[PATCH] PCI: Add information about describing PCI in ACPI

On Wed, Nov 23, 2016 at 03:23:35AM +0000, Zheng, Lv wrote:

Hi, Bjorn

Thanks for the documentation.
It really helps!

However I have a question below.

quoted

From: linux-acpi-owner at vger.kernel.org [mailto:linux-acpi-owner at vger.kernel.org] On Behalf Of Bjorn
Helgaas
Subject: [PATCH] PCI: Add information about describing PCI in ACPI

Add a writeup about how PCI host bridges should be described in ACPI
using PNP0A03/PNP0A08 devices, PNP0C02 devices, and the MCFG table.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
---
 Documentation/PCI/00-INDEX      |    2 +
 Documentation/PCI/acpi-info.txt |  136 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 138 insertions(+)
 create mode 100644 Documentation/PCI/acpi-info.txt

diff --git a/Documentation/PCI/00-INDEX b/Documentation/PCI/00-INDEX
index 147231f..0780280 100644
--- a/Documentation/PCI/00-INDEX
+++ b/Documentation/PCI/00-INDEX

@@ -1,5 +1,7 @@
 00-INDEX
 	- this file
+acpi-info.txt
+	- info on how PCI host bridges are represented in ACPI
 MSI-HOWTO.txt
 	- the Message Signaled Interrupts (MSI) Driver Guide HOWTO and FAQ.
 PCIEBUS-HOWTO.txt

diff --git a/Documentation/PCI/acpi-info.txt b/Documentation/PCI/acpi-info.txt
new file mode 100644
index 0000000..ccbcfda
--- /dev/null
+++ b/Documentation/PCI/acpi-info.txt

@@ -0,0 +1,136 @@
+	    ACPI considerations for PCI host bridges
+
+The basic requirement is that the ACPI namespace should describe
+*everything* that consumes address space unless there's another
+standard way for the OS to find it [1, 2]. ?For example, windows that
+are forwarded to PCI by a PCI host bridge should be described via ACPI
+devices, since the OS can't locate the host bridge by itself. ?PCI
+devices *below* the host bridge do not need to be described via ACPI,
+because the resources they consume are inside the host bridge windows,
+and the OS can discover them via the standard PCI enumeration
+mechanism (using config accesses to read and size the BARs).
+
+This ACPI resource description is done via _CRS methods of devices in
+the ACPI namespace [2]. ? _CRS methods are like generalized PCI BARs:
+the OS can read _CRS and figure out what resource is being consumed
+even if it doesn't have a driver for the device [3]. ?That's important
+because it means an old OS can work correctly even on a system with
+new devices unknown to the OS. ?The new devices won't do anything, but
+the OS can at least make sure no resources conflict with them.
+
+Static tables like MCFG, HPET, ECDT, etc., are *not* mechanisms for
+reserving address space!  The static tables are for things the OS
+needs to know early in boot, before it can parse the ACPI namespace.
+If a new table is defined, an old OS needs to operate correctly even
+though it ignores the table.  _CRS allows that because it is generic
+and understood by the old OS; a static table does not.

The entire document doesn't talk about the details of _CBA.
There is only one line below mentioned _CBA as an example.

Yes, that's a good point.  I'll add some more details about MCFG
and _CBA.

Bjorn