bootstd: Add a bootmeth for Android

Android boot flow is a bit different than a regular Linux distro.
Android relies on multiple partitions in order to boot.

A typical boot flow would be:
1. Parse the Bootloader Control Block (BCB, misc partition)
2. If BCB requested bootonce-bootloader, start fastboot and wait.
3. If BCB requested recovery or normal android, run the following:
3.a. Get slot (A/B) from BCB
3.b. Run AVB (Android Verified Boot) on boot partitions
3.c. Load boot and vendor_boot partitions
3.d. Load device-tree, ramdisk and boot

The AOSP documentation has more details at [1], [2], [3]

This has been implemented via complex boot scripts such as [4].
However, these boot script are neither very maintainable nor generic.
Moreover, DISTRO_DEFAULTS is being deprecated [5].

Add a generic Android bootflow implementation for bootstd.
For this initial version, only boot image v4 is supported.

[1] https://source.android.com/docs/core/architecture/bootloader
[2] https://source.android.com/docs/core/architecture/partitions
[3] https://source.android.com/docs/core/architecture/partitions/generic-boot
[4] https://source.denx.de/u-boot/u-boot/-/blob/master/include/configs/meson64_android.h
[5] https://lore.kernel.org/r/all/20230914165615.1058529-17-sjg@chromium.org/

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Julien Masson <jmasson@baylibre.com>
Tested-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

6 files changed, 614 insertions, 0 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index 2c6de3a1d84..19ca57d200b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -939,6 +939,13 @@ F:	include/bootstd.h
 F:	net/eth_bootdevice.c
 F:	test/boot/
 
+BOOTMETH_ANDROID
+M:	Mattijs Korpershoek <mkorpershoek@baylibre.com>
+S:	Maintained
+T:	git https://source.denx.de/u-boot/custodians/u-boot-dfu.git
+F:	boot/bootmeth_android.c
+F:	boot/bootmeth_android.h
+
 BTRFS
 M:	Marek Behún <kabel@kernel.org>
 R:	Qu Wenruo <wqu@suse.com>
diff --git a/boot/Kconfig b/boot/Kconfig
index ffcae840a50..4a17deaa3e2 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -494,6 +494,23 @@ config BOOTMETH_GLOBAL
 	  EFI bootmgr, since they take full control over which bootdevs are
 	  selected to boot.
 
