@redhat.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NJens Axboe <jens.axboe@oracle.com>
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