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Date:   Thu, 31 May 2018 10:10:12 +0530
From:   Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>
To:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Cc:     stewart@linux.vnet.ibm.com, svaidy@linux.vnet.ibm.com,
        ego@linux.vnet.ibm.com, mpe@ellerman.id.au
Subject: Re: [PATCH] [SCHEME 1] Add support for new idle device tree format
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On Thu, May 31, 2018 at 08:23:04AM +0530, Akshay Adiga wrote:
> This patch adds support for new device-tree format for idle state
> description.
> 
> Previously if a older kernel runs on a newer firmware, it may enable
> all available states irrespective of its capability of handling it.
> New device tree format adds a compatible flag, so that only kernel
> which has the capability to handle the version of stop state will enable
> it.
> 
> Older kernel will still see stop0 and stop0_lite in older format and we
> will depricate it after some time.
> 
> Idea is to bump up the version in firmware if we find a bug or
> regression in stop states. A fix will be provided in linux which would
> now know about the bumped up version of stop states, where as kernel
> without fixes would ignore the states.
> 
> New idle state device tree format :
>        power-mgt {
>             ...
>         ibm,enabled-stop-levels = <0xec000000>;
>         ibm,cpu-idle-state-psscr-mask = <0x0 0x3003ff 0x0 0x3003ff>;
>         ibm,cpu-idle-state-latencies-ns = <0x3e8 0x7d0>;
>         ibm,cpu-idle-state-psscr = <0x0 0x330 0x0 0x300330>;
>         ibm,cpu-idle-state-flags = <0x100000 0x101000>;
>         ibm,cpu-idle-state-residency-ns = <0x2710 0x4e20>;
>         ibm,idle-states {
>                 phandle = <0xff>;
>                     stop4 {
>                         flags = <0x207000>;
>                         compatible = "ibm,cpuidle-state-v1";
>                         psscr-mask = <0x0 0x3003ff>;
>                         handle = <0x102>;
>                         latency-ns = <0x186a0>;
>                         residency-ns = <0x989680>;
>                         psscr = <0x0 0x300374>;
>                  };
> 
>                     ...
>                     stop11 {
>                         ...
>                         compatible = "ibm,cpuoffline-state-v1";
>                         ...
> 
>            };
>            };
> 
> Signed-off-by: Akshay Adiga <akshay.adiga@linux.vnet.ibm.com>

This is intended to be a RFC patch. 
The skiboot patch for the device tree is posted here :
https://patchwork.ozlabs.org/patch/923120/