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

[ashwinc@codeaurora.org (ashwinc at codeaurora.org)]

Hi Hanjun,

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

> ---------- Forwarded message ----------
> From: Graeme Gregory <graeme.gregory@linaro.org>
>
> Add documentation for the guidelines of how to use ACPI
> on ARM64.
>
> Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
> Signed-off-by: Al Stone <al.stone@linaro.org>
> Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
> ---
>  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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