Linux-PWM Archive on lore.kernel.org
 help / color / Atom feed
From: Daniel Thompson <daniel.thompson@linaro.org>
To: Alexandru Stan <amstan@chromium.org>
Cc: "Thierry Reding" <thierry.reding@gmail.com>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Lee Jones" <lee.jones@linaro.org>,
	"Jingoo Han" <jingoohan1@gmail.com>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
	"Heiko Stuebner" <heiko@sntech.de>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Matthias Kaehlcke" <mka@chromium.org>,
	"Douglas Anderson" <dianders@chromium.org>,
	"Enric Balletbo i Serra" <enric.balletbo@collabora.com>,
	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
Date: Fri, 4 Sep 2020 12:27:32 +0100
Message-ID: <20200904112732.zdfzmjgpcds3jpzo@holly.lan> (raw)
In-Reply-To: <20200720212502.1.I4dcea1c90e9da3902d466033aa73351e19e49c49@changeid>

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 <dianders@chromium.org>
> Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
> Signed-off-by: Alexandru Stan <amstan@chromium.org>

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

  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-21  4:25 [PATCH 0/3] PWM backlight interpolation adjustments Alexandru Stan
2020-07-21  4:25 ` [PATCH 1/3] backlight: pwm_bl: Fix interpolation Alexandru Stan
2020-09-04 11:27   ` Daniel Thompson [this message]
2020-07-21  4:25 ` [PATCH 2/3] backlight: pwm_bl: Artificially add 0% during interpolation Alexandru Stan
2020-08-07  8:21   ` daniel
2020-08-13 13:45     ` Daniel Thompson
2020-09-04 11:38   ` Daniel Thompson
2020-09-07  7:50     ` Daniel Vetter
2020-09-09 14:45       ` Daniel Thompson
2020-09-09 15:03         ` Daniel Vetter
2020-09-10  7:47           ` Daniel Vetter
2020-09-09 18:42     ` Alexandru M Stan
2020-08-05 21:04 ` [PATCH 0/3] PWM backlight interpolation adjustments Alexandru M Stan

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200904112732.zdfzmjgpcds3jpzo@holly.lan \
    --to=daniel.thompson@linaro.org \
    --cc=amstan@chromium.org \
    --cc=b.zolnierkie@samsung.com \
    --cc=dianders@chromium.org \
    --cc=dri-devel@lists.freedesktop.org \
    --cc=enric.balletbo@collabora.com \
    --cc=heiko@sntech.de \
    --cc=jingoohan1@gmail.com \
    --cc=lee.jones@linaro.org \
    --cc=linux-fbdev@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-pwm@vger.kernel.org \
    --cc=mka@chromium.org \
    --cc=robh+dt@kernel.org \
    --cc=thierry.reding@gmail.com \
    --cc=u.kleine-koenig@pengutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-PWM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-pwm/0 linux-pwm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-pwm linux-pwm/ https://lore.kernel.org/linux-pwm \
		linux-pwm@vger.kernel.org
	public-inbox-index linux-pwm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-pwm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git