Re: [PATCH] pdcraid and weird IDE geometry

From: Andries Brouwer (
Date: Thu Jul 17 2003 - 10:34:13 EST

On Thu, Jul 17, 2003 at 03:58:38PM +0100, Alan Cox wrote:
> On Iau, 2003-07-17 at 15:37, Walt H wrote:
> > On the second drive, it's like this:
> > capacity = 80418240, head=255, sect = 63
> > lba = capacity / (head * sect) = 5005 int or 5005.80 float
> > lba = lba * (head * sect) = 80405325 int or 80418240.01 float
> > lba = lba - sect = 80405262 int or 80418177 float
> Would fixed point solve this. Start from capacity <<= 16 and then
> do the maths. That would put lba in 65536ths of a sector which
> should have the same result as your float maths

Ach Alan - I have not seen these earlier posts, but float or
fixed point here is just nonsense.

The purpose of
        A = B/C;
        A *= C;
can only be to round B down to the largest multiple of C below it.
Using infinite precision just turns this into
        A = B;

He needs the first sector of the last cylinder, in a setup where
cylinders have size 16*63 or so, but the surrounding software
thinks that it is 255*63, a mistake.

I don't know anything about these RAIDs, but possibly it would
help to give boot parameters for this disk.

Maybe he is victim of the completely ridiculous
        drive->head = 255;
in ide-disk.c.
(We have drive->head, the number of physical heads, and
drive->bios_head, the translation presently used by the BIOS -
or at least that is the intention. It is a bug if the former
is larger than 16. The latter may well be 255.)


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