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

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Date:   Tue, 8 Oct 2019 12:02:07 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Daniel Thompson <daniel.thompson@...aro.org>
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 08/10/2019 11.31, Daniel Thompson wrote:
> 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.
>>>
>>
>> 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.

Agreed. Do you want me to resend the series with patch 3 updated to read

"For a fixed small exponent of 3, it is more efficient to simply use two
explicit multiplications rather than calling the int_pow() library
function: Aside from the function call overhead, its implementation
using repeated squaring means it ends up doing four 64x64 multiplications."

(and obviously patch 5 dropped)?

Rasmus