From ecb9c790999fd6c5af0f44783bd0217f0b89ec2b Mon Sep 17 00:00:00 2001
From: Jan Engelhardt <jengelh@inai.de>
Date: Thu, 5 Mar 2020 13:24:25 +0100
Subject: [PATCH] acpi/x86: ignore unspecified bit positions in the ACPI global lock field
Git-commit: ecb9c790999fd6c5af0f44783bd0217f0b89ec2b
Patch-mainline: v5.7-rc1
References: bsc#1051510

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>
Acked-by: Takashi Iwai <tiwai@suse.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 d1757ceca8ae..1ae5439a9a85 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -1748,7 +1748,7 @@ 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)
-- 
2.16.4