libcamera: device_enumerator: add DeviceEnumeratorSysfs class

A udev-based device enumerator is not sufficient, since libudev is an
optional dependency, or udev might fail. In these cases, we should fall
back to using sysfs to enumerate devices.

Add a DeviceEnumeratorSysfs class which is a specialization of
DeviceEnumerator that uses sysfs to enumerate media devices on the
system.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/include/device_enumerator_sysfs.h

@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * device_enumerator_sysfs.h - sysfs-based device enumerator
+ */
+#ifndef __LIBCAMERA_DEVICE_ENUMERATOR_SYSFS_H__
+#define __LIBCAMERA_DEVICE_ENUMERATOR_SYSFS_H__
+
+#include <string>
+
+#include "device_enumerator.h"
+
+namespace libcamera {
+
+class DeviceEnumeratorSysfs final : public DeviceEnumerator
+{
+public:
+	int init();
+	int enumerate();
+
+private:
+	std::string lookupDeviceNode(int major, int minor);
+};
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_DEVICE_ENUMERATOR_SYSFS_H__ */