[PATCH 61/62] ARM: sunxi: fix build for THUMB2_KERNEL

[Rob Herring]

On Thu, Mar 20, 2014 at 5:59 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Wednesday 19 March 2014, Rob Herring wrote:
>
>> > --- a/arch/arm/mach-sunxi/headsmp.S
>> > +++ b/arch/arm/mach-sunxi/headsmp.S
>> > @@ -4,6 +4,7 @@
>> >          .section ".text.head", "ax"
>> >
>> >  ENTRY(sun6i_secondary_startup)
>> > -       msr     cpsr_fsxc, #0xd3
>> > +       mov     r0, #0xd3
>> > +       msr     cpsr_fsxc, r0
>> >         b       secondary_startup
>>
>> Secondary cores should always enter the kernel in ARM mode like the
>> primary core, right? So we need the same switching to Thumb2 as
>> head.S, but even secondary_startup doesn't do any switching. So either
>> platforms jump into the kernel with a bx and happen to work or
>> secondary boot is broken for Thumb2 kernels.
>
> Makes sense. So should we instead do this?

That would work, but I'm more worried that Thumb2 may be broken on
other platforms.

>
> .arm
> ENTRY(sun6i_secondary_startup)
>         ARM_BE8(setend   be ) /* I suppose we need this too */
>         msr     cpsr_fsxc, #0xd3
>         ldr     r0, =secondary_startup
>         bx      r0
>
> (I'm a bit confused about when to use what branch instruction, so
> forgive me if the above doesn't make sense)
>
>> Also, secondary_startup takes care of making sure the core is in SVC
>> mode, so this function shouldn't be needed in the first place.
>
> I'd rather not change this part, unless Maxime can confirm that it's
> not necessary. I would assume that it's there for a reason otherwise.

That often proves to not be the case. :) Initial SOC code is whatever
we've failed to delete from vendor BSP code.

Looking at the review comments adding SMP boot, there was discussion
about entering in HYP mode and that being dependent on u-boot support,
but nothing whether this was needed or not. So it appears to me we
probably enter the kernel in the reset default of secure supervisor
mode and this code is a nop. This code would not work if it was HYP
mode. So when u-boot does correctly setup HYP mode, this code will be
broken (although other parts of smp_ops will be too).

The first things secondary_startup do are:

-set BE mode
-if in HYP mode, install hyp vectors and drop to SVC
-set the CPSR to a know value (safe_svcmode_maskall r9)

So this code is completely redundant and unnecessary. If there were
some reason for it, then it should be in secondary_startup.

Rob