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: Keith 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/include/linux/fd.h b/include/linux/fd.h
index b6bd41d2b46..f5d194af07a 100644
--- a/include/linux/fd.h
+++ b/include/linux/fd.h
@@ -15,10 +15,16 @@ struct floppy_struct {
                         sect,           /* sectors per track */
                         head,           /* nr of heads */
                         track,          /* nr of tracks */
-                        stretch;        /* !=0 means double track steps */
+                        stretch;        /* bit 0 !=0 means double track steps */
+                                        /* bit 1 != 0 means swap sides */
+                                        /* bits 2..9 give the first sector */
+                                        /*  number (the LSB is flipped) */
 #define FD_STRETCH 1
 #define FD_SWAPSIDES 2
 #define FD_ZEROBASED 4
+#define FD_SECTBASEMASK 0x3FC
+#define FD_MKSECTBASE(s) (((s) ^ 1) << 2)
+#define FD_SECTBASE(floppy) ((((floppy)->stretch & FD_SECTBASEMASK) >> 2) ^ 1)
 
         unsigned char   gap,            /* gap1 size */