linux-kernel - Re: [PATCH 3/4] platform/x86: intel_pmc

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Date:   Wed, 26 Sep 2018 20:42:53 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     rajneesh.bhardwaj@...ux.intel.com
Cc:     Platform Driver <platform-driver-x86@...r.kernel.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>,
        Souvik Kumar Chakravarty <souvik.k.chakravarty@...el.com>
Subject: Re: [PATCH 3/4] platform/x86: intel_pmc_core: Decode Snoop / Non
 Snoop LTR

On Wed, Sep 26, 2018 at 5:19 PM Bhardwaj, Rajneesh
<rajneesh.bhardwaj@...ux.intel.com> wrote:
> On 26-Sep-18 7:23 PM, Andy Shevchenko wrote:
> > On Mon, Sep 3, 2018 at 9:05 PM Rajneesh Bhardwaj
> > <rajneesh.bhardwaj@...ux.intel.com> wrote:

> >> +static void get_ltr_scale(u32 *val)
> > What's wrong to return converted value? Actually the name should
> > reflect what it does, ie *convert* value.
>
> I can change it as per your suggestion.

Please do.

> >> +union ltr_payload {
> >> +       u32 raw_data;
> >> +       struct {
> >> +       u32 snoop_val : 10;
> >> +       u32 snoop_scale : 3;
> >> +       u32 snoop_res : 2;
> >> +       u32 snoop_req : 1;
> >> +       u32 non_snoop_val : 10;
> >> +       u32 non_snoop_scale : 3;
> >> +       u32 non_snoop_res : 2;
> >> +       u32 non_snoop_req : 1;
> >> +       } bits;
> >> +};
> > Just use normal masks and shifts.
>
> I chose union over masks and shifts to reduce code size and ensured
> correct endian-ness.

How do you ensure endianess in union if you do nothing to it here? It
just would reflect CPU endianess.

> Just for my understanding, can you please let me
> know why you feel masks/shift are better suited here?

First of all, in the very same driver shifts and masks / standalone
bits are already in use.
Like you mentioned an endianess, it would make it more clear here,
though it's still require to get a value in a proper one in the first
place.
On top of that, a compiler which might generate an awful code out of
bits defined as above.

Btw, there are helpers for that like those in bitfield.h.

--
With Best Regards,
Andy Shevchenko