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

[Matthias Fuchs]

On 03.02.2012 14:22, Marek Vasut wrote:
>> 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
> 
> 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?

First: my commit message was not very clear in ths concern.
When the SPL is called from the bootrom FIQ is turned off. When the SPL
has run and control is passed back to the bootrom (!), the bootrom
crashes, stalls or stops when FIQ is on. So we need to turn FIQ
off when the SPL has done it's job. So that's why I added the code after
board_init_ll(). I also noticed that the SPL runs fine when FIQ is never
turned on. So I could think of never turning it on at all in SPL:
170         /*
171          * set the cpu to SVC32 mode
172          */
173         mrs     r0,cpsr
174         bic     r0,r0,#0x1f
- 175         orr     r0,r0,#0xd3
+ 175         orr     r0,r0,#0x53
176         msr     cpsr,r0

This will result in CPSR = 0x53 which is the register content when
the SPL is called (setup by bootrom). So we could completely get rid of
this sequence.

And yes, for USB boot the FIQ must be turned off when SPL has done it's
work. Other boot modes do not care as far as I noticed.

> 
>> +
>> +#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).
Right. That's why the SPL code clears the 'V' bit and we got 'low
vectors'. But when the SPL code has done it's work we need to return to
'high vectors' (... in bootrom space) to keep USB from bootrom alive.
That's what my patch does. SPL run with low vectors and before it
returns to bootrom it switches back to high 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?)
Yes, by checking some GPIOs (I send an RFC/PATCH some days ago). But
there's no need to only do this when using USB boot.

> and enable high-
> vectors just before passing control back to bootrom only then?
That's what I do! Please take a look at my patch.
> 
> Though now that I think of it, high-vectors should probably be unconditionally 
> re-enabled upon entering bootrom. Can you investigate?
That's what my patch does!


Matthias