+config BOOTMETH_ANDROID
+	bool "Bootdev support for Android"
+	depends on X86 || ARM || SANDBOX
+	depends on CMDLINE
+	select ANDROID_AB
+	select ANDROID_BOOT_IMAGE
+	select CMD_BCB
+	select CMD_FASTBOOT
+	select PARTITION_TYPE_GUID
+	select PARTITION_UUIDS
+	help
+	  Enables support for booting Android using bootstd. Android requires
+	  multiple partitions (misc, boot, vbmeta, ...) in storage for booting.
+
+	  Note that only MMC bootdevs are supported at present. This is caused
+	  by AVB being limited to MMC devices only.
+
 config BOOTMETH_CROS
 	bool "Bootdev support for Chromium OS"
 	depends on X86 || ARM || SANDBOX
diff --git a/boot/Makefile b/boot/Makefile
index 84ccfeaecec..75d1cd46fab 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -66,3 +66,5 @@ obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_REQUEST) += vbe_request.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_SIMPLE) += vbe_simple.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_SIMPLE_FW) += vbe_simple_fw.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_VBE_SIMPLE_OS) += vbe_simple_os.o
+
+obj-$(CONFIG_$(SPL_TPL_)BOOTMETH_ANDROID) += bootmeth_android.o
diff --git a/boot/bootmeth_android.c b/boot/bootmeth_android.c
new file mode 100644
index 00000000000..6e8d3e615db
--- /dev/null
+++ b/boot/bootmeth_android.c
@@ -0,0 +1,553 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Bootmeth for Android
+ *
+ * Copyright (C) 2024 BayLibre, SAS
+ * Written by Mattijs Korpershoek <mkorpershoek@baylibre.com>
+ */
+#define LOG_CATEGORY UCLASS_BOOTSTD
+
+#include <android_ab.h>
+#include <android_image.h>
+#if CONFIG_IS_ENABLED(AVB_VERIFY)
+#include <avb_verify.h>
+#endif
+#include <bcb.h>
+#include <blk.h>
+#include <bootflow.h>
+#include <bootm.h>
+#include <bootmeth.h>
+#include <dm.h>
+#include <image.h>
+#include <malloc.h>
+#include <mapmem.h>
+#include <part.h>
+#include "bootmeth_android.h"
+
+#define BCB_FIELD_COMMAND_SZ 32
+#define BCB_PART_NAME "misc"
+#define BOOT_PART_NAME "boot"
+#define VENDOR_BOOT_PART_NAME "vendor_boot"
+
+/**
+ * struct android_priv - Private data
+ *
+ * This is read from the disk and recorded for use when the full Android
+ * kernel must be loaded and booted
+ *
+ * @boot_mode: Requested boot mode (normal, recovery, bootloader)
+ * @slot: Nul-terminated partition slot suffix read from BCB ("a\0" or "b\0")
+ * @header_version: Android boot image header version
+ */
+struct android_priv {
+	enum android_boot_mode boot_mode;
+	char slot[2];
+	u32 header_version;
+};
+
+static int android_check(struct udevice *dev, struct bootflow_iter *iter)
+{
+	/* This only works on mmc devices */
+	if (bootflow_iter_check_mmc(iter))
+		return log_msg_ret("mmc", -ENOTSUPP);
+
+	/*
+	 * This only works on whole devices, as multiple
+	 * partitions are needed to boot Android
+	 */
+	if (iter->part != 0)
+		return log_msg_ret("mmc part", -ENOTSUPP);
+
+	return 0;
+}
+
+static int scan_boot_part(struct udevice *blk, struct android_priv *priv)
+{
+	struct blk_desc *desc = dev_get_uclass_plat(blk);
+	struct disk_partition partition;
+	char partname[PART_NAME_LEN];
+	ulong num_blks, bufsz;
+	char *buf;
+	int ret;
+
+	sprintf(partname, BOOT_PART_NAME "_%s", priv->slot);
+	ret = part_get_info_by_name(desc, partname, &partition);
+	if (ret < 0)
+		return log_msg_ret("part info", ret);
+
+	num_blks = DIV_ROUND_UP(sizeof(struct andr_boot_img_hdr_v0), desc->blksz);
+	bufsz = num_blks * desc->blksz;
+	buf = malloc(bufsz);
+	if (!buf)
+		return log_msg_ret("buf", -ENOMEM);
+
+	ret = blk_read(blk, partition.start, num_blks, buf);
+	if (ret != num_blks) {
+		free(buf);
