Re: [PATCH] x86: ignore unspecified bit positions in the ACPI global lock field

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Thursday, March 5, 2020 1:24:25 PM CET Jan Engelhardt wrote:
> The value in "new" is constructed from "old" such that all bits defined
> as reserved by the ACPI spec[1] are left untouched. But if those bits
> do not happen to be all zero, "new < 3" will not evaluate to true.
> 
> The firmware of the laptop(s) Medion MD63490 / Akoya P15648 comes with
> garbage inside the "FACS" ACPI table. The starting value is
> old=0x4944454d, therefore new=0x4944454e, which is >= 3. Mask off
> the reserved bits.
> 
> [1] https://uefi.org/sites/default/files/resources/ACPI_6_2.pdf
> 
> References: https://bugzilla.kernel.org/show_bug.cgi?id=206553
> Signed-off-by: Jan Engelhardt <jengelh@inai.de>
> ---
>  arch/x86/kernel/acpi/boot.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 04205ce127a1..f9e84a0e2fa2 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -1740,7 +1740,7 @@ int __acpi_acquire_global_lock(unsigned int *lock)
>  		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
>  		val = cmpxchg(lock, old, new);
>  	} while (unlikely (val != old));
> -	return (new < 3) ? -1 : 0;
> +	return ((new & 0x3) < 3) ? -1 : 0;
>  }
>  
>  int __acpi_release_global_lock(unsigned int *lock)
> 

Applied as 5.7 material, thanks!