linux-kernel - Re: [PATCH 1/3] soc: qcom: rpmh: Update dirty flag only when data changes

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Message-ID: <CAE=gft7ESnpS8bYg4hmoAUtsuLiyek1D-AYEL+LWUKe=KubAJw@mail.gmail.com>
Date:   Wed, 5 Feb 2020 10:07:29 -0800
From:   Evan Green <evgreen@...omium.org>
To:     Maulik Shah <mkshah@...eaurora.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Andy Gross <agross@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Doug Anderson <dianders@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Lina Iyer <ilina@...eaurora.org>, lsrao@...eaurora.org
Subject: Re: [PATCH 1/3] soc: qcom: rpmh: Update dirty flag only when data changes

On Tue, Feb 4, 2020 at 8:14 PM Maulik Shah <mkshah@...eaurora.org> wrote:
>
>
> On 2/5/2020 6:05 AM, Evan Green wrote:
> > On Mon, Feb 3, 2020 at 10:14 PM Maulik Shah <mkshah@...eaurora.org> wrote:
> >> Currently rpmh ctrlr dirty flag is set for all cases regardless
> >> of data is really changed or not.
> >>
> >> Add changes to update it when data is updated to new values.
> >>
> >> Signed-off-by: Maulik Shah <mkshah@...eaurora.org>
> >> ---
> >>   drivers/soc/qcom/rpmh.c | 15 +++++++++++----
> >>   1 file changed, 11 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/soc/qcom/rpmh.c b/drivers/soc/qcom/rpmh.c
> >> index 035091f..c3d6f00 100644
> >> --- a/drivers/soc/qcom/rpmh.c
> >> +++ b/drivers/soc/qcom/rpmh.c
> >> @@ -139,20 +139,27 @@ static struct cache_req *cache_rpm_request(struct rpmh_ctrlr *ctrlr,
> >>   existing:
> >>          switch (state) {
> >>          case RPMH_ACTIVE_ONLY_STATE:
> >> -               if (req->sleep_val != UINT_MAX)
> >> +               if (req->sleep_val != UINT_MAX) {
> >>                          req->wake_val = cmd->data;
> >> +                       ctrlr->dirty = true;
> >> +               }
> > Don't you need to set dirty = true for ACTIVE_ONLY state always? The
> > conditional is just saying "if nobody set a sleep vote, then maintain
> > this vote when we wake back up".
>
> The ACTIVE_ONLY vote is cached as wake_val to be apply when wakeup happens.
>
> In case value didn't change,wake_val is still same as older value and
> there is no need to mark the entire cache as dirty.
>

Ah, I see it now. We don't actually cache active_only votes anywhere,
since they're one time requests. The sleep/wake votes seem to be the
only thing that gets cached.

I was thinking it might be safer to also set dirty = true just after
list_add_tail, since in the non-existing case this is a new batch that
RPMh has never seen before and should always be written. But I suppose
your checks here should cover that case, since sleep_val and wake_val
are initialized to UINT_MAX. If you think the code might evolve, it
might still be nice to add it.

While I'm looking at that, why do we have this needless INIT_LIST_HEAD?
        INIT_LIST_HEAD(&req->list);
        list_add_tail(&req->list, &ctrlr->cache);

-Evan

> --
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