+		return log_msg_ret("part read", -EIO);
+	}
+
+	if (!is_android_boot_image_header(buf)) {
+		free(buf);
+		return log_msg_ret("header", -ENOENT);
+	}
+
+	priv->header_version = ((struct andr_boot_img_hdr_v0 *)buf)->header_version;
+	free(buf);
+
+	return 0;
+}
+
+static int scan_vendor_boot_part(struct udevice *blk, struct android_priv *priv)
+{
+	struct blk_desc *desc = dev_get_uclass_plat(blk);
+	struct disk_partition partition;
+	char partname[PART_NAME_LEN];
+	ulong num_blks, bufsz;
+	char *buf;
+	int ret;
+
+	sprintf(partname, VENDOR_BOOT_PART_NAME "_%s", priv->slot);
+	ret = part_get_info_by_name(desc, partname, &partition);
+	if (ret < 0)
+		return log_msg_ret("part info", ret);
+
+	num_blks = DIV_ROUND_UP(sizeof(struct andr_vnd_boot_img_hdr), desc->blksz);
+	bufsz = num_blks * desc->blksz;
+	buf = malloc(bufsz);
+	if (!buf)
+		return log_msg_ret("buf", -ENOMEM);
+
+	ret = blk_read(blk, partition.start, num_blks, buf);
+	if (ret != num_blks) {
+		free(buf);
+		return log_msg_ret("part read", -EIO);
+	}
+
+	if (!is_android_vendor_boot_image_header(buf)) {
+		free(buf);
+		return log_msg_ret("header", -ENOENT);
+	}
+	free(buf);
+
+	return 0;
+}
+
+static int android_read_slot_from_bcb(struct bootflow *bflow, bool decrement)
+{
+	struct blk_desc *desc = dev_get_uclass_plat(bflow->blk);
+	struct android_priv *priv = bflow->bootmeth_priv;
+	struct disk_partition misc;
+	char slot_suffix[3];
+	int ret;
+
+	ret = part_get_info_by_name(desc, BCB_PART_NAME, &misc);
+	if (ret < 0)
+		return log_msg_ret("part", ret);
+
+	ret = ab_select_slot(desc, &misc, decrement);
+	if (ret < 0)
+		return log_msg_ret("slot", ret);
+
+	priv->slot[0] = BOOT_SLOT_NAME(ret);
+	priv->slot[1] = '\0';
+
+	sprintf(slot_suffix, "_%s", priv->slot);
+	ret = bootflow_cmdline_set_arg(bflow, "androidboot.slot_suffix",
+				       slot_suffix, false);
+	if (ret < 0)
+		return log_msg_ret("cmdl", ret);
+
+	return 0;
+}
+
+static int configure_serialno(struct bootflow *bflow)
+{
+	char *serialno = env_get("serial#");
+
+	if (!serialno)
+		return log_msg_ret("serial", -ENOENT);
+
+	return bootflow_cmdline_set_arg(bflow, "androidboot.serialno", serialno, false);
+}
+
+static int android_read_bootflow(struct udevice *dev, struct bootflow *bflow)
+{
+	struct blk_desc *desc = dev_get_uclass_plat(bflow->blk);
+	struct disk_partition misc;
+	struct android_priv *priv;
+	char command[BCB_FIELD_COMMAND_SZ];
+	int ret;
+
+	bflow->state = BOOTFLOWST_MEDIA;
+
+	/*
+	 * bcb_find_partition_and_load() will print errors to stdout
+	 * if BCB_PART_NAME is not found. To avoid that, check if the
+	 * partition exists first.
+	 */
+	ret = part_get_info_by_name(desc, BCB_PART_NAME, &misc);
+	if (ret < 0)
+		return log_msg_ret("part", ret);
+
+	ret = bcb_find_partition_and_load("mmc", desc->devnum, BCB_PART_NAME);
+	if (ret < 0)
+		return log_msg_ret("bcb load", ret);
+
+	ret = bcb_get(BCB_FIELD_COMMAND, command, sizeof(command));
+	if (ret < 0)
+		return log_msg_ret("bcb read", ret);
+
+	priv = malloc(sizeof(struct android_priv));
+	if (!priv)
+		return log_msg_ret("buf", -ENOMEM);
+
+	if (!strcmp("bootonce-bootloader", command)) {
+		priv->boot_mode = ANDROID_BOOT_MODE_BOOTLOADER;
+		bflow->os_name = strdup("Android (bootloader)");
+	} else if (!strcmp("boot-fastboot", command)) {
+		priv->boot_mode = ANDROID_BOOT_MODE_RECOVERY;
+		bflow->os_name = strdup("Android (fastbootd)");
+	} else if (!strcmp("boot-recovery", command)) {
+		priv->boot_mode = ANDROID_BOOT_MODE_RECOVERY;
+		bflow->os_name = strdup("Android (recovery)");
+	} else {
+		priv->boot_mode = ANDROID_BOOT_MODE_NORMAL;
+		bflow->os_name = strdup("Android");
