v4l2: v4l2_compat: Add V4L2 compatibility layer

Add libcamera V4L2 compatibility layer.

This initial implementation supports the minimal set of V4L2 operations,
which allows getting, setting, and enumerating formats, and streaming
frames from a video device. Some data about the wrapped V4L2 video
device are hardcoded.

Add a build option named 'v4l2' and adjust the build system to
selectively compile the V4L2 compatibility layer.

For now we match the V4L2 device node to a libcamera camera based on a
devnum that a pipeline handler may optionally map to a libcamera
camera.

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

src/v4l2/v4l2_compat.cpp

@@ -0,0 +1,85 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * v4l2_compat.cpp - V4L2 compatibility layer
+ */
+
+#include "v4l2_compat_manager.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdarg.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#define LIBCAMERA_PUBLIC __attribute__((visibility("default")))
+
+using namespace libcamera;
+
+#define extract_va_arg(type, arg, last)	\
+{					\
+	va_list ap;			\
+	va_start(ap, last);		\
+	arg = va_arg(ap, type);		\
+	va_end(ap);			\
+}
+
+extern "C" {
+
+LIBCAMERA_PUBLIC int open(const char *path, int oflag, ...)
+{
+	mode_t mode = 0;
+	if (oflag & O_CREAT || oflag & O_TMPFILE)
+		extract_va_arg(mode_t, mode, oflag);
+
+	return V4L2CompatManager::instance()->openat(AT_FDCWD, path,
+						     oflag, mode);
+}
+
+/* _FORTIFY_SOURCE redirects open to __open_2 */
+LIBCAMERA_PUBLIC extern __typeof(open) __open_2 __attribute__ ((alias("open")));
+
+LIBCAMERA_PUBLIC int openat(int dirfd, const char *path, int oflag, ...)
+{
+	mode_t mode = 0;
+	if (oflag & O_CREAT || oflag & O_TMPFILE)
+		extract_va_arg(mode_t, mode, oflag);
+
+	return V4L2CompatManager::instance()->openat(dirfd, path, oflag, mode);
+}
+
+LIBCAMERA_PUBLIC extern __typeof(openat) __openat_2 __attribute__ ((alias("openat")));
+
+LIBCAMERA_PUBLIC int dup(int oldfd)
+{
+	return V4L2CompatManager::instance()->dup(oldfd);
+}
+
+LIBCAMERA_PUBLIC int close(int fd)
+{
+	return V4L2CompatManager::instance()->close(fd);
+}
+
+LIBCAMERA_PUBLIC void *mmap(void *addr, size_t length, int prot, int flags,
+			    int fd, off_t offset)
+{
+	return V4L2CompatManager::instance()->mmap(addr, length, prot, flags,
+						   fd, offset);
+}
+
+LIBCAMERA_PUBLIC int munmap(void *addr, size_t length)
+{
+	return V4L2CompatManager::instance()->munmap(addr, length);
+}
+
+LIBCAMERA_PUBLIC int ioctl(int fd, unsigned long request, ...)
+{
+	void *arg;
+	extract_va_arg(void *, arg, request);
+
+	return V4L2CompatManager::instance()->ioctl(fd, request, arg);
+}
+
+}