hw/char/serial-isa.c: declare IRQ as shared in ACPI IRQ descriptor

From Windows 8.1 onwards ISA serial IRQs cannot be shared when ACPI Revision
3.0 is used in the FACP table. The reason for this is that if a 2-byte IRQ
Descriptor is used then the interrupt is considered to be high true, edge
sensitive, non-shareable. Since legacy serial ports COM1/3 and COM2/4 share
an IRQ then if more than 2 serial ports are added, Windows indicates a
conflict in Device Manager and these combinations cannot be used together.

Change the 2-byte IRQ Descriptor in the _CRS resource to a 3-byte IRQ
Descriptor indicating that the ISA serial IRQ is low true, level sensitive and
shareable. This enables all 4 legacy serial ports to be used in Windows without
conflict.

Note that it was agreed during the list discussion that this change does not
require a compatibility property since it is not part of the default machine
configuration and was already broken.

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260515150634.2637533-4-mark.caveayland@nutanix.com>

diff --git a/hw/char/serial-isa.c b/hw/char/serial-isa.c
index a4be0492c598914aa5d3b407fe9670399f1bf419..eaa4e843c028b94248cb4a2a65268dbbd5dbcbe2 100644 (file)
--- a/hw/char/serial-isa.c
+++ b/hw/char/serial-isa.c
@@ -92,7 +92,8 @@ static void serial_isa_build_aml(AcpiDevAmlIf *adev, Aml *scope)
 
     crs = aml_resource_template();
     aml_append(crs, aml_io(AML_DECODE16, isa->iobase, isa->iobase, 0x00, 0x08));
-    aml_append(crs, aml_irq_no_flags(isa->isairq));
+    aml_append(crs, aml_irq(isa->isairq, AML_LEVEL, AML_ACTIVE_LOW,
+                            AML_SHARED));
 
     dev = aml_device("COM%d", isa->index + 1);
     aml_append(dev, aml_name_decl("_HID", aml_eisaid("PNP0501")));