+	}
+	if (!bflow->os_name)
+		return log_msg_ret("os", -ENOMEM);
+
+	if (priv->boot_mode == ANDROID_BOOT_MODE_BOOTLOADER) {
+		/* Clear BCB */
+		memset(command, 0, sizeof(command));
+		ret = bcb_set(BCB_FIELD_COMMAND, command);
+		if (ret < 0) {
+			free(priv);
+			return log_msg_ret("bcb set", ret);
+		}
+		ret = bcb_store();
+		if (ret < 0) {
+			free(priv);
+			return log_msg_ret("bcb store", ret);
+		}
+
+		bflow->bootmeth_priv = priv;
+		bflow->state = BOOTFLOWST_READY;
+		return 0;
+	}
+
+	bflow->bootmeth_priv = priv;
+
+	/* For recovery and normal boot, we need to scan the partitions */
+	ret = android_read_slot_from_bcb(bflow, false);
+	if (ret < 0) {
+		log_err("read slot: %d", ret);
+		goto free_priv;
+	}
+
+	ret = scan_boot_part(bflow->blk, priv);
+	if (ret < 0) {
+		log_debug("scan boot failed: err=%d\n", ret);
+		goto free_priv;
+	}
+
+	if (priv->header_version != 4) {
+		log_debug("only boot.img v4 is supported %u\n", priv->header_version);
+		ret = -EINVAL;
+		goto free_priv;
+	}
+
+	ret = scan_vendor_boot_part(bflow->blk, priv);
+	if (ret < 0) {
+		log_debug("scan vendor_boot failed: err=%d\n", ret);
+		goto free_priv;
+	}
+
+	/* Ignoring return code: setting serial number is not mandatory for booting */
+	configure_serialno(bflow);
+
+	if (priv->boot_mode == ANDROID_BOOT_MODE_NORMAL) {
+		ret = bootflow_cmdline_set_arg(bflow, "androidboot.force_normal_boot",
+					       "1", false);
+		if (ret < 0) {
+			log_debug("normal_boot %d", ret);
+			goto free_priv;
+		}
+	}
+
+	bflow->state = BOOTFLOWST_READY;
+
+	return 0;
+
+ free_priv:
+	free(priv);
+	bflow->bootmeth_priv = NULL;
+	return ret;
+}
+
+static int android_read_file(struct udevice *dev, struct bootflow *bflow,
+			     const char *file_path, ulong addr, ulong *sizep)
+{
+	/*
+	 * Reading individual files is not supported since we only
+	 * operate on whole mmc devices (because we require multiple partitions)
+	 */
+	return log_msg_ret("Unsupported", -ENOSYS);
+}
+
+/**
+ * read_slotted_partition() - Read a partition by appending a slot suffix
+ *
+ * Most modern Android devices use Seamless Updates, where each partition
+ * is duplicated. For example, the boot partition has boot_a and boot_b.
+ * For more information, see:
+ * https://source.android.com/docs/core/ota/ab
+ * https://source.android.com/docs/core/ota/ab/ab_implement
+ *
+ * @blk: Block device to read
+ * @name: Partition name to read
+ * @slot: Nul-terminated slot suffixed to partition name ("a\0" or "b\0")
+ * @addr: Address where the partition content is loaded into
+ * Return: 0 if OK, negative errno on failure.
+ */
+static int read_slotted_partition(struct blk_desc *desc, const char *const name,
+				  const char slot[2], ulong addr)
+{
+	struct disk_partition partition;
+	char partname[PART_NAME_LEN];
+	int ret;
+	u32 n;
+
+	/* Ensure name fits in partname it should be: <name>_<slot>\0 */
+	if (strlen(name) > (PART_NAME_LEN - 2 - 1))
+		return log_msg_ret("name too long", -EINVAL);
+
+	sprintf(partname, "%s_%s", name, slot);
+	ret = part_get_info_by_name(desc, partname, &partition);
+	if (ret < 0)
+		return log_msg_ret("part", ret);
+
+	n = blk_dread(desc, partition.start, partition.size, map_sysmem(addr, 0));
+	if (n < partition.size)
+		return log_msg_ret("part read", -EIO);
+
+	return 0;
+}
+
+#if CONFIG_IS_ENABLED(AVB_VERIFY)
+static int avb_append_commandline_arg(struct bootflow *bflow, char *arg)
+{
+	char *key = strsep(&arg, "=");
+	char *value = arg;
+	int ret;
+
+	ret = bootflow_cmdline_set_arg(bflow, key, value, false);
+	if (ret < 0)
+		return log_msg_ret("avb cmdline", ret);
+
+	return 0;
