[PATCH] armv8: cpu: Implement allow_unaligned()

[Heinrich Schuchardt]

On 10/31/24 23:06, Sam Protsenko wrote:
> Usually ARMv8 platforms allow unaligned access for Normal memory. But
> some chips might not allow it by default, having SCTLR.A bit set to 1
> before U-Boot execution. One such example is Exynos850 SoC. As
> allow_unaligned() is not implemented for ARMv8 at the moment, its __weak
> implementation is used, which does nothing. That might lead to unaligned
> access abort, for example when running EFI selftest. Fix that by
> implementing allow_unaligned() for ARMv8.
>
> The issue was found when running EFI selftest on E850-96 board
> (Exynos850 based):
>
>      => bootefi selftest $fdtcontroladdr
>
>      ...
>      Executing 'HII database protocols'
>      "Synchronous Abort" handler, esr 0x96000021, far 0xbaac0991
>      ...
>      resetting ...
>
> Unaligned abort happens in u16_strnlen(), which is called from
> efi_hii_sibt_string_ucs2_block_next():
>
>      u16_strlen(blk->string_text)
>
> where 'blk' type is struct efi_hii_sibt_string_ucs2_block. Because this
> struct is packed, doing "->string_text" makes 'blk' address incremented
> by 1 byte, which makes it unaligned. Although allow_unaligned() was
> called in efi_init_early() before EFI selftest execution, it wasn't
> implemented for ARMv8 CPUs, so data abort happened.
>
> Signed-off-by: Sam Protsenko <semen.protsenko at linaro.org>
> ---
>   arch/arm/cpu/armv8/cpu.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/arch/arm/cpu/armv8/cpu.c b/arch/arm/cpu/armv8/cpu.c
> index d568efa427ab..82ecf02f4b03 100644
> --- a/arch/arm/cpu/armv8/cpu.c
> +++ b/arch/arm/cpu/armv8/cpu.c
> @@ -94,3 +94,8 @@ void armv8_setup_psci(void)
>   	secure_ram_addr(psci_arch_init)();
>   }
>   #endif
> +
> +void allow_unaligned(void)
> +{
> +	set_sctlr(get_sctlr() & ~CR_A);

Looking at the ARM documentation SCTLR_EL2 is a 64bit register.

https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/SCTLR-EL2--System-Control-Register--EL2-?lang=en

But get_sctlr() is defined as

arch/arm/include/asm/system.h:174:
static inline unsigned int get_sctlr(void)

Your code might not work as expected. Should we adjust the definition of
get_sctlr() to return a long value?

Best regards

Heinrich

> +}