docs/specs: Add ACPI GED documentation

Documents basic concepts of ACPI Generic Event device(GED)
and interface between QEMU and the ACPI BIOS.

Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20190918130633.4872-10-shameerali.kolothum.thodi@huawei.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/acpi_hw_reduced_hotplug.rst

@@ -0,0 +1,70 @@
+==================================================
+QEMU and ACPI BIOS Generic Event Device interface
+==================================================
+
+The ACPI *Generic Event Device* (GED) is a HW reduced platform
+specific device introduced in ACPI v6.1 that handles all platform
+events, including the hotplug ones. GED is modelled as a device
+in the namespace with a _HID defined to be ACPI0013. This document
+describes the interface between QEMU and the ACPI BIOS.
+
+GED allows HW reduced platforms to handle interrupts in ACPI ASL
+statements. It follows a very similar approach to the _EVT method
+from GPIO events. All interrupts are listed in  _CRS and the handler
+is written in _EVT method. However, the QEMU implementation uses a
+single interrupt for the GED device, relying on an IO memory region
+to communicate the type of device affected by the interrupt. This way,
+we can support up to 32 events with a unique interrupt.
+
+**Here is an example,**
+
+::
+
+   Device (\_SB.GED)
+   {
+       Name (_HID, "ACPI0013")
+       Name (_UID, Zero)
+       Name (_CRS, ResourceTemplate ()
+       {
+           Interrupt (ResourceConsumer, Edge, ActiveHigh, Exclusive, ,, )
+           {
+               0x00000029,
+           }
+       })
+       OperationRegion (EREG, SystemMemory, 0x09080000, 0x04)
+       Field (EREG, DWordAcc, NoLock, WriteAsZeros)
+       {
+           ESEL,   32
+       }
+       Method (_EVT, 1, Serialized)
+       {
+           Local0 = ESEL // ESEL = IO memory region which specifies the
+                         // device type.
+           If (((Local0 & One) == One))
+           {
+               MethodEvent1()
+           }
+           If ((Local0 & 0x2) == 0x2)
+           {
+               MethodEvent2()
+           }
+           ...
+       }
+   }
+
+GED IO interface (4 byte access)
+--------------------------------
+**read access:**
+
+::
+
+   [0x0-0x3] Event selector bit field (32 bit) set by QEMU.
+
+    bits:
+       0: Memory hotplug event
+       1: System power down event
+    2-31: Reserved
+
+**write_access:**
+
+Nothing is expected to be written into GED IO memory

docs/specs/index.rst

@@ -12,3 +12,4 @@ Contents:
 
    ppc-xive
    ppc-spapr-xive
+   acpi_hw_reduced_hotplug