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

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Date:   Tue, 8 Oct 2019 11:43:11 +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 Tue, Oct 08, 2019 at 12:02:07PM +0200, Rasmus Villemoes wrote:
> 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)?

Yes, please.

When you resend you can add my R-B: to all patches:

Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>


Daniel.


PS Don't mind either way but I wondered the following is clearer than
   the slightly funky multiply-and-assign expression (which isn't wrong
   but isn't very common either so my brain won't speed read it):

		retval = DIV_ROUND_CLOSEST_ULL(retval * retval * retval,
		 			       scale * scale);