--- pdcraid.bak	2003-09-08 07:37:03.000000000 +0200
+++ pdcraid.c	2003-09-08 13:21:10.000000000 +0200
@@ -399,15 +399,56 @@
 		return 0;
 	
 	
-	/* first sector of the last cluster */
+	/* first sector of the last track */
 	if (ideinfo->head==0) 
 		return 0;
 	if (ideinfo->sect==0)
 		return 0;
+	/*
+	 * A comment by Frank Rysanek <Frantisek.Rysanek@post.cz>
+	 * 
+	 * struct ide_drive_t contains a lot of information, namely:
+	 *  cyl = number of cylinders in this disk drive
+	 *  head = number of heads
+	 *  sect = sectors per "track" (per head per cylinder)
+	 *  capacity = total disk capacity in sectors.
+	 *
+	 * Common sense says that capacity = C*H*S.
+	 * Based on my recent experience debugging this driver,
+	 * this is not necessarily true!
+	 * 
+	 * It seems that my drives have a few tracks in
+	 * excess of the C*H*S integer multiplication product:
+	 * 
+	 *    capacity > C*H*S
+	 *  
+	 * It's a well known fact that modern IDE drives translate/hide
+	 * the real geometry and prefer to work with LBA-style linear
+	 * addressing, and only present some fictious geometry that
+	 * fits the old BIOS data type limitations.
+	 * 
+	 * When calculating the LBA offset of the PDC superblock,
+	 * the original formula used by this driver derives the cyls
+	 * from capacity and H*S, then produces an integer C*H*S,
+	 * and subtracts one track.
+	 *
+	 * For my drives at least, I had to change this: now the
+	 * driver accepts the miraculous reported capacity (greater 
+	 * than CHS) and subtracts one track.
+	 *
+	 * The algorithm could be different with different models of
+	 * the Promise hardware, or even with different firmware revisions.
+	 *  
+	 * This is the original formula:
+	 
 	lba = (ideinfo->capacity / (ideinfo->head*ideinfo->sect));
 	lba = lba * (ideinfo->head*ideinfo->sect);
 	lba = lba - ideinfo->sect;
 
+	 * And this is the repaired one:
+	*/
+	lba = ideinfo->capacity - ideinfo->sect;
+
 	return lba;
 }
 
@@ -425,7 +466,7 @@
 	
 	/*
 	 * Calculate the position of the superblock,
-	 * it's at first sector of the last cylinder
+	 * it's at first sector of the last track
 	 */
 	sb_offset = calc_pdcblock_offset(major,minor)/8;
 	/* The /8 transforms sectors into 4Kb blocks */