Feat: Parse FDT header and add initial FDT dt_struct scanner

This adds support for the following features required by #2:

- Map all FDT structure block
- Convert big-endian to little-endian
- Read FDT header

include/kernel/fdt.h

@@ -0,0 +1,97 @@
+#ifndef _FDT_H
+#define _FDT_H
+
+#include "types.h"
+
+#define FDT_MAGIC 0xd00dfeed
+#define FDT_VERSION 0x11
+#define FDT_DEFAULT_ADDRESS_CELL 0x2
+#define FDT_DEFAULT_SIZE_CELLS 0x1
+
+/*
+ * marks the beginning of the node's representation and is followed by the
+ * node's unit name as extra data
+ */
+#define FDT_BEGIN_NODE 0x00000001
+/*
+ * marks the end of the node's representation and has not extra data
+ * as extra data, so is if followed immediately by the next token, which may be
+ * any token except for the FDT_NOP
+ */
+#define FDT_END_NODE 0x00000002
+
+/* Marks the beginning of the representation of one property in the device-tree.
+ * It shall be followed by an extra data describing the property which consists
+ * of the property's length and name representation
+ *
+ */
+#define FDT_PROP 0x00000003
+/* FDT_NOP tokens will be ignored by the parsing device */
+#define FDT_NOP 0x00000004
+/* marks the end of the structure block. There is only one FTD_END token and has
+ * no extra data. The byte immediately after it has the offset of the beginning
+ * of the structure block equal to the value of the size header->size_dt_struct
+ * field in the device-tree
+ */
+#define FDT_END 0x00000009
+
+/*
+ * Convert from big-endian and little-endian
+ *
+ * RISC-V ISA has no explicit byte swapping instructions.
+ * Our best bet here is rely on the compiler for the best implementation
+ * available.
+ * This returns with the order of bytes reversed, for example, 0xaabb becomes
+ * 0xbbaa. Byte here always means exactly 8 bits.
+ */
+#define swapb(x) ({ __builtin_bswap32(x); })
+
+/* from:
+ * https://github.com/devicetree-org/devicetree-specification/blob/master/source/flattened-format.rst
+ */
+struct fdt_header {
+	/* contains the value 0xd00dfeed */
+	uint32_t magic;
+	/* contains total size in bytes of the device tree */
+	uint32_t totalsize;
+	/* contains the offset in bytes of the structured block */
+	uint32_t off_dt_struct;
+	/* contains the offset in bytes of the strings block */
+	uint32_t off_dt_strings;
+	/* contains the offset in bytes of the memory reservation block */
+	uint32_t off_mem_rsvmap;
+	/* contains the version of the devicetree data structure */
+	uint32_t version;
+	/* last version of the device tree with which the version used backwads
+	 * compatibility */
+	uint32_t last_comp_version;
+	/* contains the physical ID of the system's boot CPU */
+	uint32_t boot_cpuid_phys;
+	/* contains the length in bytes of the strings block section of the
+	 * devicetree blob */
+	uint32_t size_dt_strings;
+	/* contains  the length in bytes of the structure block section of the
+	 * devicetree blob */
+	uint32_t size_dt_struct;
+};
+
+struct fdt_node {
+	uint32_t *name;
+	uint32_t address;
+	uint32_t size_cells;
+	struct fdt_property *properties;
+	struct fdt_node *nodes;
+};
+
+struct fdt_property {
+	uint32_t len; /* in bytes */
+	uint32_t *value; /* byte string of length = len */
+	uint32_t *name; /* offset to the string block where the name is
+			  stored */
+};
+
+struct fdt_header *fdt_header(uintptr_t *);
+void scan_fdt(uintptr_t *);
+uint32_t _scan_fdt(const char *, uint32_t *, struct fdt_node *);
+
+#endif

src/ftd.c

@@ -0,0 +1,49 @@
+#include <kernel/fdt.h>
+#include <kernel/string.h>
+
+volatile uintptr_t *fdt_map;
+
+uint32_t
+_scan_fdt(const char *dt_strings, uint32_t *dt_struct, struct fdt_node *node)
+{
+	parent->address = FDT_DEFAULT_ADDRESS_CELL;
+	parent->size_cells = FDT_DEFAULT_SIZE_CELLS;
+
+	while (1) {
+		uint32_t token = swapb(dt_struct[0]);
+		switch (token) {
+		case FDT_NOP:
+			dt_struct++;
+			break;
+		case FDT_BEGIN_NODE:
+			break;
+		}
+	}
+}
+
+struct fdt_header *
+fdt_header(uintptr_t *fdt)
+{
+	struct fdt_header *header = (struct fdt_header *)fdt;
+	if (swapb(header->magic) != FDT_MAGIC ||
+	    swapb(header->last_comp_version) > FDT_VERSION) {
+		return NULL;
+	}
+	return header;
+}
+
+void
+scan_fdt(uintptr_t *fdt)
+{
+	fdt_map = fdt;
+	struct fdt_header *header = fdt_header(fdt);
+
+	/* get the string block */
+	const char *dt_strings =
+	    (const char *)(fdt + swapb(header->off_dt_strings));
+
+	/* get the structure block */
+	uint32_t *dt_struct = (uint32_t *)(fdt + swapb(header->off_dt_struct));
+	struct fdt_node *parent = NULL;
+	_scan_fdt(dt_strings, dt_struct);
+}