+}
+
+static int avb_append_commandline(struct bootflow *bflow, char *cmdline)
+{
+	char *arg = strsep(&cmdline, " ");
+	int ret;
+
+	while (arg) {
+		ret = avb_append_commandline_arg(bflow, arg);
+		if (ret < 0)
+			return ret;
+
+		arg = strsep(&cmdline, " ");
+	}
+
+	return 0;
+}
+
+static int run_avb_verification(struct bootflow *bflow)
+{
+	struct blk_desc *desc = dev_get_uclass_plat(bflow->blk);
+	struct android_priv *priv = bflow->bootmeth_priv;
+	const char * const requested_partitions[] = {"boot", "vendor_boot"};
+	struct AvbOps *avb_ops;
+	AvbSlotVerifyResult result;
+	AvbSlotVerifyData *out_data;
+	enum avb_boot_state boot_state;
+	char *extra_args;
+	char slot_suffix[3];
+	bool unlocked = false;
+	int ret;
+
+	avb_ops = avb_ops_alloc(desc->devnum);
+	if (!avb_ops)
+		return log_msg_ret("avb ops", -ENOMEM);
+
+	sprintf(slot_suffix, "_%s", priv->slot);
+
+	ret = avb_ops->read_is_device_unlocked(avb_ops, &unlocked);
+	if (ret != AVB_IO_RESULT_OK)
+		return log_msg_ret("avb lock", -EIO);
+
+	result = avb_slot_verify(avb_ops,
+				 requested_partitions,
+				 slot_suffix,
+				 unlocked,
+				 AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE,
+				 &out_data);
+
+	if (result != AVB_SLOT_VERIFY_RESULT_OK) {
+		printf("Verification failed, reason: %s\n",
+		       str_avb_slot_error(result));
+		avb_slot_verify_data_free(out_data);
+		return log_msg_ret("avb verify", -EIO);
+	}
+
+	if (unlocked)
+		boot_state = AVB_ORANGE;
+	else
+		boot_state = AVB_GREEN;
+
+	extra_args = avb_set_state(avb_ops, boot_state);
+	if (extra_args) {
+		/* extra_args will be modified after this. This is fine */
+		ret = avb_append_commandline_arg(bflow, extra_args);
+		if (ret < 0)
+			goto free_out_data;
+	}
+
+	ret = avb_append_commandline(bflow, out_data->cmdline);
+	if (ret < 0)
+		goto free_out_data;
+
+	return 0;
+
+ free_out_data:
+	if (out_data)
+		avb_slot_verify_data_free(out_data);
+
+	return log_msg_ret("avb cmdline", ret);
+}
+#else
+static int run_avb_verification(struct bootflow *bflow)
+{
+	int ret;
+
+	/* When AVB is unsupported, pass ORANGE state  */
+	ret = bootflow_cmdline_set_arg(bflow,
+				       "androidboot.verifiedbootstate",
+				       "orange", false);
+	if (ret < 0)
+		return log_msg_ret("avb cmdline", ret);
+
+	return 0;
+}
+#endif /* AVB_VERIFY */
+
+static int boot_android_normal(struct bootflow *bflow)
+{
+	struct blk_desc *desc = dev_get_uclass_plat(bflow->blk);
+	struct android_priv *priv = bflow->bootmeth_priv;
+	int ret;
+	ulong loadaddr = env_get_hex("loadaddr", 0);
+	ulong vloadaddr = env_get_hex("vendor_boot_comp_addr_r", 0);
+
+	ret = run_avb_verification(bflow);
+	if (ret < 0)
+		return log_msg_ret("avb", ret);
+
+	/* Read slot once more to decrement counter from BCB */
+	ret = android_read_slot_from_bcb(bflow, true);
+	if (ret < 0)
+		return log_msg_ret("read slot", ret);
+
+	ret = read_slotted_partition(desc, "boot", priv->slot, loadaddr);
+	if (ret < 0)
+		return log_msg_ret("read boot", ret);
+
+	ret = read_slotted_partition(desc, "vendor_boot", priv->slot, vloadaddr);
+	if (ret < 0)
+		return log_msg_ret("read vendor_boot", ret);
+
+	set_abootimg_addr(loadaddr);
+	set_avendor_bootimg_addr(vloadaddr);
+
+	ret = bootm_boot_start(loadaddr, bflow->cmdline);
+
+	return log_msg_ret("boot", ret);
+}
+
+static int boot_android_recovery(struct bootflow *bflow)
+{
+	int ret;
+
+	ret = boot_android_normal(bflow);
+
+	return log_msg_ret("boot", ret);
+}
+
+static int boot_android_bootloader(struct bootflow *bflow)
+{
+	int ret;
+
+	ret = run_command("fastboot usb 0", 0);
+	do_reset(NULL, 0, 0, NULL);
+
+	return log_msg_ret("boot", ret);
