Added geometry-guessing code based on the partition table

src/arch/i386/include/int13.h

 
 /** @} */ 
 
+/** A C/H/S address within a partition table entry */
+struct partition_chs {
+	/** Head number */
+	uint8_t head;
+	/** Sector number, plus high 2 bits of cylinder number */
+	uint8_t cyl_sector;
+	/** Low 8 bits of cylinder number */
+	uint8_t cyl;
+} __attribute__ (( packed ));
+
+#define PART_HEAD(chs) ( (chs).head )
+#define PART_SECTOR(chs) ( (chs).cyl_sector & 0x3f )
+#define PART_CYLINDER(chs) ( (chs).cyl | ( ( (chs).cyl_sector & 0xc0 ) << 2 ) )
+
+/** A partition table entry within the MBR */
+struct partition_table_entry {
+	/** Bootable flag */
+	uint8_t bootable;
+	/** C/H/S start address */
+	struct partition_chs chs_start;
+	/** System indicator (partition type) */
+	uint8_t type;
+	/** C/H/S end address */
+	struct partition_chs chs_end;
+	/** Linear start address */
+	uint32_t start;
+	/** Linear length */
+	uint32_t length;
+} __attribute__ (( packed ));
+
+/** A Master Boot Record */
+struct master_boot_record {
+	uint8_t pad[446];
+	/** Partition table */
+	struct partition_table_entry partitions[4];
+	/** 0x55aa MBR signature */
+	uint16_t signature;
+} __attribute__ (( packed ));
+
 extern void register_int13_drive ( struct int13_drive *drive );
 extern void unregister_int13_drive ( struct int13_drive *drive );
 extern int int13_boot ( unsigned int drive );

src/arch/i386/interface/pcbios/int13.c

 }
 
 /**
- * Register INT 13 emulated drive
+ * Guess INT 13 drive geometry
  *
  * @v drive		Emulated drive
  *
- * Registers the drive with the INT 13 emulation subsystem, and hooks
- * the INT 13 interrupt vector (if not already hooked).
- *
- * The underlying block device must be valid.  A drive number and
- * geometry will be assigned if left blank.
+ * Guesses the drive geometry by inspecting the partition table.
  */
-void register_int13_drive ( struct int13_drive *drive ) {
-	uint8_t num_drives;
+static void guess_int13_geometry ( struct int13_drive *drive ) {
+	struct master_boot_record mbr;
+	struct partition_table_entry *partition;
+	unsigned int guessed_heads = 255;
+	unsigned int guessed_sectors_per_track = 63;
 	unsigned long blocks;
 	unsigned long blocks_per_cyl;
+	unsigned int i;
 
-	/* Give drive a default geometry if none specified */
+	/* Scan through partition table and modify guesses for heads
+	 * and sectors_per_track if we find any used partitions.
+	 */
+	if ( drive->blockdev->read ( drive->blockdev, 0, 1,
+				     virt_to_user ( &mbr ) ) == 0 ) {
+		for ( i = 0 ; i < 4 ; i++ ) {
+			partition = &mbr.partitions[i];
+			if ( ! partition->type )
+				continue;
+			guessed_heads =
+				( PART_HEAD ( partition->chs_end ) + 1 );
+			guessed_sectors_per_track = 
+				PART_SECTOR ( partition->chs_end );
+			DBG ( "Guessing C/H/S xx/%d/%d based on partition "
+			      "%d\n", guessed_heads,
+			      guessed_sectors_per_track, ( i + 1 ) );
+		}
+	} else {
+		DBG ( "Could not read partition table to guess geometry\n" );
+	}
+
+	/* Apply guesses if no geometry already specified */
 	if ( ! drive->heads )
-		drive->heads = 255;
+		drive->heads = guessed_heads;
 	if ( ! drive->sectors_per_track )
-		drive->sectors_per_track = 63;
+		drive->sectors_per_track = guessed_sectors_per_track;
 	if ( ! drive->cylinders ) {
 		/* Avoid attempting a 64-bit divide on a 32-bit system */
 		blocks = ( ( drive->blockdev->blocks <= ULONG_MAX ) ?
 		blocks_per_cyl = ( drive->heads * drive->sectors_per_track );
 		assert ( blocks_per_cyl != 0 );
 		drive->cylinders = ( blocks / blocks_per_cyl );
+		if ( drive->cylinders > 1024 )
+			drive->cylinders = 1024;
 	}
+}
+
+/**
+ * Register INT 13 emulated drive
+ *
+ * @v drive		Emulated drive
+ *
+ * Registers the drive with the INT 13 emulation subsystem, and hooks
+ * the INT 13 interrupt vector (if not already hooked).
+ *
+ * The underlying block device must be valid.  A drive number and
+ * geometry will be assigned if left blank.
+ */
+void register_int13_drive ( struct int13_drive *drive ) {
+	uint8_t num_drives;
+
+	/* Give drive a default geometry if none specified */
+	guess_int13_geometry ( drive );
 
 	/* Assign drive number if none specified, update BIOS drive count */
 	get_real ( num_drives, BDA_SEG, BDA_NUM_DRIVES );