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>

2 files changed, 22 insertions, 9 deletions

diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
index 9c0b494f5e87..cf64ddf5d839 100644
--- a/drivers/block/floppy.c
+++ b/drivers/block/floppy.c
@@ -423,8 +423,15 @@ static struct floppy_raw_cmd *raw_cmd, default_raw_cmd;
  * 1581's logical side 0 is on physical side 1, whereas the Sharp's logical
  * side 0 is on physical side 0 (but with the misnamed sector IDs).
  * 'stretch' should probably be renamed to something more general, like
- * 'options'.  Other parameters should be self-explanatory (see also
- * setfdprm(8)).
+ * 'options'.
+ *
+ * Bits 2 through 9 of 'stretch' tell the number of the first sector.
+ * The LSB (bit 2) is flipped. For most disks, the first sector
+ * is 1 (represented by 0x00<<2).  For some CP/M and music sampler
+ * disks (such as Ensoniq EPS 16plus) it is 0 (represented as 0x01<<2).
+ * For Amstrad CPC disks it is 0xC1 (represented as 0xC0<<2).
+ *
+ * Other parameters should be self-explanatory (see also setfdprm(8)).
  */
 /*
             Size
@@ -2236,9 +2243,9 @@ static void setup_format_params(int track)
                         }
                 }
         }
-        if (_floppy->stretch & FD_ZEROBASED) {
+        if (_floppy->stretch & FD_SECTBASEMASK) {
                 for (count = 0; count < F_SECT_PER_TRACK; count++)
-                        here[count].sect--;
+                        here[count].sect += FD_SECTBASE(_floppy) - 1;
         }
 }
 
@@ -2649,7 +2656,7 @@ static int make_raw_rw_request(void)
         }
         HEAD = fsector_t / _floppy->sect;
 
-        if (((_floppy->stretch & (FD_SWAPSIDES | FD_ZEROBASED)) ||
+        if (((_floppy->stretch & (FD_SWAPSIDES | FD_SECTBASEMASK)) ||
              TESTF(FD_NEED_TWADDLE)) && fsector_t < _floppy->sect)
                 max_sector = _floppy->sect;
 
@@ -2679,7 +2686,7 @@ static int make_raw_rw_request(void)
         CODE2SIZE;
         SECT_PER_TRACK = _floppy->sect << 2 >> SIZECODE;
         SECTOR = ((fsector_t % _floppy->sect) << 2 >> SIZECODE) +
-            ((_floppy->stretch & FD_ZEROBASED) ? 0 : 1);
+            FD_SECTBASE(_floppy);
 
         /* tracksize describes the size which can be filled up with sectors
          * of size ssize.
@@ -3311,7 +3318,7 @@ static inline int set_geometry(unsigned int cmd, struct floppy_struct *g,
             g->head <= 0 ||
             g->track <= 0 || g->track > UDP->tracks >> STRETCH(g) ||
             /* check if reserved bits are set */
-            (g->stretch & ~(FD_STRETCH | FD_SWAPSIDES | FD_ZEROBASED)) != 0)
+            (g->stretch & ~(FD_STRETCH | FD_SWAPSIDES | FD_SECTBASEMASK)) != 0)
                 return -EINVAL;
         if (type) {
                 if (!capable(CAP_SYS_ADMIN))
@@ -3356,7 +3363,7 @@ static inline int set_geometry(unsigned int cmd, struct floppy_struct *g,
                 if (DRS->maxblock > user_params[drive].sect ||
                     DRS->maxtrack ||
                     ((user_params[drive].sect ^ oldStretch) &
-                     (FD_SWAPSIDES | FD_ZEROBASED)))
+                     (FD_SWAPSIDES | FD_SECTBASEMASK)))
                         invalidate_drive(bdev);
                 else
                         process_fd_request();
diff --git a/include/linux/fd.h b/include/linux/fd.h
index b6bd41d2b460..f5d194af07a8 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 */