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

On 1 June 2015 at 23:45, Russell King - ARM Linux
[off-list ref] wrote:

On Mon, Jun 01, 2015 at 01:22:00PM -0700, Stephen Boyd wrote:

quoted

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.