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

From: Hanjun Guo
Date: Wed Dec 31 2014 - 03:35:18 EST

On 2014å12æ31æ 04:13, ashwinc@xxxxxxxxxxxxxx wrote:
Hi Hanjun,

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

---------- Forwarded message ----------
From: Graeme Gregory <graeme.gregory@xxxxxxxxxx>

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

Signed-off-by: Graeme Gregory <graeme.gregory@xxxxxxxxxx>
Signed-off-by: Al Stone <al.stone@xxxxxxxxxx>
Signed-off-by: Hanjun Guo <hanjun.guo@xxxxxxxxxx>
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?

No, we can booting the ACPI system in EFI stub without dtb. Catalin
also pointed out this issue, I will remove this paragraph.

+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.

Good point, this exactly what ACPI spec says, we need to update this
paragraph a little bit.

+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.

and I think it is CPU idle specific here, right?

+ -- 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.

will remove it.

Thanks for reviewing it!

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