[RFC] Fix omap3 booting with thumb2 compiled kernel

[Tony Lindgren]

* Dave Martin <Dave.Martin at arm.com> [150528 09:19]:
> On Thu, May 28, 2015 at 07:21:25AM -0700, Tony Lindgren wrote:
> > --- a/arch/arm/mach-omap2/sleep34xx.S
> > +++ b/arch/arm/mach-omap2/sleep34xx.S
> > @@ -203,23 +203,8 @@ save_context_wfi:
> >  	 */
> >  	ldr	r1, kernel_flush
> >  	blx	r1
> > -	/*
> > -	 * The kernel doesn't interwork: v7_flush_dcache_all in particluar will
> > -	 * always return in Thumb state when CONFIG_THUMB2_KERNEL is enabled.
> > -	 * This sequence switches back to ARM.  Note that .align may insert a
> > -	 * nop: bx pc needs to be word-aligned in order to work.
> > -	 */
> > - THUMB(	.thumb		)
> > - THUMB(	.align		)
> > - THUMB(	bx	pc	)
> > - THUMB(	nop		)
> > -	.arm
> > -
> 
> ^ This looks unrelated to the issue?  This code is simply unnecessary
> after Russell introduced the "ret" macro in commit 6ebbf2ce43.  That
> made v7_flush_dcache_all do a proper interworking return.
> 
> It should probably be in a separate commit, or at least mentioned
> explicitly in the commit log.

Thanks, I've updated the description with the commit info above.

I'll keep the removal of the duplicate code in this patch, as it's
all related to the mode switching and we don't want to do it twice.
 
> >  	b	omap3_do_wfi
> > -
> > -/*
> > - * Local variables
> > - */
> 
> ^ Likewise this is just tidyup, not bug fixing.

Will keep that too, the local variables comments are now just too
confusing while reading the code after adding the ENDPROC. And that
too is just removal.
 
> > +ENDPROC(omap34xx_cpu_suspend)
> >  omap3_do_wfi_sram_addr:
> >  	.word omap3_do_wfi_sram
> >  kernel_flush:
> > @@ -364,10 +349,7 @@ exit_nonoff_modes:
> >   * ===================================
> >   */
> >  	ldmfd	sp!, {r4 - r11, pc}	@ restore regs and return
> > -
> > -/*
> > - * Local variables
> > - */
> > +ENDPROC(omap3_do_wfi)
> 
> The ENDPROC()s look appropriate here.

OK thanks for looking, I was hoping you'd check it as you did the
original thumb2 changes for this file :) Updated patch below.

Regards,

Tony

8< ------------------------
From: Tony Lindgren <tony at atomide.com>
Date: Thu, 28 May 2015 07:22:08 -0700
Subject: [PATCH] ARM: OMAP3: Fix booting with thumb2 kernel

We get a NULL pointer dereference on omap3 for thumb2 compiled kernels:

Internal error: Oops: 80000005 [#1] SMP THUMB2
...
[<c046497b>] (_raw_spin_unlock_irqrestore) from [<c0024375>]
(omap3_enter_idle_bm+0xc5/0x178)
[<c0024375>] (omap3_enter_idle_bm) from [<c0374e63>]
(cpuidle_enter_state+0x77/0x27c)
[<c0374e63>] (cpuidle_enter_state) from [<c00627f1>]
(cpu_startup_entry+0x155/0x23c)
[<c00627f1>] (cpu_startup_entry) from [<c06b9a47>]
(start_kernel+0x32f/0x338)
[<c06b9a47>] (start_kernel) from [<8000807f>] (0x8000807f)

The power management related assembly on omaps needs to interact with
ARM mode bootrom code, so we need to keep most of the related assembly
in ARM mode.

Turns out this error is because of missing ENDPROC for assembly code
as suggested by Stephen Boyd <sboyd at codeaurora.org>. Let's fix the
problem by adding ENDPROC in two places to sleep34xx.S.

Let's also remove the now duplicate custom code for mode switching.
This has been unnecessary since commit 6ebbf2ce437b ("ARM: convert
all "mov.* pc, reg" to "bx reg" for ARMv6+").

And let's also remove the comments about local variables, they are
now just confusing after the ENDPROC.

The reason why ENDPROC makes a difference is it sets .type and then
the compiler knows what to do with the thumb bit as explained at:

https://wiki.ubuntu.com/ARM/Thumb2PortingHowto

Reported-by: Kevin Hilman <khilman at kernel.org>
Tested-by: Kevin Hilman <khilman at linaro.org>
Signed-off-by: Tony Lindgren <tony at atomide.com>

--- a/arch/arm/mach-omap2/sleep34xx.S
+++ b/arch/arm/mach-omap2/sleep34xx.S
@@ -203,23 +203,8 @@ save_context_wfi:
 	 */
 	ldr	r1, kernel_flush
 	blx	r1
-	/*
-	 * The kernel doesn't interwork: v7_flush_dcache_all in particluar will
-	 * always return in Thumb state when CONFIG_THUMB2_KERNEL is enabled.
-	 * This sequence switches back to ARM.  Note that .align may insert a
-	 * nop: bx pc needs to be word-aligned in order to work.
-	 */
- THUMB(	.thumb		)
- THUMB(	.align		)
- THUMB(	bx	pc	)
- THUMB(	nop		)
-	.arm
-
 	b	omap3_do_wfi
-
-/*
- * Local variables
- */
+ENDPROC(omap34xx_cpu_suspend)
 omap3_do_wfi_sram_addr:
 	.word omap3_do_wfi_sram
 kernel_flush:
@@ -364,10 +349,7 @@ exit_nonoff_modes:
  * ===================================
  */
 	ldmfd	sp!, {r4 - r11, pc}	@ restore regs and return
-
-/*
- * Local variables
- */
+ENDPROC(omap3_do_wfi)
 sdrc_power:
 	.word	SDRC_POWER_V
 cm_idlest1_core: