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.

The commit also includes a new binary (bin/fel-sdboot.sunxi) with
these changes.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

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.c → fel-sdboot.S

 /*
- * (C) Copyright 2011 Henrik Nordstrom <henrik@henriknordstrom.net>
+ * Copyright (C) 2016  Bernhard Nortmann <bernhard.nortmann@web.de>
+ *
+ * Based on previous works
+ * Copyright (C) 2016  Siarhei Siamashka <siarhei.siamashka@gmail.com>
+ * Copyright (C) 2012  Henrik Nordstrom <henrik@henriknordstrom.net>
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -15,45 +19,51 @@
  * 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
+ */
 
-Build instructions:
-
-arm-none-linux-gnueabi-gcc  -g  -Os   -fno-common -ffixed-r8 -msoft-float -fno-builtin -ffreestanding -nostdinc -mno-thumb-interwork -Wall -Wstrict-prototypes -fno-stack-protector -Wno-format-nonliteral -Wno-format-security -fno-toplevel-reorder  fel-boot.c -c
-
-arm-none-linux-gnueabi-objcopy -O binary fel-boot.o fel-boot.bin
-
-mksunxiboot fel-boot.bin fel-boot.sunxi
-
-Install instructions:
-
-dd if=fel-boot.sunxi of=/dev/sdX bs=1024 seek=8
-
-*/
-
-
-void _start(void)
-{
-	unsigned int sctlr;
+SCTRL	.req	r0
+.equ	V_BIT, (1 << 13)
 
-	/*
-	 * FEL mode fails to activate in an unpredictable way without
-	 * this NOP padding. Minor changes in the code, such as checking
-	 * the PC register (PC >= 0x10000) instead of SCTLR.V or doing
-	 * jump instead of call to the FEL handler in the BROM sometimes
-	 * break on A64 and sometimes break on A10/A13/A20. Trying to
-	 * add DSB & ISB instructions and/or invalidating caches and
-	 * BTB do not seem to make any difference. Only adding a bunch
-	 * of NOP instructions in the beginning helps.
-	 */
-	asm volatile(".rept 32 \n nop \n .endr");
+.equ	BROM_ENTRY_LOW,  0x00000020
+.equ	BROM_ENTRY_HIGH, 0xFFFF0020
 
-	asm volatile("mrc p15, 0, %0, c1, c0, 0" : "=r" (sctlr));
+/*
+ * 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
 
-	if (sctlr & (1 << 13)) /* SCTLR.V */
-		((void (*)(void))0xffff0020)();
-	else
-		((void (*)(void))0x00000020)();
-}
+_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