Re: [PATCH 2/2] ARM: mmu_32: fix setting up zero page when it is in SDRAM

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Sascha,

On 31.05.23 12:01, Sascha Hauer wrote:
> We used to skip setting the zero page to faulting when SDRAM starts
> at 0x0. As bootm code now explicitly sets the zero page accessible
> before copying ATAGs there this should no longer be necessary, so
> unconditionally set the zero page to faulting during MMU startup.
> This also moves the zero page setup after the point the SDRAM has
> been mapped cachable, because otherwise the zero page setup would
> be overwritten.
> 
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> ---
>  arch/arm/cpu/mmu_32.c | 21 ++-------------------
>  1 file changed, 2 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/arm/cpu/mmu_32.c b/arch/arm/cpu/mmu_32.c
> index c4e5a3bb0a..faebba42d0 100644
> --- a/arch/arm/cpu/mmu_32.c
> +++ b/arch/arm/cpu/mmu_32.c
> @@ -459,23 +459,6 @@ static int set_vector_table(unsigned long adr)
>  	return -EINVAL;
>  }
>  
> -static void create_zero_page(void)
> -{
> -	struct resource *zero_sdram;
> -
> -	zero_sdram = request_sdram_region("zero page", 0x0, PAGE_SIZE);

I think this should remain to avoid some $platform overwriting ATAGS
or w/e with the kernel image. I know this is a discrepancy to ARM64,
but let's play it safe?

> -	if (zero_sdram) {
> -		/*
> -		 * Here we would need to set the second level page table
> -		 * entry to faulting. This is not yet implemented.
> -		 */
> -		pr_debug("zero page is in SDRAM area, currently not supported\n");
> -	} else {
> -		zero_page_faulting();
> -		pr_debug("Created zero page\n");
> -	}
> -}
> -
>  /*
>   * Map vectors and zero page
>   */
> @@ -487,7 +470,6 @@ static void vectors_init(void)
>  	 */
>  	if (!set_vector_table((unsigned long)__exceptions_start)) {
>  		arm_fixup_vectors();
> -		create_zero_page();
>  		return;
>  	}
>  
> @@ -495,7 +477,6 @@ static void vectors_init(void)
>  	 * Next try high vectors at 0xffff0000.
>  	 */
>  	if (!set_vector_table(ARM_HIGH_VECTORS)) {
> -		create_zero_page();
>  		create_vector_table(ARM_HIGH_VECTORS);
>  		return;
>  	}
> @@ -552,6 +533,8 @@ void __mmu_init(bool mmu_on)
>  
>  		remap_range((void *)pos, bank->start + bank->size - pos, MAP_CACHED);
>  	}
> +
> +	zero_page_faulting();
>  }
>  
>  /*

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