fel-sdboot: Fix header corruption workaround

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.

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

fel-sdboot.c

@@ -39,16 +39,21 @@ void _start(void)
 	unsigned int sctlr;
 
 	/*
-	 * 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.
+	 * In cases where insufficient padding is added by an old mksunxiboot,
+	 * the _start entry point 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 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.
 	 */
-	asm volatile(".rept 32 \n nop \n .endr");
+	asm volatile("b _main \n"
+		"nop \n"
+		"nop \n"
+		"nop \n"
+		"_main:");
 
 	asm volatile("mrc p15, 0, %0, c1, c0, 0" : "=r" (sctlr));