Skip reading bits from a region marked as "Preserve" if all bits will be

modified.  Some Sony and Asus laptops don't like this; the read seems to
trigger an unwanted SMI that makes the machine hang.  Diagnosed by mpi@

ok mpi@

diff --git a/sys/dev/acpi/dsdt.c b/sys/dev/acpi/dsdt.c
index 89179f9..20dd8af 100644 (file)
--- a/sys/dev/acpi/dsdt.c
+++ b/sys/dev/acpi/dsdt.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: dsdt.c,v 1.212 2014/07/12 18:48:17 tedu Exp $ */
+/* $OpenBSD: dsdt.c,v 1.213 2014/07/20 12:20:38 kettenis Exp $ */
 /*
  * Copyright (c) 2005 Jordan Hargrave <jordan@openbsd.org>
  *
@@ -2311,7 +2311,8 @@ aml_rwgas(struct aml_value *rgn, int bpos, int blen, struct aml_value *val,
                aml_bufcpy(vbit, 0, tbit, bpos, blen);
        } else {
                /* Write bits to opregion */
-               if (AML_FIELD_UPDATE(flag) == AML_FIELD_PRESERVE) {
+               if (AML_FIELD_UPDATE(flag) == AML_FIELD_PRESERVE &&
+                   (bpos != 0 || blen != tlen)) {
                        acpi_gasio(acpi_softc, ACPI_IOREAD, type, pi.addr,
                            sz, tlen >> 3, tbit);
                } else if (AML_FIELD_UPDATE(flag) == AML_FIELD_WRITEASONES) {