mediatek: mt7623: add scatter file for unbricking with SP Flash Tool

Many MediaTek SoCs can be unbricked by using the SP Flash Tool from
http://spflashtool.com/ along with a "scatter list" file, which is
just a text file listing which image gets loaded where.

We use a trivial partition layout for the tool, with the whole eMMC
image as a single "partition", which means users just need to unzip
the sysupgrade image. Doing the real partition layout would be overly
complex and would require the individual partitions to be shipped
as artifacts — or users to extract them out of the sysupgrade image
just for the tool to put them adjacent to each other on the eMMC
anyway.

The tool does require a copy of the preloader in order to operate,
even when it isn't flashing the preloader to the eMMC boot region.
So drop that into the bin directory as an artifact too.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

target/linux/mediatek/image/gen_scatterfile.sh

@@ -0,0 +1,75 @@
+#!/bin/sh
+#
+# Copyright © 2020 David Woodhouse <dwmw2@infradead.org>
+#
+# This is free software, licensed under the GNU General Public License v2.
+# See /LICENSE for more information.
+#
+# Generate as "scatter file" for use with the MediaTek SP Flash tool for
+# writing images to MediaTek boards. This can be used to write images
+# even on a bricked board which has no preloader installed, or broken
+# U-Boot.
+#
+# NOTE: At the time of writing (2020-07-20), the Linux tool linked from
+# the front page of https://spflashtool.com/ is out of date and does not
+# support MT7623. The newer v5.1916 found on the download page at
+# https://spflashtool.com/download/ has been tested on UniElec U7623 and
+# Banana Pi R2 from Linux, and does work.
+#
+
+SOC=$1
+IMAGE=${2%.gz}
+PROJECT=${3%-scatter.txt}
+DEVICENAME="$4"
+
+cat <<EOF
+# OpenWrt eMMC scatter file for ${DEVICENAME}
+# For use with SP Flash Tool: https://spflashtool.com/download/
+#
+# Unzip the file system file ${PROJECT}-${IMAGE}.gz before flashing.
+# Connect the device's USB port, although it won't appear to the host yet.
+# Load this scatter file into the SP Flash Tool, select 'Format All + Download'
+# After pressing the 'Download' button, power on the board.
+# The /dev/ttyACM0 device should appear on USB and the tool should find it.
+
+- general: MTK_PLATFORM_CFG
+  info:
+    - config_version: V1.1.2
+      platform: ${SOC}
+      project: ${PROJECT}
+      storage: EMMC
+      boot_channel: MSDC_0
+      block_size: 0x20000
+
+- partition_index: SYS0
+  partition_name: PRELOADER
+  file_name: ${PROJECT}-preloader.bin
+  is_download: true
+  type: SV5_BL_BIN
+  linear_start_addr: 0x0
+  physical_start_addr: 0x0
+  partition_size: 0x40000
+  region: EMMC_BOOT_1
+  storage: HW_STORAGE_EMMC
+  boundary_check: true
+  is_reserved: false
+  operation_type: BOOTLOADERS
+  d_type: FALSE
+  reserve: 0x00
+
+- partition_index: SYS1
+  partition_name: OPENWRT
+  file_name: ${PROJECT}-${IMAGE}
+  is_download: true
+  type: NORMAL_ROM
+  linear_start_addr: 0x00000
+  physical_start_addr: 0x00000
+  partition_size: 0x4000000
+  region: EMMC_USER
+  storage: HW_STORAGE_EMMC
+  boundary_check: true
+  is_reserved: false
+  operation_type: UPDATE
+  d_type: FALSE
+  reserve: 0x00
+EOF

target/linux/mediatek/image/mt7623.mk

@@ -1,4 +1,5 @@
 KERNEL_LOADADDR := 0x80008000
+DEVICE_VARS += UBOOT_TARGET
 
 ifneq ($(CONFIG_BANANA_PI_BOOT_PARTSIZE),)
 BOOTFS_BLOCK_SIZE := 1024
@@ -25,6 +26,15 @@ define Build/banana-pi-sdcard
 		$(CONFIG_TARGET_ROOTFS_PARTSIZE)
 endef
 
+define Build/preloader
+	$(CP) $(STAGING_DIR_IMAGE)/$1-preloader.bin $@
+endef
+
+define Build/scatterfile
+	./gen_scatterfile.sh $(subst mt,MT,$(SUBTARGET)) "$1" \
+		$(subst -scatter.txt,,$(notdir $@)) "$(DEVICE_TITLE)" > $@
+endef
+
 define Device/bpi_bananapi-r2
   DEVICE_VENDOR := Bpi
   DEVICE_MODEL := Banana Pi R2
@@ -34,6 +44,9 @@ define Device/bpi_bananapi-r2
   UBOOT_TARGET := mt7623n_bpir2
   IMAGES := img.gz
   IMAGE/img.gz := banana-pi-sdcard | gzip | append-metadata
+  ARTIFACT/preloader.bin := preloader $$(UBOOT_TARGET)
+  ARTIFACT/scatter.txt := scatterfile $$(firstword $$(FILESYSTEMS))-$$(firstword $$(IMAGES))
+  ARTIFACTS = preloader.bin scatter.txt
   SUPPORTED_DEVICES := bananapi,bpi-r2
 endef
 TARGET_DEVICES += bpi_bananapi-r2