ARM: mxs: Setup stack in JTAG mode

In case the MX23/MX28 is switched into JTAG mode via the BootMode select
switches, the BootROM bypasses the CPU core registers initialization.
This in turn means that the Stack Pointer (SP) register is not set as
it is in every other mode of operation, but instead is only zeroed out.

To prevent U-Boot SPL from crashing in this obscure JTAG mode, configure
the SP to point at the CONFIG_SYS_INIT_SP_ADDR if the SP is zeroed out.

Note that in case the SP is already configured, we must preserve that exact
SP value and must not modify it. This is important since in every other mode
but the JTAG mode, the SPL returns into the BootROM and BootROM in turn loads
U-Boot itself. If the SP were to be corrupted, the BootROM won't be able to
continue it's operation after returned from SPL and the system would crash.

Finally, add the JTAG mode switch identifier, so it's not recognised as
Unknown mode.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>

arch/arm/cpu/arm926ejs/mxs/start.S

@@ -150,6 +150,15 @@ IRQ_STACK_START_IN:
 
 _reset:
 	/*
+	 * If the CPU is configured in "Wait JTAG connection mode", the stack
+	 * pointer is not configured and is zero. This will cause crash when
+	 * trying to push data onto stack right below here. Load the SP and make
+	 * it point to the end of OCRAM if the SP is zero.
+	 */
+	cmp	sp, #0x00000000
+	ldreq	sp, =CONFIG_SYS_INIT_SP_ADDR
+
+	/*
 	 * Store all registers on old stack pointer, this will allow us later to
 	 * return to the BootROM and let the BootROM load U-Boot into RAM.
 	 *

arch/arm/include/asm/arch-mxs/sys_proto.h

@@ -46,6 +46,7 @@ static const struct mxs_pair mxs_boot_modes[] = {
 	{ 0x02, 0x1f, "SSP SPI #1, master, NOR" },
 	{ 0x03, 0x1f, "SSP SPI #2, master, NOR" },
 	{ 0x04, 0x1f, "NAND" },
+	{ 0x06, 0x1f, "JTAG" },
 	{ 0x08, 0x1f, "SSP SPI #3, master, EEPROM" },
 	{ 0x09, 0x1f, "SSP SD/MMC #0" },
 	{ 0x0a, 0x1f, "SSP SD/MMC #1" },
@@ -60,6 +61,7 @@ static const struct mxs_pair mxs_boot_modes[] = {
 	{ 0x13, 0x1f, "SSP SPI #3, master, 1V8 NOR" },
 	{ 0x04, 0x1f, "NAND, 3V3" },
 	{ 0x14, 0x1f, "NAND, 1V8" },
+	{ 0x06, 0x1f, "JTAG" },
 	{ 0x08, 0x1f, "SSP SPI #3, master, 3V3 EEPROM" },
 	{ 0x18, 0x1f, "SSP SPI #3, master, 1V8 EEPROM" },
 	{ 0x09, 0x1f, "SSP SD/MMC #0, 3V3" },