diff --git a/sys/conf/files.sparc64 b/sys/conf/files.sparc64
index 3a0f6f03f04d..00b7c45f972e 100644
--- a/sys/conf/files.sparc64
+++ b/sys/conf/files.sparc64
@@ -88,6 +88,7 @@ sparc64/pci/psycho.c		optional	pci
 sparc64/sbus/dma_sbus.c		optional	sbus
 sparc64/sbus/sbus.c		optional	sbus
 sparc64/sbus/lsi64854.c		optional	sbus
+sparc64/sparc64/ata_machdep.c	optional	atadisk
 sparc64/sparc64/autoconf.c	standard
 sparc64/sparc64/bus_machdep.c	standard
 sparc64/sparc64/cache.c		standard
diff --git a/sys/sparc64/include/md_var.h b/sys/sparc64/include/md_var.h
index 34a3437ac06a..69c6d698c5fb 100644
--- a/sys/sparc64/include/md_var.h
+++ b/sys/sparc64/include/md_var.h
@@ -60,4 +60,17 @@ cpu_block_zero_t spitfire_block_zero;
 extern	cpu_block_copy_t *cpu_block_copy;
 extern	cpu_block_zero_t *cpu_block_zero;
 
+/*
+ * Given that the Sun disk label only uses 16-bit fields for cylinders,
+ * heads and sectors we might need to adjust the geometry of large IDE
+ * disks.
+ * We have to have a knowledge that a device_t is a struct device * here
+ * to avoid including too many things from this file.
+ */
+struct disk;
+struct device;
+void sparc64_ad_firmware_geom_adjust(struct device *dev, struct disk *disk);
+#define	ad_firmware_geom_adjust(dev, dsk)				\
+	sparc64_ad_firmware_geom_adjust(dev, dsk)
+
 #endif /* !_MACHINE_MD_VAR_H_ */
diff --git a/sys/sparc64/sparc64/ata_machdep.c b/sys/sparc64/sparc64/ata_machdep.c
new file mode 100644
index 000000000000..20729bbaa20d
--- /dev/null
+++ b/sys/sparc64/sparc64/ata_machdep.c
@@ -0,0 +1,52 @@
+/*-
+ * Copyright (c) 2008 Marius Strobl <marius@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <geom/geom_disk.h>
+#include <machine/md_var.h>
+
+void
+sparc64_ad_firmware_geom_adjust(device_t dev, struct disk *disk)
+{
+
+	/*
+	 * The Sun disk label only uses 16-bit fields for cylinders,
+	 * heads and sectors so the geometry of large IDE disks has
+	 * to be adjusted.  If the disk is > 32GB at 16 heads and 63
+	 * sectors, the sectors have to be adjusted to 255.  If the
+	 * the disk is even > 128GB, additionally adjust the heads
+	 * to 255.
+	 * XXX the OpenSolaris dad(7D) driver limits the mediasize
+	 * to 128GB.
+	 */
+	if (disk->d_mediasize > (off_t)65535 * 16 * 63 * disk->d_sectorsize)
+		disk->d_fwsectors = 255;
+	if (disk->d_mediasize > (off_t)65535 * 16 * 255 * disk->d_sectorsize)
+		disk->d_fwheads = 255;
+}