+}
+
+static int android_boot(struct udevice *dev, struct bootflow *bflow)
+{
+	struct android_priv *priv = bflow->bootmeth_priv;
+	int ret;
+
+	switch (priv->boot_mode) {
+	case ANDROID_BOOT_MODE_NORMAL:
+		ret = boot_android_normal(bflow);
+		break;
+	case ANDROID_BOOT_MODE_RECOVERY:
+		ret = boot_android_recovery(bflow);
+		break;
+	case ANDROID_BOOT_MODE_BOOTLOADER:
+		ret = boot_android_bootloader(bflow);
+		break;
+	default:
+		printf("ANDROID: Unknown boot mode %d. Running fastboot...\n",
+		       priv->boot_mode);
+		boot_android_bootloader(bflow);
+		/* Tell we failed to boot since boot mode is unknown */
+		ret = -EFAULT;
+	}
+
+	return ret;
+}
+
+static int android_bootmeth_bind(struct udevice *dev)
+{
+	struct bootmeth_uc_plat *plat = dev_get_uclass_plat(dev);
+
+	plat->desc = "Android boot";
+	plat->flags = BOOTMETHF_ANY_PART;
+
+	return 0;
+}
+
+static struct bootmeth_ops android_bootmeth_ops = {
+	.check		= android_check,
+	.read_bootflow	= android_read_bootflow,
+	.read_file	= android_read_file,
+	.boot		= android_boot,
+};
+
+static const struct udevice_id android_bootmeth_ids[] = {
+	{ .compatible = "u-boot,android" },
+	{ }
+};
+
+U_BOOT_DRIVER(bootmeth_android) = {
+	.name		= "bootmeth_android",
+	.id		= UCLASS_BOOTMETH,
+	.of_match	= android_bootmeth_ids,
+	.ops		= &android_bootmeth_ops,
+	.bind		= android_bootmeth_bind,
+};
diff --git a/boot/bootmeth_android.h b/boot/bootmeth_android.h
new file mode 100644
index 00000000000..a57fad9f011
--- /dev/null
+++ b/boot/bootmeth_android.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Bootmethod for Android
+ *
+ * Copyright (C) 2024 BayLibre, SAS
+ * Written by Mattijs Korpershoek <mkorpershoek@baylibre.com>
+ */
+
+enum android_boot_mode {
+	ANDROID_BOOT_MODE_NORMAL = 0,
+
+	/*
+	 * Android "recovery" is a special boot mode that uses another ramdisk.
+	 * It can be used to "factory reset" a board or to flash logical partitions
+	 * It operates in 2 modes: adb or fastbootd
+	 * To enter recovery from Android, we can do:
+	 * $ adb reboot recovery
+	 * $ adb reboot fastboot
+	 */
+	ANDROID_BOOT_MODE_RECOVERY,
+
+	/*
+	 * Android "bootloader" is for accessing/reflashing physical partitions
+	 * Typically, this will launch a fastboot process in U-Boot.
+	 * To enter "bootloader" from Android, we can do:
+	 * $ adb reboot bootloader
+	 */
+	ANDROID_BOOT_MODE_BOOTLOADER,
+};
diff --git a/doc/develop/bootstd.rst b/doc/develop/bootstd.rst
index 34631089ae0..bdda90fae3f 100644
--- a/doc/develop/bootstd.rst
+++ b/doc/develop/bootstd.rst
@@ -95,6 +95,7 @@ bootflows.
 
 Note: it is possible to have a bootmeth that uses a partition or a whole device
 directly, but it is more common to use a filesystem.
+For example, the Android bootmeth uses a whole device.
 
 Note that some bootmeths are 'global', meaning that they select the bootdev
 themselves. Examples include VBE and EFI boot manager. In this case, they
@@ -277,6 +278,9 @@ script_offset_f
 script_size_f
     Size of the script to load, e.g. 0x2000
 
+vendor_boot_comp_addr_r
+    Address to which to load the vendor_boot Android image, e.g. 0xe0000000
+
 Some variables are set by script bootmeth:
 
 devtype
@@ -418,6 +422,7 @@ Bootmeth drivers are provided for:
    - EFI boot using bootefi from disk
    - VBE
    - EFI boot using boot manager
+   - Android bootflow (boot image v4)
 
 
 Command interface
@@ -786,6 +791,7 @@ To do
 Some things that need to be done to completely replace the distro-boot scripts:
 
 - implement extensions (devicetree overlays with add-on boards)
+- implement legacy (boot image v2) android boot flow
 
 Other ideas: