linux-kernel - Re: [PATCH v3 3/3] backlight: pwm

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Message-ID: <20201104155412.GR4488@dell>
Date:   Wed, 4 Nov 2020 15:54:12 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Alexandru Stan <amstan@...omium.org>
Cc:     Heiko Stuebner <heiko@...ech.de>, Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Douglas Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v3 3/3] backlight: pwm_bl: Fix interpolation

On Wed, 21 Oct 2020, Alexandru Stan wrote:

> The previous behavior was a little unexpected, its properties/problems:
> 1. It was designed to generate strictly increasing values (no repeats)
> 2. It had quantization errors when calculating step size. Resulting in
> unexpected jumps near the end of some segments.
> 
> Example settings:
> 	brightness-levels = <0 1 2 4 8 16 32 64 128 256>;
> 	num-interpolated-steps = <16>;
> 
> Whenever num-interpolated-steps was larger than the distance
> between 2 consecutive brightness levels the table would get really
> discontinuous. The slope of the interpolation would stick with
> integers only and if it was 0 the whole line segment would get skipped.
> 
> The distances between 1 2 4 and 8 would be 1 (property #1 fighting us),
> and only starting with 16 it would start to interpolate properly.
> 
> Property #1 is not enough. The goal here is more than just monotonically
> increasing. We should still care about the shape of the curve. Repeated
> points might be desired if we're in the part of the curve where we want
> to go slow (aka slope near 0).
> 
> Problem #2 is plainly a bug. Imagine if the 64 entry was 63 instead,
> the calculated slope on the 32-63 segment will be almost half as it
> should be.
> 
> The most expected and simplest algorithm for interpolation is linear
> interpolation, which would handle both problems.
> Let's just implement that!
> 
> Take pairs of points from the brightness-levels array and linearly
> interpolate between them. On the X axis (what userspace sees) we'll
> now have equally sized intervals (num-interpolated-steps sized,
> as opposed to before where we were at the mercy of quantization).
> 
> END

I removed this.

> Signed-off-by: Alexandru Stan <amstan@...omium.org>
> ---
> 
>  drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------
>  1 file changed, 31 insertions(+), 39 deletions(-)

Applied, thanks.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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