libcamera: Add support for V4L2 requests

The V4L2 requests API provides support to atomically tie controls to a
set of buffers. This is especially common for m2m devices. Such a
request is represented by an fd that is allocated via
MEDIA_IOC_REQUEST_ALLOC and then passed to the various V4L2 functions.

Implement a V4L2Request class to wrap such an fd and add the
corresponding utility functions.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/media_device.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <map>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -18,6 +19,7 @@
 #include <libcamera/base/unique_fd.h>
 
 #include "libcamera/internal/media_object.h"
+#include "libcamera/internal/v4l2_request.h"
 
 namespace libcamera {
 
@@ -57,6 +59,11 @@ public:
 
 	std::vector<MediaEntity *> locateEntities(unsigned int function);
 
+	int allocateRequests(unsigned int count,
+			     std::vector<std::unique_ptr<V4L2Request>> *requests);
+
+	bool supportsRequests();
+
 protected:
 	std::string logPrefix() const override;
 
@@ -87,6 +94,7 @@ private:
 	UniqueFD fd_;
 	bool valid_;
 	bool acquired_;
+	std::optional<bool> supportsRequests_;
 
 	std::map<unsigned int, MediaObject *> objects_;
 	std::vector<MediaEntity *> entities_;

include/libcamera/internal/meson.build

@@ -44,6 +44,7 @@ libcamera_internal_headers = files([
     'sysfs.h',
     'v4l2_device.h',
     'v4l2_pixelformat.h',
+    'v4l2_request.h',
     'v4l2_subdevice.h',
     'v4l2_videodevice.h',
     'vector.h',

include/libcamera/internal/v4l2_device.h

@@ -24,6 +24,7 @@
 #include <libcamera/controls.h>
 
 #include "libcamera/internal/formats.h"
+#include "libcamera/internal/v4l2_request.h"
 
 namespace libcamera {
 
@@ -37,8 +38,8 @@ public:
 
 	const ControlInfoMap &controls() const { return controls_; }
 
-	ControlList getControls(Span<const uint32_t> ids);
-	int setControls(ControlList *ctrls);
+	ControlList getControls(Span<const uint32_t> ids, const V4L2Request *request = nullptr);
+	int setControls(ControlList *ctrls, const V4L2Request *request = nullptr);
 
 	const struct v4l2_query_ext_ctrl *controlInfo(uint32_t id) const;
 

include/libcamera/internal/v4l2_request.h

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2025, Ideas On Board
+ *
+ * V4L2 Request
+ */
+
+#pragma once
+
+#include <string>
+
+#include <linux/videodev2.h>
+
+#include <libcamera/base/event_notifier.h>
+#include <libcamera/base/log.h>
+#include <libcamera/base/signal.h>
+#include <libcamera/base/unique_fd.h>
+
+namespace libcamera {
+
+class V4L2Request : protected Loggable
+{
+public:
+	bool isValid() const { return fd_.isValid(); }
+	int fd() const { return fd_.get(); }
+
+	int reinit();
+	int queue();
+
+	V4L2Request(UniqueFD &&fd);
+
+	Signal<V4L2Request *> requestDone;
+
+private:
+	LIBCAMERA_DISABLE_COPY_AND_MOVE(V4L2Request)
+
+	void requestReady();
+	std::string logPrefix() const override;
+
+	UniqueFD fd_;
+	EventNotifier fdNotifier_;
+};
+
+} /* namespace libcamera */

include/libcamera/internal/v4l2_videodevice.h

@@ -33,6 +33,7 @@
 #include "libcamera/internal/formats.h"
 #include "libcamera/internal/v4l2_device.h"
 #include "libcamera/internal/v4l2_pixelformat.h"
+#include "libcamera/internal/v4l2_request.h"
 
 namespace libcamera {
 
@@ -217,7 +218,7 @@ public:
 	int importBuffers(unsigned int count);
 	int releaseBuffers();
 
-	int queueBuffer(FrameBuffer *buffer);
+	int queueBuffer(FrameBuffer *buffer, const V4L2Request *request = nullptr);
 	Signal<FrameBuffer *> bufferReady;
 
 	int streamOn();