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

[Marek Vasut <marek.vasut@gmail.com>]

> 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@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

Add this section just past _reset into:

170         /*
171          * set the cpu to SVC32 mode
172          */
173         mrs     r0,cpsr
174         bic     r0,r0,#0x1f
175         orr     r0,r0,#0xd3
176         msr     cpsr,r0

And only if you really need this. Why do you need to disable FIQ?

> +
> +#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

High-vectors break the current implementation. That IS WRONG. The RAM memory 
detection routine will not work if you enable high vectors since it depends on 
adjusting the jumptable at 0x0 (aka. low vectors).

Why do you need to enable high vectors? Can't you detect that USB boot is 
happening (can mx28 report boot reason like mxc chips do?) and enable high-
vectors just before passing control back to bootrom only then?

Though now that I think of it, high-vectors should probably be unconditionally 
re-enabled upon entering bootrom. Can you investigate?

> +
>  	pop	{r0-r12,r14}
>  	bx	lr