Show how a flattend file tree is traversed.

dtb_get_top_level_property.c

/*
 * Find value of top level property in device tree.
 *
 * Copyright (c) 2017 Heinrich Schuchardt <xypron.glpk@gmx.de>
 *
 * SPDX-License-Identifier:     GPL-2.0+
 */

#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <libfdt.h>

/* This should be sufficent for */
#define BUFFERSIZE 0x100000

/*
 * Convert FDT value to host endianness.
 *
 * @val		FDT value
 * @return	converted value
 */
static uint32_t f2h(fdt32_t val) {
	char *buf = (char *)&val;
	char i;

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
	i = buf[0]; buf[0] = buf[3]; buf[3] = i;
	i = buf[1]; buf[1] = buf[2]; buf[2] = i;
#endif
	return *(uint32_t *)buf;
}

/*
 * Return the value of a property of an FDT node.
 *
 * @node	FDT node
 * @name	name of the property
 * @return	value of the property
 */
static const char *get_property(const char *node, const char *property)
{
	struct fdt_header *header = (struct fdt_header *)node;
	const fdt32_t *fdt;
	const char *strings;
	
	if (FDT_MAGIC != f2h(header->magic)) {
		printf("Wrong magic\n");
		return NULL;
	}

	fdt = (fdt32_t *)(node + f2h(header->off_dt_struct)); 
	strings = node + f2h(header->off_dt_strings);

	for (;;) {
		switch (f2h(fdt[0])) {
			case FDT_BEGIN_NODE: {
				char *c = (char *)&fdt[1];
				size_t i;

				for (i = 0; c[i]; ++i);
				fdt = &fdt[2 + (i >> 2)];
				break;
			}
			case FDT_PROP: {
				struct fdt_property *prop =
					(struct fdt_property *)fdt;
				const char *label =
					&strings[f2h(prop->nameoff)];

				if (!strcmp(label, property))
					return (char *)&fdt[3];

				fdt = &fdt[3 + ((f2h(prop->len) + 3) >> 2)];
				break;
			}
			case FDT_NOP:
				fdt = &fdt[1];
				break;
			default:
				return NULL;
		}
	}
}

int main(int argc, const char *argv[])
{
	FILE *file;
	char *buf;
	size_t n;

	if (argc < 3) {
		printf("Usage\n  %s FILE PROPERTY\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	file = fopen(argv[1], "r");
	if (!file) {
		perror("fopen");
		exit(EXIT_FAILURE);
	}

	buf = malloc(BUFFERSIZE);
	if (!buf) {
		perror("malloc");
		exit(EXIT_FAILURE);
	}
	memset(buf, 0, BUFFERSIZE);

	n = fread(buf, 1, BUFFERSIZE, file);
	if (!n) {
		perror("fread");
		goto out;
	}
	if (n == BUFFERSIZE) {
		fprintf(stderr, "Internal buffer is too small\n");
		goto out;
	}

	printf("%s = %s\n", argv[2], get_property(buf, argv[2]));	
out:
	fclose(file);
	free(buf);
	exit(EXIT_SUCCESS);
}