[PATCH] ARM: Enter CPU in ARM state for cpu_resume

[Ard Biesheuvel]

On 1 June 2015 at 23:45, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Mon, Jun 01, 2015 at 01:22:00PM -0700, Stephen Boyd wrote:
>> The standard boot protocol on ARM requires CPUs to be entered in
>> the ARM state, unless they don't support the ARM instruction set
>> (see Documentation/arm/Booting). On THUMB2 kernels, we assume
>> the firmware can determine what state to enter the kernel in, but
>> some firmwares don't honor the thumb bit. Make the cpu_resume
>> symbol an ARM symbol, so that firmwares that honor the thumb bit
>> will enter the kernel in ARM state and firmwares that don't honor
>> the thumb bit will be able to enter the kernel in ARM state
>> without more changes.
>>
>> This fixes a problem reported by Kevin Hilman where the ifc6410
>> fails to boot for THUMB2 kernels because the platform's firmware
>> always enters the kernel in ARM mode from deep idle states.
>
> Please do this differently.  The default should be (as we do with
> the SMP secondary entry path) to assume that the firmware does the
> right thing.
>
> So, if we want an ARM-mode entry point, please use:
>
> +               .arm
> +ENTRY(cpu_resume_arm)
> + THUMB(        badr    r9, 1f          )       @ Kernel is entered in ARM.
> + THUMB(        bx      r9              )       @ If this is a Thumb-2 kernel,
> + THUMB(        .thumb                  )       @ switch to Thumb now.
> + THUMB(1:                      )
>
> Don't forget an ENDPROC() for the new symbol.  Buggy platforms then
> use cpu_resume_arm instead of cpu_resume.
>

OK, I think that was Stephen intention at first, but I suggested this instead.

The point is that it is safer and more tidy to make these entry points
ARM only throughout, and switch to Thumb2 only if THUMB2_KERNEL. This
way, since all firmwares (except ARMv7-M, but let's disregard that for
now) are known to be able to enter/resume into the kernel in ARM mode,
this is more robust in the face of new platforms and firmware
revisions of existing platforms.