fel-sdboot: Fix header corruption workaround, implement in assembly

Now that we have a better understanding of what's causing the issue
that prevented entering FEL sometimes, we can adjust the workaround
code to a proper solution, i.e. skip over the problematic location.

Since the code amounts to less than a dozen ARM instructions, I've
decided to rewrite it as assembly code - fel-sdboot.S replaces the
former fel-sdboot.c.

Signed-off-by: Bernhard Nortmann <[email protected]>

Makefile

@@ -135,7 +135,7 @@ fel-pio.nm: fel-pio.elf
 jtag-loop.elf: jtag-loop.c jtag-loop.lds
 	$(CROSS_COMPILE)gcc  -g  $(ARM_ELF_FLAGS)  $< -nostdlib -o $@ -T jtag-loop.lds -Wl,-N
 
-fel-sdboot.elf: fel-sdboot.c fel-sdboot.lds
+fel-sdboot.elf: fel-sdboot.S fel-sdboot.lds
 	$(CROSS_COMPILE)gcc  -g  $(ARM_ELF_FLAGS)  $< -nostdlib -o $@ -T fel-sdboot.lds -Wl,-N
 
 boot_head_sun3i.elf: boot_head.S boot_head.lds

fel-sdboot.S

@@ -0,0 +1,69 @@
+/*
+ * Copyright (C) 2016  Bernhard Nortmann <[email protected]>
+ *
+ * Based on previous works
+ * Copyright (C) 2016  Siarhei Siamashka <[email protected]>
+ * Copyright (C) 2012  Henrik Nordstrom <[email protected]>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ *
+ */
+
+/*
+ * This file is a utility stub (bootloader code) to force the device into
+ * FEL mode, by jumping directly to the corresponding (N-)BROM entry point.
+ *
+ * Build instructions:
+ *   make fel-sdboot.sunxi
+ *
+ *   If needed, adjust CROSS_COMPILE and MKSUNXIBOOT according to your
+ *   toolchain, e.g.
+ *   make fel-sdboot.sunxi CROSS_COMPILE=armv7a-hardfloat-linux-gnueabi- \
+ *   MKSUNXIBOOT=/usr/local/bin/mksunxiboot
+ *
+ *
+ * Install instructions:
+ *   dd if=fel-sdboot.sunxi of=/dev/sdX bs=1024 seek=8
+ */
+
+SCTRL	.req	r0
+.equ	V_BIT, (1 << 13)
+
+.equ	BROM_ENTRY_LOW,  0x00000020
+.equ	BROM_ENTRY_HIGH, 0xFFFF0020
+
+/*
+ * In cases where insufficient padding is added by an old mksunxiboot,
+ * _start may be 0x20, which means that the instruction at 0x28 could get
+ * corrupted by the BROM - see https://patchwork.ozlabs.org/patch/622173/
+ *
+ * Apply a workaround to avoid (= skip over) that memory location.
+ * _main would be at 0x30 in that particular case. With newer (properly
+ * fixed) versions of mksunxiboot, this code ends up at higher addresses
+ * and will be moot, but harmless.
+ */
+_start:
+	b	_main
+	nop
+	nop
+	nop
+
+_main:
+	mrc	p15, 0, SCTRL, c1, c0, 0
+        tst     SCTRL, #V_BIT			@ test SCTRL.V
+	moveq	lr, #BROM_ENTRY_LOW
+	ldrne	lr, =BROM_ENTRY_HIGH
+	bx	lr