[PATCH v6 5/5] fw_cfg: document ACPI device node information

[Gabriel L. Somlo]

Signed-off-by: Gabriel Somlo <somlo@xxxxxxx>
Reviewed-by: Laszlo Ersek <lersek@xxxxxxxxxx>
Reviewed-by: Marc Marà <markmb@xxxxxxxxxx>
---
 docs/specs/fw_cfg.txt | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
index 2099ad9..5414140 100644
--- a/docs/specs/fw_cfg.txt
+++ b/docs/specs/fw_cfg.txt
@@ -84,6 +84,15 @@ Selector Register address: Base + 8 (2 bytes)
 Data Register address:     Base + 0 (8 bytes)
 DMA Address address:       Base + 16 (8 bytes)
 
+== ACPI Interface ==
+
+The fw_cfg device is defined with ACPI ID "QEMU0002". Since we expect
+ACPI tables to be passed into the guest through the fw_cfg device itself,
+the guest-side firmware can not use ACPI to find fw_cfg. However, once the
+firmware is finished setting up ACPI tables and hands control over to the
+guest kernel, the latter can use the fw_cfg ACPI node for a more accurate
+inventory of in-use IOport or MMIO regions.
+
 == Firmware Configuration Items ==
 
 === Signature (Key 0x0000, FW_CFG_SIGNATURE) ===
-- 
2.4.3