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Subject: Re: [RFC 0/3] Integrate Support for self-save and determine
To:     Ram Pai <linuxram@us.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org,
        mpe@ellerman.id.au, svaidy@linux.ibm.com, ego@linux.vnet.ibm.com,
        pratik.sampat@in.ibm.com
References: <20191204093255.11849-1-psampat@linux.ibm.com>
 <20200103230551.GB5562@oc0525413822.ibm.com>
From:   Pratik Sampat <psampat@linux.ibm.com>
Date:   Mon, 6 Jan 2020 15:15:30 +0530
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Thanks for your comments Ram,
>> A list of preferred SPRs are maintained in the kernel which contains two
>> properties:
>> 1. supported_mode: Helps in identifying if it strictly supports self
>>                     save or restore or both.
> Will be good to capture the information that, 'supported_mode' gets
> initialized using the information from the device tree.
>
>> 2. preferred_mode: Calls out what mode is preferred for each SPR. It
>>                     could be strictly self save or restore, or it can also
>>                     determine the preference of  mode over the other if both
>>                     are present by encapsulating the other in bitmask from
>>                     LSB to MSB.
> and 'preferred_mode'  is statically initialized.
>
Sure thing, I'll mention that.

>> Below is a table to show the Scenario::Consequence when the self save and
>> self restore modes are available or disabled in different combinations as
>> perceived from the device tree thus giving complete backwards compatibly
>> regardless of an older firmware running a newer kernel or vise-versa.
>>
>> SR = Self restore; SS = Self save
>>
>> .-----------------------------------.----------------------------------------.
>> |             Scenario              |                Consequence             |
>> :-----------------------------------+----------------------------------------:
>> | Legacy Firmware. No SS or SR node | Self restore is called for all         |
>> |                                   | supported SPRs                         |
>> :-----------------------------------+----------------------------------------:
>> | SR: !active SS: !active           | Deep stop states disabled              |
>> :-----------------------------------+----------------------------------------:
>> | SR: active SS: !active            | Self restore is called for all         |
>> |                                   | supported SPRs                         |
>> :-----------------------------------+----------------------------------------:
>> | SR: active SS: active             | Goes through the preferences for each  |
>> |                                   | SPR and executes of the modes          |
>> |                                   | accordingly. Currently, Self restore is|
>> |                                   | called for all the SPRs except PSSCR   |
>> |                                   | which is self saved                    |
>> :-----------------------------------+----------------------------------------:
>> | SR: active(only HID0) SS: active  | Self save called for all supported     |
>> |                                   | registers expect HID0 (as HID0 cannot  |
>> |                                   | be self saved currently)               |
> Not clear, how this will be conveyed to the hypervisor? Through the
> device tree or through some other means?
>
This information will be conveyed through the device tree. I'll frame a sentence
that makes this more explicit.

>> :-----------------------------------+----------------------------------------:
>> | SR: !active SS: active            | currently will disable deep states as  |
>> |                                   | HID0 is needed to be self restored and |
>> |                                   | cannot be self saved                   |
>> '-----------------------------------'----------------------------------------'
>>
>> Pratik Rajesh Sampat (3):
>>    powerpc/powernv: Interface to define support and preference for a SPR
>>    powerpc/powernv: Introduce Self save support
>>    powerpc/powernv: Parse device tree, population of SPR support
>>
>>   arch/powerpc/include/asm/opal-api.h        |   3 +-
>>   arch/powerpc/include/asm/opal.h            |   1 +
>>   arch/powerpc/platforms/powernv/idle.c      | 431 ++++++++++++++++++---
>>   arch/powerpc/platforms/powernv/opal-call.c |   1 +
>>   4 files changed, 379 insertions(+), 57 deletions(-)
>>
>> -- 
>> 2.21.0