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[86.9.19.6]) by smtp.googlemail.com with ESMTPSA id 5sm4910909wrc.76.2019.06.21.06.10.19 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Fri, 21 Jun 2019 06:10:20 -0700 (PDT) Subject: Re: [PATCH 4/4] backlight: pwm_bl: Set scale type for brightness curves specified in the DT To: Matthias Kaehlcke , Thierry Reding , Lee Jones , Jingoo Han , Bartlomiej Zolnierkiewicz Cc: linux-pwm@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, Enric Balletbo i Serra , Douglas Anderson , Brian Norris , Pavel Machek , Jacek Anaszewski References: <20190613194326.180889-1-mka@chromium.org> <20190613194326.180889-5-mka@chromium.org> From: Daniel Thompson Message-ID: <9ea1bb40-95a6-7a67-a8a6-ecc77a70e547@linaro.org> Date: Fri, 21 Jun 2019 14:10:19 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190613194326.180889-5-mka@chromium.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13/06/2019 20:43, Matthias Kaehlcke wrote: > Check if a brightness curve specified in the device tree is linear or > not and set the corresponding property accordingly. This makes the > scale type available to userspace via the 'scale' sysfs attribute. > > To determine if a curve is linear it is compared to a interpolated linear > curve between min and max brightness. The curve is considered linear if > no value deviates more than +/-5% of ${brightness_range} from their > interpolated value. > > Signed-off-by: Matthias Kaehlcke > --- > drivers/video/backlight/pwm_bl.c | 25 +++++++++++++++++++++++++ > 1 file changed, 25 insertions(+) > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c > index f067fe7aa35d..912407b6d67f 100644 > --- a/drivers/video/backlight/pwm_bl.c > +++ b/drivers/video/backlight/pwm_bl.c > @@ -404,6 +404,26 @@ int pwm_backlight_brightness_default(struct device *dev, > } > #endif > > +static bool pwm_backlight_is_linear(struct platform_pwm_backlight_data *data) > +{ > + unsigned int nlevels = data->max_brightness + 1; > + unsigned int min_val = data->levels[0]; > + unsigned int max_val = data->levels[nlevels - 1]; > + unsigned int slope = (100 * (max_val - min_val)) / nlevels; Why 100 (rather than a power of 2)? It would also be good to have a comment here saying what the maximum quantization error is. Doesn't have to be over complex just mentioning something like the following (assuming you agree that its true ;-) ): Multiplying by XXX means that even in pathalogical cases such as (max_val - min_val) == nlevels then the error at max_val is less than 1%. With a suitable comment in the fixed point code: Acked-by: Daniel Thompson Daniel. > + unsigned int margin = (max_val - min_val) / 20; /* 5% */ > + int i; > + > + for (i = 1; i < nlevels; i++) { > + unsigned int linear_value = min_val + ((i * slope) / 100); > + unsigned int delta = abs(linear_value - data->levels[i]); > + > + if (delta > margin) > + return false; > + } > + > + return true; > +} > + > static int pwm_backlight_initial_power_state(const struct pwm_bl_data *pb) > { > struct device_node *node = pb->dev->of_node; > @@ -567,6 +587,11 @@ static int pwm_backlight_probe(struct platform_device *pdev) > > pb->levels = data->levels; > } > + > + if (pwm_backlight_is_linear(data)) > + props.scale = BACKLIGHT_SCALE_LINEAR; > + else > + props.scale = BACKLIGHT_SCALE_NON_LINEAR; > } else if (!data->max_brightness) { > /* > * If no brightness levels are provided and max_brightness is >