[Qemu-devel] [PATCH-for-4.2 v9 10/12] docs/specs: Add ACPI GED documentation

[Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>]

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>
---
 docs/specs/acpi_hw_reduced_hotplug.txt | 60 ++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 docs/specs/acpi_hw_reduced_hotplug.txt

diff --git a/docs/specs/acpi_hw_reduced_hotplug.txt b/docs/specs/acpi_hw_reduced_hotplug.txt
new file mode 100644
index 0000000000..46839be5ff
--- /dev/null
+++ b/docs/specs/acpi_hw_reduced_hotplug.txt
@@ -0,0 +1,60 @@
+QEMU<->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 like the _EVT method
+from GPIO events. All interrupts are listed in  _CRS and the handler
+is written in _EVT method. However, Qemu implementation uses a single
+interrupt for the GED device, relying on 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:
+        1:  Memory hotplug event
+        2:  System power down event
+     3-31:  Reserved
+
+write_access:
+    Nothing is expected to be written into GED IO memory
-- 
2.17.1