[U-Boot] [PATCH 2/6] ARM: add assembly routine to switch to non-secure state

[Christoffer Dall]

On Mon, May 06, 2013 at 03:17:46PM +0200, Andre Przywara wrote:
> While actually switching to non-secure state is one thing, the
> more important part of this process is to make sure that we still
> have full access to the interrupt controller (GIC).
> The GIC is fully aware of secure vs. non-secure state, some
> registers are banked, others may be configured to be accessible from
> secure state only.
> To be as generic as possible, we get the GIC memory mapped address
> based on the PERIPHBASE register. We check explicitly for
> ARM Cortex-A7 and A15 cores, assuming an A9 otherwise, as for those
> cores we know the offsets for the GIC CPU interface from the
> PERIPHBASE content. Other cores could be added as needed.
> 
> With the GIC accessible, we:
> a) allow private interrupts to be delivered to the core
>    (GICD_IGROUPR0 = 0xFFFFFFFF)
> b) enable the CPU interface (GICC_CTLR[0] = 1)
> c) set the priority filter to allow non-secure interrupts
>    (GICC_PMR = 0x80)
> 
> After having switched to non-secure state, we also enable the
> non-secure GIC CPU interface, since this register is banked.
> 
> Also we allow access to all coprocessor interfaces from non-secure
> state by writing the appropriate bits in the NSACR register.
> 
> For reasons obvious later we only use caller saved registers r0-r3.

You probably want to put that in a comment in the code, and it would
also be super helpful to explain the obvious part here, because most
readers don't look "forward in time" to understand this patch...

> 
> Signed-off-by: Andre Przywara <andre.przywara at linaro.org>
> ---
>  arch/arm/cpu/armv7/start.S | 47 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
> 
> diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
> index da48b36..e63e892 100644
> --- a/arch/arm/cpu/armv7/start.S
> +++ b/arch/arm/cpu/armv7/start.S
> @@ -572,3 +572,50 @@ fiq:
>  
>  #endif /* CONFIG_USE_IRQ */
>  #endif /* CONFIG_SPL_BUILD */
> +
> +#ifdef CONFIG_ARMV7_VIRT
> +/* Routine to initialize GIC CPU interface and switch to nonsecure state.
> + */
> +.globl _nonsec_gic_switch
> +_nonsec_gic_switch:
> +	mrc	p15, 4, r2, c15, c0, 0		@ r2 = PERIPHBASE

You should probably check if bits [7:0] == 0x00 here, otherwise you may
end up doing some very strange stuff - if any of those bits are set you
can just error out at this point, but it should be gracefully handled.

Also since it's core specific, you'd probably want to check that before
using it.

> +	add	r3, r2, #0x1000			@ GIC dist i/f offset

Since this is core specific, could the offset please go in an
appropriate header file?  It will also eliminate the need for the
comment if you just have a proper define (i.e. GIC_DIST_OFFSET ...)

> +	mvn	r1, #0
> +	str	r1, [r3, #0x80]			@ allow private interrupts

Aren't you makeing an assumption about the number of available
interrupts here?  You should read the ITLinesNumber field from the
GICD_TYPER if I'm not mistaking.

I also think it would be much cleaner if you again used a define for the
0x80 offset.

Also, don't you need to set some enable fields on the GICD_CTLR here to
enable group 1 interrupts?

> +
> +	mrc	p15, 0, r0, c0, c0, 0		@ MIDR
> +	bfc	r0, #16, #8			@ mask out variant, arch
> +	bfc	r0, #0, #4			@ and revision
> +	movw	r1, #0xc070
> +	movt	r1, #0x4100
> +	cmp	r0, r1				@ check for Cortex-A7
> +	orr	r1, #0xf0

wow, nice bit fiddling.  It may be quite a bit easier to read if you
simply had defines for the bitmasks and real values and just did a load
and placed a literal section accordingly.

> +	cmpne	r0, r1				@ check for Cortex-A15
> +	movne	r1, #0x100			@ GIC CPU offset for A9

So I read the ARMV7_VIRT config flag as something you can only enable on
a board that actually supports the virtulization extensions, which the
A9 doesn't so this should probably error out instead (or do an endless
loop, or ignore the case in the code, ...).

> +	moveq	r1, #0x2000			@ GIC CPU offset for A15/A7

Again, I think defines for these are appropriate.

> +	add	r3, r2, r1			@ r3 = GIC CPU i/f addr
> +
> +	mov	r1, #1
> +	str	r1, [r3, #0]			@ set GICC_CTLR[enable]
> +	mov	r1, #0x80

Why are you not setting this to #0xff ?

> +	str	r1, [r3, #4]			@ set GICC_PIMR[7]
> +

here it seems we're moving into non-gic related stuff in a function
called _nonsec_gic_switch

> +	movw	r1, #0x3fff
> +	movt	r1, #0x0006
> +	mcr	p15, 0, r1, c1, c1, 2		@ NSACR = all copros to non-sec
> +
> +	ldr	r1, =_start
> +	mcr	p15, 0, r1, c12, c0, 0		@ set VBAR
> +	mcr	p15, 0, r1, c12, c0, 1		@ set MVBAR

I thought you already took care of the MVBAR during init?
> +
> +	isb
> +	smc	#0				@ call into MONITOR mode
> +	isb					@ clobbers r0 and r1

This isb shouldn't be necessary if you just did an exception return?

> +
> +	mov	r1, #1
> +	str	r1, [r3, #0]			@ set GICC_CTLR[enable]

you're doing more than setting the enable bit: you're setting the EOImodeNS,
IRQBypDisGrp1, and FIQBypDisGrp1, as you should.

> +	add	r2, r2, #0x1000			@ GIC dist i/f offset
> +	str	r1, [r2]			@ allow private interrupts

It seems a bit brittle to rely on the smc handler to not clobber r2 and
r3, and it may an annoying thing to track down.  You could just
re-generate the the gic base address from the cp15 register.  Your
choice.

> +
> +	mov	pc, lr
> +#endif /* CONFIG_ARMV7_VIRT */
> -- 
> 1.7.12.1
>