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[86.9.19.6]) by smtp.gmail.com with ESMTPSA id 124sm10860162wmd.31.2020.09.04.04.27.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Sep 2020 04:27:34 -0700 (PDT) Date: Fri, 4 Sep 2020 12:27:32 +0100 From: Daniel Thompson To: Alexandru Stan Cc: Thierry Reding , Uwe =?utf-8?Q?Kleine-K=C3=B6nig?= , Lee Jones , Jingoo Han , Bartlomiej Zolnierkiewicz , Heiko Stuebner , Rob Herring , Matthias Kaehlcke , Douglas Anderson , Enric Balletbo i Serra , dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org Subject: Re: [PATCH 1/3] backlight: pwm_bl: Fix interpolation Message-ID: <20200904112732.zdfzmjgpcds3jpzo@holly.lan> References: <20200721042522.2403410-1-amstan@chromium.org> <20200720212502.1.I4dcea1c90e9da3902d466033aa73351e19e49c49@changeid> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200720212502.1.I4dcea1c90e9da3902d466033aa73351e19e49c49@changeid> Sender: linux-pwm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pwm@vger.kernel.org On Mon, Jul 20, 2020 at 09:25:20PM -0700, Alexandru Stan wrote: > 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. > > Example settings: > brightness-levels = <0 1 2 4 8 16 32 64 128 256>; > num-interpolated-steps = <16>; > > The distances between 1 2 4 and 8 would be 1, and only starting with 16 > it would start to interpolate properly. > > Let's change it so there's always interpolation happening, even if > there's no enough points available (read: values in the table would > appear more than once). This should match the expected behavior much > more closely. > > Reviewed-by: Douglas Anderson > Reviewed-by: Matthias Kaehlcke > Signed-off-by: Alexandru Stan Apologies for the delay. Patch 2/3 meant I had some thinking to do... and then the holiday's took their toll. Overall this looks good, just some quibbles about broken 64-bit maths. > --- > > drivers/video/backlight/pwm_bl.c | 70 ++++++++++++++------------------ > 1 file changed, 31 insertions(+), 39 deletions(-) > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c > index 82b8d7594701..5193a72305a2 100644 > --- a/drivers/video/backlight/pwm_bl.c > +++ b/drivers/video/backlight/pwm_bl.c > @@ -235,8 +235,7 @@ static int pwm_backlight_parse_dt(struct device *dev, > struct platform_pwm_backlight_data *data) > { > struct device_node *node = dev->of_node; > - unsigned int num_levels = 0; > - unsigned int levels_count; > + unsigned int num_levels; > unsigned int num_steps = 0; > struct property *prop; > unsigned int *table; > @@ -265,12 +264,11 @@ static int pwm_backlight_parse_dt(struct device *dev, > if (!prop) > return 0; > > - data->max_brightness = length / sizeof(u32); > + num_levels = length / sizeof(u32); > > /* read brightness levels from DT property */ > - if (data->max_brightness > 0) { > - size_t size = sizeof(*data->levels) * data->max_brightness; > - unsigned int i, j, n = 0; > + if (num_levels > 0) { > + size_t size = sizeof(*data->levels) * num_levels; > > data->levels = devm_kzalloc(dev, size, GFP_KERNEL); > if (!data->levels) > @@ -278,7 +276,7 @@ static int pwm_backlight_parse_dt(struct device *dev, > > ret = of_property_read_u32_array(node, "brightness-levels", > data->levels, > - data->max_brightness); > + num_levels); > if (ret < 0) > return ret; > > @@ -303,7 +301,13 @@ static int pwm_backlight_parse_dt(struct device *dev, > * between two points. > */ > if (num_steps) { > - if (data->max_brightness < 2) { > + unsigned int num_input_levels = num_levels; > + unsigned int i; > + u32 x1, x2, x; > + u32 y1, y2; > + s64 dx, dy; dx should be 32-bit. It will be truncated to 32-bit when passed to div_s64() so this type is actively misleading about how the maths works. > + > + if (num_input_levels < 2) { > dev_err(dev, "can't interpolate\n"); > return -EINVAL; > } > @@ -313,14 +317,7 @@ static int pwm_backlight_parse_dt(struct device *dev, > * taking in consideration the number of interpolated > * steps between two levels. > */ > - for (i = 0; i < data->max_brightness - 1; i++) { > - if ((data->levels[i + 1] - data->levels[i]) / > - num_steps) > - num_levels += num_steps; > - else > - num_levels++; > - } > - num_levels++; > + num_levels = (num_input_levels - 1) * num_steps + 1; > dev_dbg(dev, "new number of brightness levels: %d\n", > num_levels); > > @@ -332,24 +329,25 @@ static int pwm_backlight_parse_dt(struct device *dev, > table = devm_kzalloc(dev, size, GFP_KERNEL); > if (!table) > return -ENOMEM; > - > - /* Fill the interpolated table. */ > - levels_count = 0; > - for (i = 0; i < data->max_brightness - 1; i++) { > - value = data->levels[i]; > - n = (data->levels[i + 1] - value) / num_steps; > - if (n > 0) { > - for (j = 0; j < num_steps; j++) { > - table[levels_count] = value; > - value += n; > - levels_count++; > - } > - } else { > - table[levels_count] = data->levels[i]; > - levels_count++; > + /* > + * Fill the interpolated table[x] = y > + * by draw lines between each (x1, y1) to (x2, y2). > + */ > + dx = num_steps; > + for (i = 0; i < num_input_levels - 1; i++) { > + x1 = i * dx; > + x2 = x1 + dx; > + y1 = data->levels[i]; > + y2 = data->levels[i + 1]; > + dy = y2 - y1; This is an u32 expression being assigned to a s64. I could be rusty on my fixed point maths but won't this promote too late for the 64-bitness of dy to be useful? Daniel. > + > + for (x = x1; x < x2; x++) { > + table[x] = y1 + > + div_s64(dy * (x - x1), dx); > } > } > - table[levels_count] = data->levels[i]; > + /* Fill in the last point, since no line starts here. */ > + table[x2] = y2; > > /* > * As we use interpolation lets remove current > @@ -358,15 +356,9 @@ static int pwm_backlight_parse_dt(struct device *dev, > */ > devm_kfree(dev, data->levels); > data->levels = table; > - > - /* > - * Reassign max_brightness value to the new total number > - * of brightness levels. > - */ > - data->max_brightness = num_levels; > } > > - data->max_brightness--; > + data->max_brightness = num_levels - 1; > } > > return 0; > -- > 2.27.0