board: Add new Samsung "stemmy" board based on ST-Ericsson U8500

The ST-Ericsson U8500 SoC has been used in mass-production for
some Android smartphones released around 2012.
In particular, Samsung has released more than 5 different
smartphones based on U8500, e.g.

  - Samsung Galaxy S III mini (GT-I8190) "golden"
  - Samsung Galaxy S Advance (GT-I9070)  "janice"
  - Samsung Galaxy Xcover 2 (GT-S7710)   "skomer"

and a few others.

Mainline Linux has great support for the Ux500 SoC, so these
smartphones can also run Linux mainline quite well.

Unfortunately, the original Samsung bootloader used on these devices
has limitations that prevent booting Linux mainline directly.
It keeps the L2 cache enabled, which causes Linux to crash very early,
shortly after decompressing the kernel.

Using U-Boot allows to circumvent these limitations. We can let the
Samsung bootloader chain-load U-Boot and U-Boot locks the L2 cache
before booting into Linux. U-Boot has several other advantages
- it supports device-trees directly and we are no longer limited to
flashing Android boot images through Samsung's proprietary download
mode.

The Samsung "stemmy" board covers all Samsung devices based on U8500.
Add minimal support for "stemmy". For now only UART is supported but
this will be extended later.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

board/ste/stemmy/Kconfig

@@ -0,0 +1,12 @@
+if TARGET_STEMMY
+
+config SYS_BOARD
+	default "stemmy"
+
+config SYS_VENDOR
+	default "ste"
+
+config SYS_CONFIG_NAME
+	default "stemmy"
+
+endif

board/ste/stemmy/MAINTAINERS

@@ -0,0 +1,6 @@
+STEMMY BOARD
+M:	Stephan Gerhold <stephan@gerhold.net>
+S:	Maintained
+F:	board/ste/stemmy/
+F:	include/configs/stemmy.h
+F:	configs/stemmy_defconfig

board/ste/stemmy/Makefile

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+obj-y	:= stemmy.o

board/ste/stemmy/README

@@ -0,0 +1,49 @@
+ST-Ericsson U8500 Samsung "stemmy" board
+========================================
+
+The "stemmy" board supports Samsung smartphones released with
+the ST-Ericsson NovaThor U8500 SoC, e.g.
+
+	- Samsung Galaxy S III mini (GT-I8190)	"golden"
+	- Samsung Galaxy S Advance (GT-I9070)	"janice"
+	- Samsung Galaxy Xcover 2 (GT-S7710)	"skomer"
+
+and likely others as well (untested).
+
+At the moment, U-Boot is intended to be chain-loaded from
+the original Samsung bootloader, not replacing it entirely.
+
+Installation
+------------
+
+1. Setup cross compiler, e.g. export CROSS_COMPILE=arm-none-eabi-
+2. make stemmy_defconfig
+3. make
+
+For newer devices (golden and skomer), the U-Boot binary has to be packed into
+an Android boot image. janice boots the raw U-Boot binary from the boot partition.
+
+4. Obtain mkbootimg, e.g. https://android.googlesource.com/platform/system/core/+/refs/tags/android-7.1.2_r37/mkbootimg/mkbootimg
+5. mkbootimg \
+    --kernel=u-boot.bin \
+    --base=0x00000000 \
+    --kernel_offset=0x00100000 \
+    --ramdisk_offset=0x02000000 \
+    --tags_offset=0x00000100 \
+    --output=u-boot.img
+
+6. Enter Samsung download mode (press Power + Home + Volume Down)
+7. Flash U-Boot image to Android boot partition using Heimdall:
+     https://gitlab.com/BenjaminDobell/Heimdall
+
+   heimdall flash --Kernel u-boot.(bin|img)
+
+8. After reboot U-Boot prompt should appear via UART.
+
+UART
+----
+
+UART is available through the micro USB port, similar to the Carkit standard.
+With a ~619kOhm resistor between ID and GND, 1.8V RX/TX is available at D+/D-.
+
+Make sure to connect the UART cable *before* turning on the phone.

board/ste/stemmy/stemmy.c

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2019 Stephan Gerhold <stephan@gerhold.net>
+ */
+#include <common.h>
+
+DECLARE_GLOBAL_DATA_PTR;
+
+int dram_init(void)
+{
+	gd->ram_size = get_ram_size(CONFIG_SYS_SDRAM_BASE, CONFIG_SYS_SDRAM_SIZE);
+	return 0;
+}
+
+int board_init(void)
+{
+	return 0;
+}