Subject: [PATCH] ARM: avoid mis-detecting some V7 cores in the decompressor

From: [email-masked] (Stephen Boyd)
Date: Tue, 4 Jun 2013 14:45:01 -0700

On 06/04, Nicolas Pitre wrote:
> On Tue, 4 Jun 2013, Stephen Boyd wrote:
> 
> > On 06/04, Nicolas Pitre wrote:
> > > 
> > > I've looked at the code and I think that #1 in your initial options is 
> > > probably best here.  I agree with Russell about #2 being way too complex 
> > > for only this case.
> > > 
> > > So, right before calling into cache_on, you could test if r4 - 16K >= pc 
> > > and r4 < pc + (_end - .) then skip cache_on.
> > > 
> > > Something like this untested patch:
> > 
> > So this would cause the decompression to run without the cache on
> > if we have to relocate the decompression code to avoid
> > overwriting ourselves?
> 
> No.  What my example patch does is to simply skip setting up a page 
> table and turning on the cache if the page table ends up in the 
> code/data to be relocated.
> 
> > It seems that the memcpy is fairly quick on my hardware in comparison 
> > to the decompression so moving the cache_on() call to right before we 
> > run decompression keeps things pretty fast. It's very possible 
> > different hardware will have different results.
> 
> "Fairly quick" is still not optimal.
> 
> > This is what I meant by option #1. I suppose
> > we can make it smarter and conditionalize it on if we relocated
> > or not?
> 
> Here's what I,m suggesting:

Yes this looks closer to what I'm saying.

> 
> From f1ec55e8189c06b89d2f196929f01fe3fc40f908 Mon Sep 17 00:00:00 2001
> From: Nicolas Pitre <[email-masked]>
> Date: Tue, 4 Jun 2013 17:01:30 -0400
> Subject: [PATCH] ARM: zImage: don't overwrite ourself with a page table
> 
> When zImage is loaded into RAM at a low address but TEXT_OFFSET
> is set higher, we risk overwriting ourself with the page table
> needed to turn on the cache as it is located relative to the relocation
> address.  Let's defer the cache setup after relocation in that case.
> 
> Signed-off-by: Nicolas Pitre <[email-masked]>
> 
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index 9a94f344df..773bc35f92 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -178,11 +178,23 @@ not_angel:
>  		mov	r4, pc
>  		and	r4, r4, #0xf8000000
>  		add	r4, r4, #TEXT_OFFSET
> +		bl	cache_on
>  #else
>  		ldr	r4, =zreladdr
> -#endif
>  
> -		bl	cache_on
> +		/*
> +		 * Set up a page table only if we don't overwrite ourself.
> +		 * That means r4 < pc && r4 - 4K > &_end.
> +		 * Given that r4 > &_en is most unfrequent, we add a rough
> +		 * additional 1MB of room for a possible appended DTB.
> +		 */
> +		mov	r0, pc
> +		cmp	r0, r4
> +		ldrcc	r0, LC0+32
> +		addcc	r0, r0, pc
> +		cmpcc	r4, r0
> +		blcs	cache_on
> +#endif

But this looks backwards? Shouldn't we put it in the
CONFIG_AUTO_ZRELADDR case?

>  
>  restart:	adr	r0, LC0
>  		ldmia	r0, {r1, r2, r3, r6, r10, r11, r12}
> @@ -464,6 +476,16 @@ not_relocated:	mov	r0, #0
>  		cmp	r2, r3
>  		blo	1b
>  
> +#if defined(CONFIG_AUTO_ZRELADDR) && defined(CONFIG_CPU_CP15)
> +		/*
> +		 * Did we skip the cache setup earlier?
> +		 * Do it now if so.
> +		 */
> +		mrc     p15, 0, r0, c1, c0, 0	@ read control register
> +		tst	r0, #1			@ MMU bit set?
> +		bleq	cache_on		@ no: set it up
> +#endif

Too bad we can't store one more variable into LC0 that says we
turned the caches on. Then we could read that here and detect it
without CP15 accessors required.

> +
>  /*
>   * The C runtime environment should now be setup sufficiently.
>   * Set up some pointers, and start decompressing.
> @@ -512,6 +534,7 @@ LC0:		.word	LC0			@ r1
>  		.word	_got_start		@ r11
>  		.word	_got_end		@ ip
>  		.word	.L_user_stack_end	@ sp
> +		.word	_end - restart + 16384 + 1024*1024
>  		.size	LC0, . - LC0
>  
>  #ifdef CONFIG_ARCH_RPC

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