libcamera: Make libudev optional

libcamera depends on libudev for device enumeration. It is however
useful to allow building documentation without requiring the dependency
to be installed. Make the libudev dependency optional and compile the
udev-based device enumerator out when libudev is not present.

Note that while libcamera will compile without libudev, it will not be
able to enumerate devices. A sysfs-based device enumerator is planned as
a fallback but not implemented yet.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/device_enumerator_udev.cpp

@@ -0,0 +1,163 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2018-2019, Google Inc.
+ *
+ * device_enumerator_udev.cpp - udev-based device enumerator
+ */
+
+#include "device_enumerator_udev.h"
+
+#include <fcntl.h>
+#include <libudev.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <unistd.h>
+
+#include <libcamera/event_notifier.h>
+
+#include "log.h"
+
+namespace libcamera {
+
+LOG_DECLARE_CATEGORY(DeviceEnumerator)
+
+DeviceEnumeratorUdev::DeviceEnumeratorUdev()
+	: udev_(nullptr)
+{
+}
+
+DeviceEnumeratorUdev::~DeviceEnumeratorUdev()
+{
+	delete notifier_;
+
+	if (monitor_)
+		udev_monitor_unref(monitor_);
+	if (udev_)
+		udev_unref(udev_);
+}
+
+int DeviceEnumeratorUdev::init()
+{
+	int ret;
+
+	if (udev_)
+		return -EBUSY;
+
+	udev_ = udev_new();
+	if (!udev_)
+		return -ENODEV;
+
+	monitor_ = udev_monitor_new_from_netlink(udev_, "udev");
+	if (!monitor_)
+		return -ENODEV;
+
+	ret = udev_monitor_filter_add_match_subsystem_devtype(monitor_, "media",
+							      nullptr);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+}
+
+int DeviceEnumeratorUdev::enumerate()
+{
+	struct udev_enumerate *udev_enum = nullptr;
+	struct udev_list_entry *ents, *ent;
+	int ret;
+
+	udev_enum = udev_enumerate_new(udev_);
+	if (!udev_enum)
+		return -ENOMEM;
+
+	ret = udev_enumerate_add_match_subsystem(udev_enum, "media");
+	if (ret < 0)
+		goto done;
+
+	ret = udev_enumerate_scan_devices(udev_enum);
+	if (ret < 0)
+		goto done;
+
+	ents = udev_enumerate_get_list_entry(udev_enum);
+	if (!ents)
+		goto done;
+
+	udev_list_entry_foreach(ent, ents) {
+		struct udev_device *dev;
+		const char *devnode;
+		const char *syspath = udev_list_entry_get_name(ent);
+
+		dev = udev_device_new_from_syspath(udev_, syspath);
+		if (!dev) {
+			LOG(DeviceEnumerator, Warning)
+				<< "Failed to get device for '"
+				<< syspath << "', skipping";
+			continue;
+		}
+
+		devnode = udev_device_get_devnode(dev);
+		if (!devnode) {
+			udev_device_unref(dev);
+			ret = -ENODEV;
+			goto done;
+		}
+
+		addDevice(devnode);
+
+		udev_device_unref(dev);
+	}
+done:
+	udev_enumerate_unref(udev_enum);
+	if (ret < 0)
+		return ret;
+
+	ret = udev_monitor_enable_receiving(monitor_);
+	if (ret < 0)
+		return ret;
+
+	int fd = udev_monitor_get_fd(monitor_);
+	notifier_ = new EventNotifier(fd, EventNotifier::Read);
+	notifier_->activated.connect(this, &DeviceEnumeratorUdev::udevNotify);
+
+	return 0;
+}
+
+std::string DeviceEnumeratorUdev::lookupDeviceNode(int major, int minor)
+{
+	struct udev_device *device;
+	const char *name;
+	dev_t devnum;
+	std::string deviceNode = std::string();
+
+	devnum = makedev(major, minor);
+	device = udev_device_new_from_devnum(udev_, 'c', devnum);
+	if (!device)
+		return std::string();
+
+	name = udev_device_get_devnode(device);
+	if (name)
+		deviceNode = name;
+
+	udev_device_unref(device);
+
+	return deviceNode;
+}
+
+void DeviceEnumeratorUdev::udevNotify(EventNotifier *notifier)
+{
+	struct udev_device *dev = udev_monitor_receive_device(monitor_);
+	std::string action(udev_device_get_action(dev));
+	std::string deviceNode(udev_device_get_devnode(dev));
+
+	LOG(DeviceEnumerator, Debug)
+		<< action << " device " << udev_device_get_devnode(dev);
+
+	if (action == "add") {
+		addDevice(deviceNode);
+	} else if (action == "remove") {
+		removeDevice(deviceNode);
+	}
+
+	udev_device_unref(dev);
+}
+
+} /* namespace libcamera */