[PATCH RFC 1/2] Documentation: arm: define DT bindings for system suspend

[jszhang@marvell.com (Jisheng Zhang)]

Dear Lorenzo,

On Wed, 21 Jan 2015 05:56:11 -0800
Lorenzo Pieralisi <lorenzo.pieralisi@arm.com> wrote:

> On Wed, Jan 21, 2015 at 01:35:07PM +0000, Jisheng Zhang wrote:
> > Dear Sudeep,
> > 
> > On Wed, 21 Jan 2015 05:21:39 -0800
> > Jisheng Zhang <jszhang@marvell.com> wrote:
> > 
> > > Dear Sudeep,
> > > 
> > > On Wed, 21 Jan 2015 03:35:54 -0800
> > > Sudeep Holla <sudeep.holla@arm.com> wrote:
> > > 
> > > > ARM based platforms implement unique ways to enter system suspend
> > > > (i.e. Suspend to RAM). The mechanism and the parameters defining the
> > > > system state vary on a per-platform basis forcing the OS to handle it
> > > > in very platform specific way.
> > > > 
> > > > Since ARM 32-bit systems had machine specific code, no attempts to
> > > > standardize are being made as it provides easy way to implement
> > > > suspend operations in a platform specific manner. However, this
> > > > approach not only makes maintainance more difficult as the number of
> > > > platforms supported increases but also not feasible for ARM64.
> > > > 
> > > > This DT binding aims at standardizing the system suspend for ARM
> > > > platforms. ARM64 platforms mandates entry-method property in DT for
> > > > this system suspend node.
> > > > 
> > > > On system implementing PSCI as an enable-method to enter system
> > > > suspend, the PSCI CPU suspend method is used on versions upto v0.2
> > > > and requires the power_state parameter to be passed to the PSCI CPU
> > > > suspend function.
> > > > 
> > > > This parameter is platform specific, therefore must be provided by
> > > > firmware to the OS in order to enable proper call sequence.
> > > > 
> > > > This ARM system suspend DT bindings rely on a property
> > > > (i.e. arm,psci-suspend-param) in the PSCI DT bindings that describes
> > > > how the PSCI CPU suspend power_state parameter should be defined in
> > > > DT.
> > > > 
> > > > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> > > > ---
> > > >  Documentation/devicetree/bindings/arm/psci.txt     | 11 +++
> > > >  .../devicetree/bindings/arm/system-suspend.txt     | 93
> > > > ++++++++++++++++++++++ 2 files changed, 104 insertions(+)
> > > >  create mode 100644
> > > > Documentation/devicetree/bindings/arm/system-suspend.txt
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/arm/psci.txt
> > > > b/Documentation/devicetree/bindings/arm/psci.txt index
> > > > 5aa40ede0e99..bd3977a2a333 100644 ---
> > > > a/Documentation/devicetree/bindings/arm/psci.txt +++
> > > > b/Documentation/devicetree/bindings/arm/psci.txt @@ -61,6 +61,14 @@
> > > > Device tree nodes that require usage of PSCI CPU_SUSPEND function (ie
> > > > idle Definition: power_state parameter to pass to the PSCI suspend
> > > > call. 
> > > > +PSCI v0.2 and earlier versions don't have explicit operation for
> > > > system +suspend. However, one can implement system suspend using
> > > > CPU_SUSPEND by +ensuring every other core except the one executing
> > > > the CPU_SUSPEND call +has called into PSCI through a CPU_OFF call.
> > > 
> > > If users explicitly hot-unplug other cores when system load is low to
> > > save power, then we want to suspend at some point, how does the
> > > firmware know this case?
> > 
> > Sorry for confusion. I mean 
> > 
> > If users explicitly hot-unplug other cores when system load is low to save
> > power, then at some point cpuidle want to suspend the cluster, how does
> > the distinguish this case with suspend the system to ram.
> 
> Through the arm,psci-suspend-param DT property, ie PSCI CPU_SUSPEND
> power_state parameter.
> 
> Did you read the patch :) ?

Yep, I do read the patch ;) To be honest, I implemented the s2ram similar as
the patch does. But according to PSCI v0.2, "arm,psci-suspend-param = <0x1010000>"
means suspend the cluster. I'm not sure I understand it correctly, "can implement
system suspend using CPU_SUSPEND by ensuring every other core except the one
executing the CPU_SUSPEND call has called into PSCI through a CPU_OFF call" intend
to ask firmware to 

suspend the system if other cores has called into PSCI through a CPU_OFF

or

suspend the cpu cluster if other cores are not CPU_OFF.


I extend the PSCI CPU_SUSPEND function's to use power_state bit[26] to tell firmware
whether suspend to ram or not.

Could you please correct me if I misunderstand something?

Thank you very much,
Jisheng

> 
> > > In my private tree, I extend the PSCI spec to tell firmware we want to
> > > suspend the system.
> > 
> > I extend the PSCI suspend function to help firmware distinguish suspend
> > system to ram and other suspend cores/clusters.
> 
> See above.
> 
> Thanks,
> Lorenzo
>