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

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Message-ID: <20191009174622.GN6390@tuxbook-pro>
Date:   Wed, 9 Oct 2019 10:46:22 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Evan Green <evgreen@...omium.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 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.

Or am I missing something?

Regards,
Bjorn

> But either way it's correct, so really it's fine if you ignore me :)
> -Evan