[PATCH v5 18/18] Documentation: ACPI for ARM64

Hi Hanjun,

Overall the document looks good to us. Some minor clarifications below.

---------- Forwarded message ----------
From: Graeme Gregory <redacted>

Add documentation for the guidelines of how to use ACPI
on ARM64.

Signed-off-by: Graeme Gregory <redacted>
Signed-off-by: Al Stone <redacted>
Signed-off-by: Hanjun Guo <redacted>
---
 Documentation/arm64/arm-acpi.txt |  323
++++++++++++++++++++++++++++++++++++++
 1 file changed, 323 insertions(+)
 create mode 100644 Documentation/arm64/arm-acpi.txt

[..]

+Relationship with Device Tree
+-----------------------------

[..]

+When booting using ACPI tables, the /chosen node in DT will still be
parsed
+to extract the kernel command line and initrd path.  No other section of
the
+DT will be used.

Is this still true?

+Programmable Power Control Resources
+------------------------------------
+Programmable power control resources include such resources as
voltage/current
+providers (regulators) and clock sources.
+
+The kernel assumes that power control of these resources is represented
with
+Power Resource Objects (ACPI section 7.1).  The ACPI core will then
handle
+correctly enabling and disabling resources as they are needed.  In order
to
+get that to work, ACPI assumes each device has defined D-states and that
these
+can be controlled through the optional ACPI methods _PS0, _PS1, _PS2, and
_PS3;
+in ACPI, _PS0 is the method to invoke to turn a device full on, and _PS3
is for
+turning a device full off.
+
+The kernel ACPI code will also assume that the _PS? methods follow the
normal
+ACPI rules for such methods:
+
+   -- If either _PS0 or _PS3 is implemented, then the other method must
also
+      be implemented.
+
+   -- If a device requires usage or setup of a power resource when on,
the ASL
+      should organize that it is allocated/enabled using the _PS0 method.
+
+   -- Resources allocated or enabled in the _PS0 method should be
disabled
+      or de-allocated in the _PS3 method.
+
+   -- Firmware will leave the resources in a reasonable state before
handing
+      over control to the kernel.
+

We found this section could be improved a bit by explicitly calling out
the options for handling device PM. Platform vendor has two choices.
Resources can be managed in _PSx routine which gets called on entry to Dx.
 Or they can be declared separately as power resources with their own _ON
and _OFF methods.  They are then tied back to D-states for a particular
device via _PRx which specifies which power resources a device needs to be
on while in Dx.  Kernel then tracks number of devices using a power
resource and calls _ON/_OFF as needed.

+Such code in _PS? methods will of course be very platform specific.  But,
+this allows the driver to abstract out the interface for operating the
device
+and avoid having to read special non-standard values from ACPI tables.
Further,
+abstracting the use of these resources allows the hardware to change over
time
+without requiring updates to the driver.
+

I think its been mentioned in the past and you planned to add it here: we
should explicitly state that with ACPI, the kernel clock/vreg framework
are not expected to be used at all.

+
+Clocks
+------
+ACPI makes the assumption that clocks are initialized by the firmware --
+UEFI, in this case -- to some working value before control is handed over
+to the kernel.  This has implications for devices such as UARTs, or SoC
+driven LCD displays, for example.
+
+When the kernel boots, the clock is assumed to be set to reasonable
+working value.  If for some reason the frequency needs to change -- e.g.,
+throttling for power management -- the device driver should expect that
+process to be abstracted out into some ACPI method that can be invoked

Exception to this is CPU clocks where CPPC provides a much richer
interface than just blindly invoking some method.

+(please see the ACPI specification for further recommendations on
standard
+methods to be expected).  If is not, there is no direct way for ACPI to
+control the clocks.
+
+

[..]

+ASWG
+----
+The following areas are not yet fully defined for ARM in the 5.1 version
+of the ACPI specification and are expected to be worked through in the
+UEFI ACPI Specification Working Group (ASWG):
+
+   -- ACPI based CPU topology
+   -- ACPI based Power management

Should clarify this to idle management rather than generic power management.

+   -- CPU idle control based on PSCI
+   -- CPU performance control (CPPC)

There is no ongoing work on CPPC. Additional enhancements may be explored
in the future, but spec is viable as is.

Regards,
Ashwin

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