linux-kernel - Re: [PATCH v2 0/2] Avoid regmap debugfs collisions in qcom llcc driver

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Message-ID: <CAE=gft53-N+kWZKQO6YRAT0NBX_zrGYkqTUWOGrK2mT5Krf+3w@mail.gmail.com>
Date:   Wed, 9 Oct 2019 10:59:30 -0700
From:   Evan Green <evgreen@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Stephen Boyd <swboyd@...omium.org>, Andy Gross <agross@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Venkata Narendra Kumar Gutta <vnkgutta@...eaurora.org>
Subject: Re: [PATCH v2 0/2] Avoid regmap debugfs collisions in qcom llcc driver

On Wed, Oct 9, 2019 at 10:46 AM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> On Wed 09 Oct 09:01 PDT 2019, Evan Green wrote:
>
> > On Tue, Oct 8, 2019 at 6:58 PM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > Quoting Bjorn Andersson (2019-10-08 16:55:04)
> > > > On Tue 08 Oct 16:45 PDT 2019, Stephen Boyd wrote:
> > > > >     @@ drivers/soc/qcom/llcc-slice.c
> > > > >
> > > > >       static struct llcc_drv_data *drv_data = (void *) -EPROBE_DEFER;
> > > > >
> > > > >     --static const struct regmap_config llcc_regmap_config = {
> > > > >     +-static struct regmap_config llcc_regmap_config = {
> > > > >      -        .reg_bits = 32,
> > > > >      -        .reg_stride = 4,
> > > > >      -        .val_bits = 32,
> > > > >     @@ drivers/soc/qcom/llcc-slice.c: static struct regmap *qcom_llcc_init_mmio(struct
> > > > >       {
> > > > >               struct resource *res;
> > > > >               void __iomem *base;
> > > > >     -+        static struct regmap_config llcc_regmap_config = {
> > > > >     ++        struct regmap_config llcc_regmap_config = {
> > > >
> > > > Now that this isn't static I like the end result better. Not sure about
> > > > the need for splitting it in two patches, but if Evan is happy I'll take
> > > > it.
> > > >
> > >
> > > Well I split it into bug fix and micro-optimization so backport choices
> > > can be made. But yeah, I hope Evan is happy enough to provide a
> > > reviewed-by tag!
> >
> > It's definitely better without the static local since it no longer has
> > the cognitive trap, but I still don't really get why we're messing
> > with the global v. local aspect of it. We're now inconsistent with
> > every other caller of this function, and for what exactly? We've
> > traded some data space for a call to memset() and some instructions. I
> > would have thought anecdotally that memory was the cheaper thing (ie
> > cpu speeds stopped increasing awhile ago, but memory is still getting
> > cheaper).
> >
>
> The reason for making the structure local is because it's being modified
> per instance, meaning it would still work as long as
> qcom_llcc_init_mmio() is never called concurrently for two llcc
> instances. But the correctness outweighs the performance degradation of
> setting it up on the stack in my view.
>

I hadn't considered the concurrency aspect of the change, since I had
anchored myself on the static local. I'm convinced. Might be worth
mentioning that in the commit message.

For the series:
Reviewed-by: Evan Green <evgreen@...omium.org>