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-rw-r--r--docs/specs/acpi_pci_hotplug.txt12
1 files changed, 10 insertions, 2 deletions
diff --git a/docs/specs/acpi_pci_hotplug.txt b/docs/specs/acpi_pci_hotplug.txt
index 1883d63061..a839434f31 100644
--- a/docs/specs/acpi_pci_hotplug.txt
+++ b/docs/specs/acpi_pci_hotplug.txt
@@ -27,8 +27,16 @@ events. Read-only.
PCI device eject (IO port 0xae08-0xae0b, 4-byte access):
----------------------------------------
-Used by ACPI BIOS _EJ0 method to request device removal. One bit per slot.
-Reads return 0.
+Write: Used by ACPI BIOS _EJ0 method to request device removal.
+One bit per slot.
+
+Read: Hotplug features register. Used by platform to identify features
+available. Current base feature set (no bits set):
+ - Read-only "up" register @0xae00, 4-byte access, bit per slot
+ - Read-only "down" register @0xae04, 4-byte access, bit per slot
+ - Read/write "eject" register @0xae08, 4-byte access,
+ write: bit per slot eject, read: hotplug feature set
+ - Read-only hotplug capable register @0xae0c, 4-byte access, bit per slot
PCI removability status (IO port 0xae0c-0xae0f, 4-byte access):
-----------------------------------------------