[snip]

Hard to say.  The whole notion of "geometry" is a little confused
these days.  The geometry shown by the 2.6 kernel is consistent with
that reported by the "extended INT13" BIOS interface, which allows 16
bits for each of C/H/S.  This is, in some sense, the "real" geometry.

The other geometry, which happens to be the one you need if you plan
to interoperate with other operating systems, is reported by the
"legacy INT13" BIOS interface.  This interface allows only 10 bits for
the cylinder count (and has no "linear" mode), so modern BIOSes
typically "lie" and report large head and sector counts to allow more
of the drive to appear within the first 1024 cylinders.

The 2.6 behavior was causing a headache for me, so I fixed it.

First, I wrote a patch for the Linux EDD module and sent it to the
maintainer (Matt Domsch at Dell).  He cleaned it up and submitted it
to Andrew Morton, who sent it to Linus.  It is included in 2.6.5-rc*.

So begin by updating to a 2.6.5-rc* or -mm* kernel.  Load the EDD
module ("modprobe edd").

This will create entries under /sys/firmware/edd/int13_dev80.  The
entries I added are named "legacy_cylinders", "legacy_heads", and
"legacy_sectors".  These merely reflect the geometry reported by the
legacy INT13 interface.

Ignore the cylinder count.  Read the head count (H) and sector count
(S).  Increment H by one (long story).

Read /sys/block/hda/size, and divide that by S and then by H to
produce a cylinder count (C).

Finally, run a command like this:

  echo "bios_cyl:C bios_head:H bios_sect:S" > /proc/ide/hda/settings

...where C, H, and S are as computed.  This will reset the values
which the kernel reports in HDIO_GETGEO, causing your partitioning
tools to produce results the same way DOS fdisk would (which is, of
course, what you want).

Works for me; see <http://unattended.sourceforge.net/>.

Note that simply assuming H and S are 255 and 63, respectively, will
usually work but not always.  IBM Thinkpads, for instance, use H=240.

 - Pat
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