linux-kernel - Re: [PATCH v8 2/3] soc: qcom: rpmh: Update dirty flag only when data changes

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Message-ID: <CAE=gft6VDMoTZ4mW7d7scUCtDowfJiCbOzx_1FaFkoz8tm99DQ@mail.gmail.com>
Date:   Thu, 27 Feb 2020 10:18:06 -0800
From:   Evan Green <evgreen@...omium.org>
To:     Maulik Shah <mkshah@...eaurora.org>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Andy Gross <agross@...nel.org>,
        Doug Anderson <dianders@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Lina Iyer <ilina@...eaurora.org>, lsrao@...eaurora.org
Subject: Re: [PATCH v8 2/3] soc: qcom: rpmh: Update dirty flag only when data changes

On Thu, Feb 27, 2020 at 12:57 AM 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 dirty flag when data is
> changed to newer values.
>
> Also move dirty flag updates to happen from within cache_lock and remove
> unnecessary INIT_LIST_HEAD() call and a default case from switch.
>
> Fixes: 600513dfeef3 ("drivers: qcom: rpmh: cache sleep/wake state requests")
> Signed-off-by: Maulik Shah <mkshah@...eaurora.org>
> Reviewed-by: Srinivas Rao L <lsrao@...eaurora.org>
> ---
>  drivers/soc/qcom/rpmh.c | 29 ++++++++++++++++-------------
>  1 file changed, 16 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/soc/qcom/rpmh.c b/drivers/soc/qcom/rpmh.c
> index eb0ded0..3f5d9eb 100644
> --- a/drivers/soc/qcom/rpmh.c
> +++ b/drivers/soc/qcom/rpmh.c
> @@ -133,26 +133,30 @@ static struct cache_req *cache_rpm_request(struct rpmh_ctrlr *ctrlr,
>
>         req->addr = cmd->addr;
>         req->sleep_val = req->wake_val = UINT_MAX;
> -       INIT_LIST_HEAD(&req->list);

Thanks!

>         list_add_tail(&req->list, &ctrlr->cache);
>
>  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;
> +               }
>                 break;
>         case RPMH_WAKE_ONLY_STATE:
> -               req->wake_val = cmd->data;
> +               if (req->wake_val != cmd->data) {
> +                       req->wake_val = cmd->data;
> +                       ctrlr->dirty = true;
> +               }
>                 break;
>         case RPMH_SLEEP_STATE:
> -               req->sleep_val = cmd->data;
> -               break;
> -       default:
> +               if (req->sleep_val != cmd->data) {
> +                       req->sleep_val = cmd->data;
> +                       ctrlr->dirty = true;
> +               }
>                 break;
>         }
>
> -       ctrlr->dirty = true;
>  unlock:
>         spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>
> @@ -287,6 +291,7 @@ static void cache_batch(struct rpmh_ctrlr *ctrlr, struct batch_cache_req *req)
>
>         spin_lock_irqsave(&ctrlr->cache_lock, flags);
>         list_add_tail(&req->list, &ctrlr->batch_cache);
> +       ctrlr->dirty = true;
>         spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>  }
>
> @@ -323,6 +328,7 @@ static void invalidate_batch(struct rpmh_ctrlr *ctrlr)
>         list_for_each_entry_safe(req, tmp, &ctrlr->batch_cache, list)
>                 kfree(req);
>         INIT_LIST_HEAD(&ctrlr->batch_cache);
> +       ctrlr->dirty = true;
>         spin_unlock_irqrestore(&ctrlr->cache_lock, flags);
>  }
>
> @@ -456,13 +462,9 @@ static int send_single(struct rpmh_ctrlr *ctrlr, enum rpmh_state state,
>  int rpmh_flush(struct rpmh_ctrlr *ctrlr)
>  {
>         struct cache_req *p;
> +       unsigned long flags;
>         int ret;
>
> -       if (!ctrlr->dirty) {
> -               pr_debug("Skipping flush, TCS has latest data.\n");
> -               return 0;
> -       }
> -
>         /* First flush the cached batch requests */
>         ret = flush_batch(ctrlr);
>         if (ret)
> @@ -488,7 +490,9 @@ int rpmh_flush(struct rpmh_ctrlr *ctrlr)
>                         return ret;
>         }
>
> +       spin_lock_irqsave(&ctrlr->cache_lock, flags);
>         ctrlr->dirty = false;
> +       spin_unlock_irqrestore(&ctrlr->cache_lock, flags);

You're acquiring a lock around an operation that's already inherently
atomic, which is not right. If the comment earlier in this function is
still correct that "Nobody else should be calling this function other
than system PM, hence we can run without locks", then you can simply
remove this hunk and the part moving ->dirty = true into
invalidate_batch.

However, if rpmh_flush() can now be called in a scenario where
pre-emption is enabled or multiple cores are alive, then ctrlr->cache
is no longer adequately protected. You'd need to add a lock
acquire/release around the list iteration above, and fix up the
comment.
-Evan