linux-kernel - Re: [PATCH v6 3/4] leds: core: add documentation for color extension

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Date:	Fri, 04 Mar 2016 10:05:23 +0100
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Heiner Kallweit <hkallweit1@...il.com>
Cc:	linux-leds@...r.kernel.org,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/4] leds: core: add documentation for color extension

Hi Heiner,

Please split the sysfs-class-led part to a separate patch.

On 03/02/2016 08:31 PM, Heiner Kallweit wrote:
> Document the color extension in Documentation/leds/leds-class.txt
>
> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
> ---
> v2:
> - introduced to patch series
> v3:
> - document extension in more detail
> v4:
> - Better explain why flag LED_SET_HUE_SAT is needed
> v5:
> - no changes
> v6:
> - add docu for this feature to Documentation/ABI/testing/sysfs-class-led
> ---
>   Documentation/ABI/testing/sysfs-class-led | 11 +++++++++++
>   Documentation/leds/leds-class.txt         | 27 ++++++++++++++++++++++-----
>   2 files changed, 33 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-class-led b/Documentation/ABI/testing/sysfs-class-led
> index 3646ec8..20357d5 100644
> --- a/Documentation/ABI/testing/sysfs-class-led
> +++ b/Documentation/ABI/testing/
> @@ -7,6 +7,17 @@ Description:
>   		have hardware brightness support so will just be turned on for
>   		non-zero brightness settings. The value is between 0 and
>   		/sys/class/leds/<led>/max_brightness.
> +		If a driver uses the RGB LED feature then this attribute is
> +		interpreted as a HSV color model value:
> +		<0000000F><HHHHHHHH><SSSSSSSS><VVVVVVVV>
> +		Usage of the least byte is identical to monochrome mode.
> +		Saturation can be 0-255 and hue 0-251 (Colour circle is mapped
> +		to 0-252). If hue and saturation both are 0 the current colour
> +		is preserved and only the brightness is set. This ensures
> +		backwards compatibility with monochrome mode, e.g. for
> +		led_set_brightness() calls from triggers.
> +		Flag F enables setting hue and saturation even if both are 0.
> +		For details and examples see Documentation/leds/leds-class.txt
>
>   What:		/sys/class/leds/<led>/max_brightness
>   Date:		March 2006
> diff --git a/Documentation/leds/leds-class.txt b/Documentation/leds/leds-class.txt
> index d406d98..b1e4cba 100644
> --- a/Documentation/leds/leds-class.txt
> +++ b/Documentation/leds/leds-class.txt
> @@ -8,6 +8,22 @@ LED is defined in max_brightness file. The brightness file will set the brightne
>   of the LED (taking a value 0-max_brightness). Most LEDs don't have hardware
>   brightness support so will just be turned on for non-zero brightness settings.
>
> +If a driver uses the colour extension of the LED core then the brightness

s/colour/RGB/

> +file can be used to set hue / saturation / value. The brightness value is
> +interpreted as: <0000000F><HHHHHHHH><SSSSSSSS><VVVVVVVV>
> +Usage of the least byte is identical to monochrome mode. Saturation can be
> +0-255 and hue 0-251 (Colour circle is mapped to 0-252).
> +If hue and saturation both are 0 the current colour is preserved and only
> +the brightness is set. This ensures backwards compatibility with monochrome
> +mode, e.g. for led_set_brightness() calls from triggers.
> +However we might want to have the option to set all HSV components, even
> +if hue and saturation both are 0 (e.g. via brightness sysfs attribute).
> +Use case: Set color to white (hue = 0 and saturation = 0).
> +Therefore the default behaviour can be overridden with flag F (LED_SET_HUE_SAT).
> +If this flag is set then hue and saturation are not checked for being 0 and
> +the color components are set unconditionally. Example:
> +0x010000ff sets the LED to white color with full brightness.
> +

I had to play a bit with RGB<->HSV conversion to get the flavour of
the HSV color model, and indeed, changing the brightness byte alone
influences only the intensity of the perceived color, despite that
all RGB components are changed. Effectively I find this description
sufficient.

>   The class also introduces the optional concept of an LED trigger. A trigger
>   is a kernel based source of led events. Triggers can either be simple or
>   complex. A simple trigger isn't configurable and is designed to slot into
> @@ -45,11 +61,12 @@ Is currently of the form:
>
>   "devicename:colour:function"
>
> -There have been calls for LED properties such as colour to be exported as
> -individual led class attributes. As a solution which doesn't incur as much
> -overhead, I suggest these become part of the device name. The naming scheme
> -above leaves scope for further attributes should they be needed. If sections
> -of the name don't apply, just leave that section blank.
> +If the colour extension is used hsv / rgb can be used instead of a specific

s/colour/RGB/

> +colour.  There have been calls for LED properties such as colour to be
> +exported as individual led class attributes. As a solution which doesn't
> +incur as much overhead, I suggest these become part of the device name.
> +The naming scheme above leaves scope for further attributes should they be
> +needed. If sections of the name don't apply, just leave that section blank.
>
>
>   Brightness setting API
>