[U-Boot] [PATCH] mx28: fix SPL code to make USB booting work

[Matthias Fuchs]

This patch fixes booting i.MX28 CPUs via USB download.
In this mode the CPU's bootrom implements a USB HID device that
accepts a bootstream.

When downloading the bootstream via USB, first the SPL code is
received and executed. Then the u-boot image is received and
called.

The USB bootmode is interrupt driven.

This patch fixes two things:

1) The ARM's fast interrupt mode is disabled when the SPL code
has been run. This is the default state when called by the bootrom.

2) The exception vector location is set back to bootrom space to
make the USB interrupts work again. The SPL code needs to change this
option for the ram size probing.

Signed-off-by: Matthias Fuchs <matthias.fuchs at esd.eu>
---
 arch/arm/cpu/arm926ejs/mx28/start.S |   17 +++++++++++++++++
 1 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/arch/arm/cpu/arm926ejs/mx28/start.S b/arch/arm/cpu/arm926ejs/mx28/start.S
index 2cd4d73..4116bb1 100644
--- a/arch/arm/cpu/arm926ejs/mx28/start.S
+++ b/arch/arm/cpu/arm926ejs/mx28/start.S
@@ -185,6 +185,23 @@ _reset:
 
 	bl	board_init_ll
 
+	/*
+	 * turn of fast interrupt mode (required by bootrom for USB boot)
+	 */
+	mrs	r0,cpsr
+	bic	r0,r0,#0x80
+	msr	cpsr,r0
+
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT
+	/*
+	 * set exception vector location back to bootrom space.
+	 * (required by bootrom for USB boot)
+	 */
+	mrc	p15, 0, r0, c1, c0, 0
+	orr	r0, r0, #0x00002000	/* set bit 13 'V' */
+	mcr	p15, 0, r0, c1, c0, 0
+#endif
+
 	pop	{r0-r12,r14}
 	bx	lr
 
-- 
1.6.1