libcamera: sysfs: Add helper to lookup sysfs path of a character device

Add a helper function to lookup the sysfs path of a character device.
Store the function in a new libcamera::sysfs namespace as there is not
class to host it.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

include/libcamera/internal/meson.build

@@ -25,6 +25,7 @@ libcamera_internal_headers = files([
     'process.h',
     'pub_key.h',
     'semaphore.h',
+    'sysfs.h',
     'thread.h',
     'utils.h',
     'v4l2_controls.h',

include/libcamera/internal/sysfs.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2020, Google Inc.
+ *
+ * sysfs.h - Miscellaneous utility functions to access sysfs
+ */
+#ifndef __LIBCAMERA_INTERNAL_SYSFS_H__
+#define __LIBCAMERA_INTERNAL_SYSFS_H__
+
+#include <string>
+
+namespace libcamera {
+
+namespace sysfs {
+
+std::string charDevPath(const std::string &deviceNode);
+
+} /* namespace sysfs */
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_INTERNAL_SYSFS_H__ */

src/libcamera/meson.build

@@ -41,6 +41,7 @@ libcamera_sources = files([
     'semaphore.cpp',
     'signal.cpp',
     'stream.cpp',
+    'sysfs.cpp',
     'thread.cpp',
     'timer.cpp',
     'utils.cpp',

src/libcamera/sysfs.cpp

@@ -0,0 +1,52 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2020, Google Inc.
+ *
+ * sysfs.cpp - Miscellaneous utility functions to access sysfs
+ */
+
+#include "libcamera/internal/sysfs.h"
+
+#include <sstream>
+#include <sys/stat.h>
+#include <sys/sysmacros.h>
+
+#include "libcamera/internal/log.h"
+
+/**
+ * \file sysfs.h
+ * \brief Miscellaneous utility functions to access sysfs
+ */
+
+namespace libcamera {
+
+LOG_DEFINE_CATEGORY(SysFs);
+
+namespace sysfs {
+
+/**
+ * \brief Retrieve the sysfs path for a character device
+ * \param[in] deviceNode Path to character device node
+ * \return The sysfs path on success or an empty string on failure
+ */
+std::string charDevPath(const std::string &deviceNode)
+{
+	struct stat st;
+	int ret = stat(deviceNode.c_str(), &st);
+	if (ret < 0) {
+		ret = -errno;
+		LOG(SysFs, Error)
+			<< "Unable to stat '" << deviceNode << "': "
+			<< strerror(-ret);
+		return {};
+	}
+
+	std::ostringstream dev("/sys/dev/char/", std::ios_base::ate);
+	dev << major(st.st_rdev) << ":" << minor(st.st_rdev);
+
+	return dev.str();
+}
+
+} /* namespace sysfs */
+
+} /* namespace libcamera */