libcamera: sysfs: Add helper to lookup device firmware node path

A system's firmware description is recorded differently in sysfs
depending if the system uses DT or ACPI. Add a helper to abstract
this, allowing users not to care which of the two are used.

For DT-based systems, the path is the full name of the DT node that
represents the device. For ACPI-based systems, the path is the absolute
namespace path to the ACPI object that represents the device. In both
cases, the path is guaranteed to be unique and persistent as long as the
system firmware is not modified.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/src/libcamera/sysfs.cpp b/src/libcamera/sysfs.cpp
index 398df2c2..6c8e9554 100644
--- a/src/libcamera/sysfs.cpp
+++ b/src/libcamera/sysfs.cpp
@@ -7,10 +7,12 @@
 
 #include "libcamera/internal/sysfs.h"
 
+#include <fstream>
 #include <sstream>
 #include <sys/stat.h>
 #include <sys/sysmacros.h>
 
+#include "libcamera/internal/file.h"
 #include "libcamera/internal/log.h"
 
 /**
@@ -47,6 +49,62 @@ std::string charDevPath(const std::string &deviceNode)
 	return dev.str();
 }
 
+/**
+ * \brief Retrieve the path of the firmware node for a device
+ * \param[in] device Path in sysfs to search
+ *
+ * Physical devices in a system are described by the system firmware. Depending
+ * on the type of platform, devices are identified using different naming
+ * schemes. The Linux kernel abstract those differences with "firmware nodes".
+ * This function retrieves the firmware node path corresponding to the
+ * \a device.
+ *
+ * For DT-based systems, the path is the full name of the DT node that
+ * represents the device. For ACPI-based systems, the path is the absolute
+ * namespace path to the ACPI object that represents the device. In both cases,
+ * the path is guaranteed to be unique and persistent as long as the system
+ * firmware is not modified.
+ *
+ * \return The firmware node path on success or an empty string on failure
+ */
+std::string firmwareNodePath(const std::string &device)
+{
+	std::string fwPath, node;
+
+	/* Lookup for DT-based systems */
+	node = device + "/of_node";
+	if (File::exists(node)) {
+		char *ofPath = realpath(node.c_str(), nullptr);
+		if (!ofPath)
+			return {};
+
+		static const char prefix[] = "/sys/firmware/devicetree";
+		if (strncmp(ofPath, prefix, strlen(prefix)) == 0)
+			fwPath = ofPath + strlen(prefix);
+		else
+			fwPath = ofPath;
+
+		free(ofPath);
+
+		return fwPath;
+	}
+
+	/* Lookup for ACPI-based systems */
+	node = device + "/firmware_node/path";
+	if (File::exists(node)) {
+		std::ifstream file(node);
+		if (!file.is_open())
+			return {};
+
+		std::getline(file, fwPath);
+		file.close();
+
+		return fwPath;
+	}
+
+	return {};
+}
+
 } /* namespace sysfs */
 
 } /* namespace libcamera */