diff --git a/hardware_interface.h b/hardware_interface.h
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+// Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_HARDWARE_INTERFACE_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_HARDWARE_INTERFACE_H__
+
+#include <string>
+
+namespace chromeos_update_engine {
+
+// The hardware interface allows access to the following parts of the system,
+// closely related to the hardware:
+//  * crossystem exposed properties: firmware, hwid, etc.
+//  * Physical disk: partition booted from and partition name conversions.
+// These stateless functions are tied together in this interface to facilitate
+// unit testing.
+class HardwareInterface {
+ public:
+  // Returns the currently booted device. "/dev/sda3", for example.
+  // This will not interpret LABEL= or UUID=. You'll need to use findfs
+  // or something with equivalent funcionality to interpret those.
+  virtual const std::string BootDevice() = 0;
+
+  // Returns the kernel device associated with the given boot device,
+  // for example, this function returns "/dev/sda2" if |boot_device| is
+  // "/dev/sda3".
+  // To obtain the current booted kernel device, the suggested calling
+  // convention is KernelDeviceOfBootDevice(BootDevice()).
+  // This function works by doing string modification on |boot_device|.
+  // Returns empty string on failure.
+  virtual const std::string KernelDeviceOfBootDevice(
+      const std::string& boot_device) = 0;
+
+  // TODO(deymo): Move other hardware-dependant functions to this interface:
+  // GetECVersion, GetFirmwareVersion, GetHardwareClass, IsNormalBootMode and
+  // IsOfficialBuild.
+
+  virtual ~HardwareInterface() {}
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_HARDWARE_INTERFACE_H__