kernel: add parser finding rootfs after CFE bootfs

It's required for BCM4908. It cannot use "bcm-wfi-fw" parser because
that one requires *two* JFFS2 partitions which is untested / unsupported
on the BCM4908 architecture. With a single JFFS2 partition "bcm-wfi-fw"
parser will:
1. Fail to find "vmlinux.lz" as it doesn't follow "1-openwrt" file
2. Create partitions that don't precisely match bootfs layout

The new parser is described in details in the MTD_SPLIT_CFE_BOOTFS
symbol help message.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/bcm4908/config-5.4

@@ -138,7 +138,8 @@ CONFIG_MTD_NAND_BRCMNAND=y
 CONFIG_MTD_NAND_CORE=y
 CONFIG_MTD_NAND_ECC_SW_HAMMING=y
 CONFIG_MTD_RAW_NAND=y
-CONFIG_MTD_SPLIT_BCM_WFI_FW=y
+CONFIG_MTD_SPLIT_CFE_BOOTFS=y
+# CONFIG_MTD_SPLIT_SQUASHFS_ROOT is not set
 CONFIG_MTD_UBI=y
 CONFIG_MTD_UBI_BEB_LIMIT=20
 CONFIG_MTD_UBI_BLOCK=y

target/linux/generic/files/drivers/mtd/mtdsplit/Kconfig

@@ -25,6 +25,18 @@ config MTD_SPLIT_BCM_WFI_FW
 	depends on MTD_SPLIT_SUPPORT
 	select MTD_SPLIT
 
+config MTD_SPLIT_CFE_BOOTFS
+	bool "Parser finding rootfs appended to the CFE bootfs"
+	depends on MTD_SPLIT_SUPPORT && ARCH_BCM4908
+	select MTD_SPLIT
+	help
+	  cferom on BCM4908 (and bcm63xx) uses JFFS2 bootfs partition
+	  for storing kernel, cferam and some device specific files.
+	  There isn't any straight way of storing rootfs so it gets
+	  appended to the JFFS2 bootfs partition. Kernel needs to find
+	  it and run init from it. This parser is responsible for
+	  finding appended rootfs.
+
 config MTD_SPLIT_SEAMA_FW
 	bool "Seama firmware parser"
 	depends on MTD_SPLIT_SUPPORT

target/linux/generic/files/drivers/mtd/mtdsplit/Makefile

@@ -1,5 +1,6 @@
 obj-$(CONFIG_MTD_SPLIT)		+= mtdsplit.o
 obj-$(CONFIG_MTD_SPLIT_BCM_WFI_FW) += mtdsplit_bcm_wfi.o
+obj-$(CONFIG_MTD_SPLIT_CFE_BOOTFS) += mtdsplit_cfe_bootfs.o
 obj-$(CONFIG_MTD_SPLIT_SEAMA_FW) += mtdsplit_seama.o
 obj-$(CONFIG_MTD_SPLIT_SQUASHFS_ROOT) += mtdsplit_squashfs.o
 obj-$(CONFIG_MTD_SPLIT_UIMAGE_FW) += mtdsplit_uimage.o

target/linux/generic/files/drivers/mtd/mtdsplit/mtdsplit_cfe_bootfs.c

@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2021 Rafał Miłecki <rafal@milecki.pl>
+ */
+
+#include <linux/init.h>
+#include <linux/jffs2.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/partitions.h>
+#include <linux/of.h>
+#include <linux/slab.h>
+
+#include "mtdsplit.h"
+
+#define je16_to_cpu(x) ((x).v16)
+#define je32_to_cpu(x) ((x).v32)
+
+#define NR_PARTS		1
+
+static int mtdsplit_cfe_bootfs_parse(struct mtd_info *mtd,
+				     const struct mtd_partition **pparts,
+				     struct mtd_part_parser_data *data)
+{
+	struct jffs2_raw_dirent node;
+	enum mtdsplit_part_type type;
+	struct mtd_partition *parts;
+	size_t rootfs_offset;
+	size_t retlen;
+	size_t offset;
+	int err;
+
+	/* Don't parse backup partitions */
+	if (strcmp(mtd->name, "firmware"))
+		return -EINVAL;
+
+	/* Find the end of JFFS2 bootfs partition */
+	offset = 0;
+	do {
+		err = mtd_read(mtd, offset, sizeof(node), &retlen, (void *)&node);
+		if (err || retlen != sizeof(node))
+			break;
+
+		if (je16_to_cpu(node.magic) != JFFS2_MAGIC_BITMASK)
+			break;
+
+		offset += je32_to_cpu(node.totlen);
+		offset = (offset + 0x3) & ~0x3;
+	} while (offset < mtd->size);
+
+	/* Find rootfs partition that follows the bootfs */
+	err = mtd_find_rootfs_from(mtd, mtd->erasesize, mtd->size, &rootfs_offset, &type);
+	if (err)
+		return err;
+
+	parts = kzalloc(NR_PARTS * sizeof(*parts), GFP_KERNEL);
+	if (!parts)
+		return -ENOMEM;
+
+	if (type == MTDSPLIT_PART_TYPE_UBI)
+		parts[0].name = UBI_PART_NAME;
+	else
+		parts[0].name = ROOTFS_PART_NAME;
+	parts[0].offset = rootfs_offset;
+	parts[0].size = mtd->size - rootfs_offset;
+
+	*pparts = parts;
+
+	return NR_PARTS;
+}
+
+static const struct of_device_id mtdsplit_cfe_bootfs_of_match_table[] = {
+	{ .compatible = "brcm,bcm4908-firmware" },
+	{},
+};
+MODULE_DEVICE_TABLE(of, mtdsplit_cfe_bootfs_of_match_table);
+
+static struct mtd_part_parser mtdsplit_cfe_bootfs_parser = {
+	.owner = THIS_MODULE,
+	.name = "cfe-bootfs",
+	.of_match_table = mtdsplit_cfe_bootfs_of_match_table,
+	.parse_fn = mtdsplit_cfe_bootfs_parse,
+};
+
+module_mtd_part_parser(mtdsplit_cfe_bootfs_parser);