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 > 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 -- Qualcomm Innovation Center, Inc The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project -- -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
