linux-kernel - Re: [PATCH 3/5] backlight: pwm_bl: drop use of int

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Message-ID: <20191008093145.kgx6ytkbycmmkist@holly.lan>
Date:   Tue, 8 Oct 2019 10:31:45 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-pwm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] backlight: pwm_bl: drop use of int_pow()

On Mon, Oct 07, 2019 at 08:43:31PM +0200, Rasmus Villemoes wrote:
> On 07/10/2019 17.28, Daniel Thompson wrote:
> > On Thu, Sep 19, 2019 at 04:06:18PM +0200, Rasmus Villemoes wrote:
> > 
> > It feels like there is some rationale missing in the description here.
> > 
> > What is the benefit of replacing the explicit int_pow() with the
> > implicit multiplications?
> > 
> > 
> > Daniel.
> > 
> > 
> >>
> >> We could (and a following patch will) change to use a power-of-2 scale,
> >> but for a fixed small exponent of 3, there's no advantage in using
> >> repeated squaring.
> 
>    ^^^^^^^^^^^^^^^^^^                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> Apart from the function call overhead (and resulting register pressure
> etc.), using int_pow is less efficient (for an exponent of 3, it ends up
> doing four 64x64 multiplications instead of just two). But feel free to
> drop it, I'm not going to pursue it further - it just seemed like a
> sensible thing to do while I was optimizing the code anyway.
> 
> [At the time I wrote the patch, this was also the only user of int_pow
> in the tree, so it also allowed removing int_pow altogether.]

To be honest the change is fine but the patch description doesn't make
sense if the only current purpose of the patch is as a optimization.


Daniel.