[PATCH v2] docs: ACPI: enumeration: add PCI hierarchy representation
From: Flavio Suligoi
Date: Fri Nov 20 2020 - 06:12:02 EST
For "fixed" PCI devices, such as chips directly soldered
on the main board (ethernet, Wi-Fi, serial ports, etc.),
it is possible to find an ACPI enumeration.
This allows to add useful properties to these devices.
Just for an example: the property "gpio-line-names" can be
added to the pins of a GPIO expander on the PCI bus.
In order to find the ACPI name of a PCI device, it's necessary
to disassemble the BIOS ACPI tables (in particular the DSDT)
and also to analyze the PCI bus topology of the board.
This patch, with a practical example, show how to do this.
Signed-off-by: Flavio Suligoi <f.suligoi@xxxxxxx>
---
.../firmware-guide/acpi/enumeration.rst | 154 ++++++++++++++++++
1 file changed, 154 insertions(+)
diff --git a/Documentation/firmware-guide/acpi/enumeration.rst b/Documentation/firmware-guide/acpi/enumeration.rst
index c13fee8b02ba..9f0d5c854fa4 100644
--- a/Documentation/firmware-guide/acpi/enumeration.rst
+++ b/Documentation/firmware-guide/acpi/enumeration.rst
@@ -461,3 +461,157 @@ Otherwise, the _DSD itself is regarded as invalid and therefore the "compatible"
property returned by it is meaningless.
Refer to :doc:`DSD-properties-rules` for more information.
+
+PCI hierarchy representation
+============================
+
+Sometimes could be useful to enumerate a PCI device, knowing its position on the
+PCI bus.
+
+For example, some systems use PCI devices soldered directly on the mother board,
+in a fixed position (ethernet, Wi-Fi, serial ports, etc.). In this conditions it
+is possible to refer to these PCI devices knowing their position on the PCI bus
+topology.
+
+To identify a PCI device, a complete hierarchical description is required, from
+the chipset root port to the final device, through all the intermediate
+bridges/switches of the board.
+
+For example, let us assume to have a system with a PCIe serial port, an
+Exar XR17V3521, soldered on the main board. This UART chip also includes
+16 GPIOs and we want to add the property ``gpio-line-names`` [1] to these pins.
+In this case, the ``lspci`` output for this component is::
+
+ 07:00.0 Serial controller: Exar Corp. XR17V3521 Dual PCIe UART (rev 03)
+
+The complete ``lspci`` output (manually reduced in length) is::
+
+ 00:00.0 Host bridge: Intel Corp... Host Bridge (rev 0d)
+ ...
+ 00:13.0 PCI bridge: Intel Corp... PCI Express Port A #1 (rev fd)
+ 00:13.1 PCI bridge: Intel Corp... PCI Express Port A #2 (rev fd)
+ 00:13.2 PCI bridge: Intel Corp... PCI Express Port A #3 (rev fd)
+ 00:14.0 PCI bridge: Intel Corp... PCI Express Port B #1 (rev fd)
+ 00:14.1 PCI bridge: Intel Corp... PCI Express Port B #2 (rev fd)
+ ...
+ 05:00.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
+ 06:01.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
+ 06:02.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
+ 06:03.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
+ 07:00.0 Serial controller: Exar Corp. XR17V3521 Dual PCIe UART (rev 03) <-- Exar
+ ...
+
+The bus topology is::
+
+ -[0000:00]-+-00.0
+ ...
+ +-13.0-[01]----00.0
+ +-13.1-[02]----00.0
+ +-13.2-[03]--
+ +-14.0-[04]----00.0
+ +-14.1-[05-09]----00.0-[06-09]--+-01.0-[07]----00.0 <-- Exar
+ | +-02.0-[08]----00.0
+ | \-03.0-[09]--
+ ...
+ \-1f.1
+
+To describe this Exar device on the PCI bus, we must start from the ACPI name
+of the chipset bridge (also called "root port") with address::
+
+ Bus: 0 - Device: 14 - Function: 1
+
+To find this information is necessary disassemble the BIOS ACPI tables, in
+particular the DSDT (see also [2])::
+
+ mkdir ~/tables/
+ cd ~/tables/
+ acpidump > acpidump
+ acpixtract -a acpidump
+ iasl -e ssdt?.* -d dsdt.dat
+
+Now, in the dsdt.dsl, we have to search the device whose address is related to
+0x14 (device) and 0x01 (function). In this case we can find the following
+device::
+
+ Scope (_SB.PCI0)
+ {
+ ... other definitions follow ...
+ Device (RP02)
+ {
+ Method (_ADR, 0, NotSerialized) // _ADR: Address
+ {
+ If ((RPA2 != Zero))
+ {
+ Return (RPA2) /* \RPA2 */
+ }
+ Else
+ {
+ Return (0x00140001)
+ }
+ }
+ ... other definitions follow ...
+
+and the _ADR method [3] returns exactly the device/function couple that
+we are looking for. With this information and analyzing the above ``lspci``
+output (both the devices list and the devices tree), we can write the following
+ACPI description for the Exar PCIe UART, also adding the list of its GPIO line
+names::
+
+ Scope (_SB.PCI0.RP02)
+ {
+ Device (BRG1) //Bridge
+ {
+ Name (_ADR, 0x0000)
+
+ Device (BRG2) //Bridge
+ {
+ Name (_ADR, 0x00010000)
+
+ Device (EXAR)
+ {
+ Name (_ADR, 0x0000)
+
+ Name (_DSD, Package ()
+ {
+ ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+ Package ()
+ {
+ Package ()
+ {
+ "gpio-line-names",
+ Package ()
+ {
+ "mode_232",
+ "mode_422",
+ "mode_485",
+ "misc_1",
+ "misc_2",
+ "misc_3",
+ "",
+ "",
+ "aux_1",
+ "aux_2",
+ "aux_3",
+ }
+ }
+ }
+ })
+ }
+ }
+ }
+ }
+
+The location "_SB.PCI0.RP02" is obtained by the above investigation in the
+dsdt.dsl table, whereas the device names "BRG1", "BRG2" and "EXAR" are
+created analyzing the position of the Exar UART in the PCI bus topology.
+
+References
+==========
+
+[1] Documentation/firmware-guide/acpi/gpio-properties.rst
+
+[2] Documentation/admin-guide/acpi/initrd_table_override.rst
+
+[3] ACPI Specifications, Version 6.3 - Paragraph 6.1.1 _ADR Address)
+ https://uefi.org/sites/default/files/resources/ACPI_6_3_May16.pdf,
+ referenced 2020-11-18
--
2.25.1