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2QEMU and ACPI BIOS Generic Event Device interface
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4
5The ACPI *Generic Event Device* (GED) is a HW reduced platform
6specific device introduced in ACPI v6.1 that handles all platform
7events, including the hotplug ones. GED is modelled as a device
8in the namespace with a _HID defined to be ACPI0013. This document
9describes the interface between QEMU and the ACPI BIOS.
10
11GED allows HW reduced platforms to handle interrupts in ACPI ASL
12statements. It follows a very similar approach to the _EVT method
13from GPIO events. All interrupts are listed in  _CRS and the handler
14is written in _EVT method. However, the QEMU implementation uses a
15single interrupt for the GED device, relying on an IO memory region
16to communicate the type of device affected by the interrupt. This way,
17we can support up to 32 events with a unique interrupt.
18
19**Here is an example,**
20
21::
22
23   Device (\_SB.GED)
24   {
25       Name (_HID, "ACPI0013")
26       Name (_UID, Zero)
27       Name (_CRS, ResourceTemplate ()
28       {
29           Interrupt (ResourceConsumer, Edge, ActiveHigh, Exclusive, ,, )
30           {
31               0x00000029,
32           }
33       })
34       OperationRegion (EREG, SystemMemory, 0x09080000, 0x04)
35       Field (EREG, DWordAcc, NoLock, WriteAsZeros)
36       {
37           ESEL,   32
38       }
39       Method (_EVT, 1, Serialized)
40       {
41           Local0 = ESEL // ESEL = IO memory region which specifies the
42                         // device type.
43           If (((Local0 & One) == One))
44           {
45               MethodEvent1()
46           }
47           If ((Local0 & 0x2) == 0x2)
48           {
49               MethodEvent2()
50           }
51           ...
52       }
53   }
54
55GED IO interface (4 byte access)
56--------------------------------
57**read access:**
58
59::
60
61   [0x0-0x3] Event selector bit field (32 bit) set by QEMU.
62
63    bits:
64       0: Memory hotplug event
65       1: System power down event
66       2: NVDIMM hotplug event
67       3: CPU hotplug event
68    4-31: Reserved
69
70**write_access:**
71
72Nothing is expected to be written into GED IO memory
73