RFC: Devicetree-ACPI hybrid mode

Hi All,

Currently as soon as the kernel boots with a populated DT provided then
the arch/arm64 code sets acpi_disabled=1 and the complete ACPI subsystem
gets disabled. On WoA Snapdragon laptops where the factory Windows OS
actually boots using these tables this is not necessarily desirable.

It might still be interesting to at least parse the ACPI tables and make
the ACPI fwnodes available for device-drivers to use. I call this DT-ACPI
hybrid mode.

This mainly is an experiment for now and possibly a method for accelerating
the ongoing effort to run Linux on currently available Snapdragon laptops.

On current laptops Linux cannot boot using ACPI due to some information
missing from the ACPI tables. People are working on changing this so that
for future WoA Snapdragon laptops Linux can boot using ACPI only without
requiring Devicetree.


There are a couple of scenarios where DT-ACPI hybrid mode is useful:

a) This leads to a populated /sys/firmware/acpi/tables allowing one to run
acpidump, which is useful to grab info from the ACPI tables when e.g.
creating a DT for a new laptop model. As a bonus /sys/firmware/acpi/bgrt
is also populated allowing the boot-splash to show the vendor logo.

b) It might be useful for device-drivers to be able to access ACPI data
for the device even when running in DT mode. E.g. Srini Kandagatla first
got me thinking about this because he wants to use the ACPI MIPI SDCA
tables for audio codec routing when booting Linux on Windows Qualcomm X2
(Glymur) laptops.

c) It is also possible to go truely hybrid and use ACPI to instantiate
some of the kernel device objects representing the hardware. For example
the last patch in this RFC series switches to using ACPI instantiation for
the I2C clients for the keyboard and touchpad on the Snapdragon X1E Lenovo
ThinkPad T14s gen 6.

d) This may help identify shortcomings in the current ACPI tables which
need to be fixed to allow future laptop generations to use ACPI only.


Upstreaming of these patches (to upstream or not to upstream?).

1. The first couple of patches in this series mainly implement a) + b) from
above. This seems like something genuinely useful to have; and except for
missing DT-bindings for hybrid mode this seems mostly ready to go upstream.

2. I see c) as a way to slowly evolve support for current Snapdragon laptops
to use more and more info from ACPI and get closer to a point where we only
need a single DT describing the SoC and any info related to laptop model
specific bits outside of the SoC can be read from the ACPI tables.

As mentioned above work is being done to have Linux boot on future laptop
generations using ACPI only, so all this applies to currently available
Snapdragon laptop generations only.

The question is what to do wrt upstreaming patches necessary for c) though
(patches 7-12) are we going to allow new Devicetree files for not yet
supported laptop models to partially rely on ACPI?

The current demo ACPI usage in this RFC series just instantiates I2C-HID
devices from ACPI. More interesting would be to hookup the embedded
controller (EC) handling in the ACPI tables instead of having to write
a special EC driver for each laptop model separately. For the EC parts
I believe that it might be worthwhile to implement c).

This new DT-ACPI hybrid mode works as follows:

1. A new global ACPI subsys flag called acpi_dt_hybrid is introduced which
can be set to 1 combined with acpi_disabled=1. When this is done, then
despite acpi_disabled being set the ACPI tables will still get parsed and
/sys/firmware/acpi (tables) and /sys/bus/acpi/devices (fwnodes) will still
get populated. No devices will be instantiated, no fwnodes will get
attached to any other (e.g. PCI, USB) devices and no other actions will
be taken.

2. Add acpi=hybrid kernel-commandline option to the arch/arm64 code.
TODO: Add a DT-binding for selecting hybrid mode from Devicetree.

3. drivers/acpi/glue.c changes to support devices with an of_node as
primary fwnode having a "acpi-path" string property and when present look
up the ACPI fwnode for that path, e.g. "\\_SB.GIO0", "\\_SB.I2C1" and
set that fwnode as secondary node.

4. pinctrl-msm changes to map special WoA ACPI table virtual GPIO numbers
for PDC pins back to regular TLMM GPIO numbers.

5. Tiny drivers/i2c/i2c-core-acpi.c change to make it instantiate ACPI
described I2C clients under a DT instantiated I2C adapter if that
adapter has an ACPI secondary fwnode.

With this I can drop the DT description of the ThinkPad T14s gen 6
keyboard and touchpad and instead have these be instantiated by ACPI,
as shown in patch 12/12.

Comments, thoughts ?

Regards,

Hans


Hans de Goede (12):
  ACPI: Introduce DT-ACPI hybrid mode
  arm64: acpi: Cleanup acpi=[on|off|force] handling
  arm64: acpi: add acpi=hybrid support
  ACPI: Add helpers for dealing with ACPI fwnode as secondary fwnode
  ACPI: glue: Implement setting secondary-fwnode for DT-ACPI hybrid mode
  ACPI: scan: Retry acpi_device_notify() in DT-ACPI hybrid mode
  ACPI: Make device_match_acpi_handle() also check the secondary fwnode
  irqchip/gic-v3: Always call acpi_set_irq_model()
  pinctrl: qcom: Add support for WoA ACPI tables virtual TLMM pin
    numbers
  i2c: acpi: Also register ACPI i2c_clients for adapters with a
    secondary ACPI fwnode
  i2c: qcom-geni: Fall back to i2c_acpi_find_bus_speed()
  arm64: dts: qcom: x1e78100-thinkpad-t14s: Move keyb and touchpad to
    ACPI enumeration

 .../qcom/x1e78100-lenovo-thinkpad-t14s.dtsi   |  61 +-----
 arch/arm64/include/asm/acpi.h                 |   1 +
 arch/arm64/kernel/acpi.c                      |  47 +++--
 arch/loongarch/include/asm/acpi.h             |   1 +
 arch/riscv/include/asm/acpi.h                 |   3 +-
 arch/x86/include/asm/acpi.h                   |   1 +
 drivers/acpi/acpi_apd.c                       |   3 +
 drivers/acpi/acpi_memhotplug.c                |   3 +
 drivers/acpi/acpi_processor.c                 |   3 +
 drivers/acpi/arm64/init.c                     |   2 +
 drivers/acpi/bus.c                            |   6 +-
 drivers/acpi/glue.c                           |  70 ++++++-
 drivers/acpi/property.c                       |  13 ++
 drivers/acpi/scan.c                           |  33 +++
 drivers/acpi/tables.c                         |   4 +-
 drivers/base/core.c                           |   4 +-
 drivers/firmware/efi/efi-bgrt.c               |   2 +-
 drivers/i2c/busses/i2c-qcom-geni.c            |  10 +-
 drivers/i2c/i2c-core-acpi.c                   |   8 +-
 drivers/irqchip/irq-gic-v3.c                  |  29 ++-
 drivers/pinctrl/qcom/Makefile                 |   4 +-
 drivers/pinctrl/qcom/pinctrl-msm-acpi.c       | 196 ++++++++++++++++++
 drivers/pinctrl/qcom/pinctrl-msm.c            |  47 ++++-
 drivers/pinctrl/qcom/pinctrl-msm.h            |  35 ++++
 include/acpi/acpi_bus.h                       |  16 ++
 include/linux/acpi.h                          |  10 +
 26 files changed, 502 insertions(+), 110 deletions(-)
 create mode 100644 drivers/pinctrl/qcom/pinctrl-msm-acpi.c

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2.54.0