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    Subject[PATCH 5.4 092/232] acpi/x86: ignore unspecified bit positions in the ACPI global lock field
    Date
    From: Jan Engelhardt <jengelh@inai.de>

    commit ecb9c790999fd6c5af0f44783bd0217f0b89ec2b upstream.

    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

    Link: https://bugzilla.kernel.org/show_bug.cgi?id=206553
    Cc: All applicable <stable@vger.kernel.org>
    Signed-off-by: Jan Engelhardt <jengelh@inai.de>
    Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kernel/acpi/boot.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/arch/x86/kernel/acpi/boot.c
    +++ b/arch/x86/kernel/acpi/boot.c
    @@ -1740,7 +1740,7 @@ int __acpi_acquire_global_lock(unsigned
    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)

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