linux-kernel - Re: [PATCHv2] soc: qcom: llcc: Support chipsets that can write to llcc registers

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Date:   Thu, 03 Sep 2020 21:17:35 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Isaac J. Manjarres" <isaacm@...eaurora.org>,
        linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCHv2] soc: qcom: llcc: Support chipsets that can write to
 llcc registers

On 2020-09-03 19:16, Doug Anderson wrote:
> Hi,
> 
> On Thu, Sep 3, 2020 at 2:58 AM Sai Prakash Ranjan
> <saiprakash.ranjan@...eaurora.org> wrote:
>> 
>> Hi,
>> 
>> On 2020-08-18 21:07, Sai Prakash Ranjan wrote:
>> > Hi Doug,
>> >
>> >>
>> >> I guess to start, it wasn't obvious (to me) that there were two
>> >> choices and we were picking one.  Mentioning that the other
>> >> alternative was way-based allocation would help a lot.  Even if you
>> >> can't fully explain the differences between the two, adding something
>> >> to the commit message indicating that this is a policy decision (in
>> >> other words, both work but each have their tradeoffs) would help.
>> >> Something like this, if it's correct:
>> >>
>> >> In general we try to enable capacity based allocation (instead of the
>> >> default way based allocation) since that gives us better performance
>> >> with the current software / hardware configuration.
>> >>
>> >
>> > Thanks, I will add it for next version. Let me also go poke some arch
>> > teams
>> > to understand if we actually do gain something with this selection, who
>> > knows
>> > we might get some additional details as well.
>> >
>> 
>> I got some information from arch team today, to quote them exactly:
>> 
>> 1) What benefits capacity based allocation brings over the default way
>> based allocation?
>> 
>> "Capacity based allows finer grain partition. It is not about improved
>> performance but more flexibility in configuration."
>> 
>> 2) Retain through power collapse, doesn’t it burn more power?
>> 
>> "This feature is similar to the standard feature of retention. Yes, 
>> when
>> we
>> have cache in retention mode it burns more power but it keeps the 
>> values
>> so
>> that when we wake up we can get more cache hits."
>> 
>> 
>> If its good enough, then I will add this info to the commit msg and 
>> post
>> next version.
> 
> Sounds fine to me.  I was mostly looking for a high level idea of what
> was happening here.  I am at least a little curious about the
> retention bit.  Is that retention during S3, or during some sort of
> Runtime PM?  Any idea how much power is burned?  Unless the power is
> miniscule it seems hard to believe that it would be a net win to keep
> a cache powered up during S3 unless you're planning on waking up a
> lot.
> 

The retention setting is based on sub cache id(SCID), so I think its for
runtime pm, the power numbers weren't provided. But I believe these
decisions are made after solid testing and not some random 
approximations.

Thanks,
Sai

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