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

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Date:   Thu, 3 Sep 2020 08:54:04 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.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

Hi,

On Thu, Sep 3, 2020 at 8:47 AM Sai Prakash Ranjan
<saiprakash.ranjan@...eaurora.org> wrote:
>
> 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.

Right, I believe it was tested, I just wonder if it was tested on a
phone vs. a laptop.  A phone is almost constantly waking up to deal
with stuff (which is why my phone battery barely lasts till the end of
the day).  Phones also usually have some type of self refresh on their
panels so they can be suspended even when they look awake which means
even more constant wakeups.  A laptop (especially without panel self
refresh) may have very different usage models.  I'm trying to confirm
that this setting is appropriate for both classes of devices or if it
has been only measured / optimized for the cell phone use case.

-Doug