[PATCH] ARM: head.S: explicitly map DT even if it lives in the first physical section

[Guillaume Tucker]

On 16/11/2020 14:56, Ard Biesheuvel wrote:
> The early ATAGS/DT mapping code uses SECTION_SHIFT to mask low order
> bits of R2, and decides that no ATAGS/DTB were provided if the resulting
> value is 0x0.
> 
> This means that on systems where DRAM starts at 0x0 (such as Raspberry
> Pi), no explicit mapping of the DT will be created if R2 points into the
> first 1 MB section of memory. This was not a problem before, because the
> decompressed kernel is loaded at the base of DRAM and mapped using
> sections as well, and so as long as the DT is referenced via a virtual
> address that uses the same translation (the linear map, in this case),
> things work fine.
> 
> However, commit 149a3ffe62b9dbc3 ("9012/1: move device tree mapping out of
> linear region") changes this, and now the DT is referenced via a virtual
> address that is disjoint from the linear mapping of DRAM, and so we need
> the early code to create the DT mapping unconditionally.
> 
> So let's create the early DT mapping for any value of R2 != 0x0.
> 
> Fixes: 149a3ffe62b9dbc3 ("9012/1: move device tree mapping out of linear region")
> Reported-by: Guillaume Tucker <guillaume.tucker at collabora.com>

This was initially found by a kernelci.org automated bisection,
so please also include this trailer if it's OK with you:

  Reported-by: "kernelci.org bot" <bot at kernelci.org>

As explained on the other thread, this fixes the early kernel
boot issue but then it fails to load the ramdisk.  I'll try to
see if there's any issue on the bootloader side.

Thanks,
Guillaume

> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm/kernel/head.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 28687fd1240a..7f62c5eccdf3 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -265,10 +265,10 @@ __create_page_tables:
>  	 * We map 2 sections in case the ATAGs/DTB crosses a section boundary.
>  	 */
>  	mov	r0, r2, lsr #SECTION_SHIFT
> -	movs	r0, r0, lsl #SECTION_SHIFT
> +	cmp	r2, #0
>  	ldrne	r3, =FDT_FIXED_BASE >> (SECTION_SHIFT - PMD_ORDER)
>  	addne	r3, r3, r4
> -	orrne	r6, r7, r0
> +	orrne	r6, r7, r0, lsl #SECTION_SHIFT
>  	strne	r6, [r3], #1 << PMD_ORDER
>  	addne	r6, r6, #1 << SECTION_SHIFT
>  	strne	r6, [r3]
>