gemini: Enable flash boot on reference design type

The flash layout on the Storlink reference design for Gemini
is using 2 MB flash for the kernel, and it also insists on
overwriting the partition table with default values on every
boot. The same is true for the SQ201. This poses a problem
on recent OpenWrt firmware as the base zImage is bigger
than 2 MB.

At the same time there is a ramdisk partition of 6 MB that we
don't really need. The partition table looks like this:

Creating 7 MTD partitions on "30000000.flash":
0x000000000000-0x000000016000 : "BOOT"
0x000000120000-0x000000320000 : "Kern"
0x000000320000-0x000000920000 : "Ramdisk"
0x000000920000-0x000000f20000 : "Application"
0x000000f20000-0x000000f40000 : "VCTL"
0x000000f40000-0x000000fe0000 : "CurConf"
0x000000fe0000-0x000001000000 : "FIS directory"

On boot the "Kern" partition is copied to RAM @0x01600000
and the "Ramdisk" partition is copied to RAM @0x00800000.
Then the kernel is executed.

The idea with this patch is to extend the "Kern" partition
with the "Ramdisk" partition to get a full 8 MB to use
for the kernel. Then we put the OpenWrt JFFS2 rootfs
inside the "Application" partition.

We create a small assembly loop that we prepend to the
"Kern" image that will copy the "Kern" from 0x0160000
and the "Ramdisk" from 0x00800000 and put them in
consecutive space at 0x00400000 and execute it from
there, using "Application" as rootfs.

We generate 3 main files:
- zImage - contains the assembly bootstrap loop and
  the first part of the generated kernel image
- rd.gz - contains the second part of the generated
  kernel image
- hddapp.tgz - contains the root filesystem

On the SQ201 I flash these manually using the native boot
loader PLATO, "Y" alternative for the zImage, "R" for
the rd.gz image and "A" for hddapp.tgz.

This works fine and I can now boot to prompt on the SQ201
with nothing but flash.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

target/linux/gemini/image/Makefile

@@ -7,6 +7,10 @@
 include $(TOPDIR)/rules.mk
 include $(INCLUDE_DIR)/image.mk
 
+define Build/copy-kernel
+	$(MAKE) -C copy-kernel CROSS_COMPILE=$(TARGET_CROSS)
+endef
+
 # Cook a "WRGG" image, this board is apparently one in the D-Link
 # WRGG family and uses the exact same firmware format as other
 # D-Link devices.
@@ -76,18 +80,29 @@ endef
 
 # Create the default image format used by the StorLink reference design
 # SL93512r, Raidsonic NAS4220B and Itian Square One SQ201
-# with the squashfs and overlay inside the "rd.gz" file.
-# We pad it out to 6144K which is the size of the initramfs partition.
+# with the squashfs and overlay inside the "application" partition.
+#
+# These devices have a hard-coded partition table that the boot loader
+# constantly reflashes back, so we need to work around it like this:
 #
-# The "application" partition is just blank. You can put anything
-# there when using OpenWRT. We just use that to create the
-# "sysupgrade" firmware image.
+# 0x000000120000-0x000000320000 : "Kern" - small copy routine and first
+#                                 part of the kernel goes here
+# 0x000000320000-0x000000920000 : "Ramdisk" - second part of the kernel and
+#                                 some padding goes here
+# 0x000000920000-0x000000f20000 : "Application" - rootfs goes here
 define Build/storlink-default-images
 	mkdir -p $@.tmp
 
-	mv $@ $@.tmp/rd.gz
-	dd if=/dev/zero of=$@.tmp/hddapp.tgz bs=6144k count=1
-	cp $(IMAGE_KERNEL) $@.tmp/zImage
+	$(call Build/copy-kernel)
+	# "App" partition is the rootfs
+	mv $@ $@.tmp/hddapp.tgz
+	# 256 bytes copy routine
+	dd if=copy-kernel/copy-kernel.bin of=$@.tmp/zImage
+	$(call Image/pad-to,$@.tmp/zImage,512)
+	# Copy first part of the kernel into zImage
+	dd if=$(IMAGE_KERNEL) of=$@.tmp/zImage bs=1 seek=512 count=2096640
+	# Put the rest of the kernel into the "ramdisk"
+	dd if=$(IMAGE_KERNEL) of=$@.tmp/rd.gz bs=1 skip=2096640 count=6144k conv=sync
 	cp ./ImageInfo-$(1) $@.tmp/ImageInfo
 
 	sed -i -e "s/DATESTR/`date +%Y%m%d $(if $(SOURCE_DATE_EPOCH),--date "@$(SOURCE_DATE_EPOCH)")`/g" $@.tmp/ImageInfo

target/linux/gemini/patches-4.19/0011-ARM-dts-Fix-up-SQ201-flash-access.patch

@@ -20,7 +20,7 @@ Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
  
  	chosen {
 -		bootargs = "console=ttyS0,115200n8 root=/dev/sda1 rw rootwait";
-+		bootargs = "console=ttyS0,115200n8 root=/dev/mtdblock2 rw rootfstype=squashfs,jffs2 rootwait";
++		bootargs = "console=ttyS0,115200n8 root=/dev/mtdblock3 rw rootfstype=squashfs,jffs2 rootwait";
  		stdout-path = &uart0;
  	};