[PATCH v5 18/18] Documentation: ACPI for ARM64
ashwinc at codeaurora.org
ashwinc at codeaurora.org
Tue Dec 30 12:13:58 PST 2014
Overall the document looks good to us. Some minor clarifications below.
> ---------- Forwarded message ----------
> From: Graeme Gregory <graeme.gregory at linaro.org>
> Add documentation for the guidelines of how to use ACPI
> on ARM64.
> Signed-off-by: Graeme Gregory <graeme.gregory at linaro.org>
> Signed-off-by: Al Stone <al.stone at linaro.org>
> Signed-off-by: Hanjun Guo <hanjun.guo at 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
> +to extract the kernel command line and initrd path. No other section of
> +DT will be used.
Is this still true?
> +Programmable Power Control Resources
> +Programmable power control resources include such resources as
> +providers (regulators) and clock sources.
> +The kernel assumes that power control of these resources is represented
> +Power Resource Objects (ACPI section 7.1). The ACPI core will then
> +correctly enabling and disabling resources as they are needed. In order
> +get that to work, ACPI assumes each device has defined D-states and that
> +can be controlled through the optional ACPI methods _PS0, _PS1, _PS2, and
> +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
> +ACPI rules for such methods:
> + -- If either _PS0 or _PS3 is implemented, then the other method must
> + 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
> + or de-allocated in the _PS3 method.
> + -- Firmware will leave the resources in a reasonable state before
> + 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
> +and avoid having to read special non-standard values from ACPI tables.
> +abstracting the use of these resources allows the hardware to change over
> +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.
> +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
> +methods to be expected). If is not, there is no direct way for ACPI to
> +control the clocks.
> +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.
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