[v4,2/2] documention: leds: Add multicolor class documentation

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+
+Multi Color LED handling under Linux
+=================================================
+
+Author: Dan Murphy <dmurphy@ti.com>
+
+Description
+-----------
+There are varying monochrome LED colors available for application.  These
+LEDs can be used as a single use case LED or can be mixed with other color
+LEDs to produce the full spectrum of color.  Color LEDs that are grouped
+can be presented under a single LED node with individual color control.
+The multicolor class groups these LEDs and allows dynamically setting the value
+of a single LED or setting the brightness values of the LEDs in the group and
+updating the LEDs virtually simultaneously.
+
+Multicolor Class Control
+-------------------------
+The multicolor class presents the LED groups under a directory called "colors".
+This directory is a child under the LED parent node created by the led_class
+framework.  The led_class framework is documented in led-class.txt within this
+documentation directory.
+
+Each colored LED is given its own directory.  These directories can be, but not
+limited to red, green, blue, white, amber, yellow and violet.  Under these
+directories the brightness and max_brightness files are presented for each LED.
+
+Under the "colors" directory there are two files created: "sync" and
+"sync_enable". The sync_enable file controls whether the LED brightness
+value is set real time or if the LED brightness value setting is deferred until
+the "sync" file is written.  If sync_enable is set then writing to each LED
+"brightness" file will store the brightness value.  Once the "sync" file is
+written then each LED color defined in the node will write the brightness of
+the LED in the device driver.
+
+If "sync_enable" is not set then writing the brightness value of the LED to the
+device driver is done immediately.  Writing the "sync" file has no affect.
+
+Directory Layout Example
+------------------------
+root:/sys/class/leds/rgb:grouped_leds# ls -lR colors/
+colors/:
+drwxr-xr-x    2 root     root             0 Jun 28 20:21 blue
+drwxr-xr-x    2 root     root             0 Jun 28 20:21 green
+drwxr-xr-x    2 root     root             0 Jun 28 20:21 red
+--w-------    1 root     root          4096 Jun 28 20:21 sync
+-rw-------    1 root     root          4096 Jun 28 20:22 sync_enable
+
+colors/blue:
+-rw-------    1 root     root          4096 Jun 28 20:21 brightness
+-r--------    1 root     root          4096 Jun 28 20:27 max_brightness
+
+colors/green:
+-rw-------    1 root     root          4096 Jun 28 20:22 brightness
+-r--------    1 root     root          4096 Jun 28 20:27 max_brightness
+
+colors/red:
+-rw-------    1 root     root          4096 Jun 28 20:21 brightness
+-r--------    1 root     root          4096 Jun 28 20:27 max_brightness
+
+Example of Writing LEDs with Sync Enabled
+-----------------------------------------
+Below the red, green and blue LEDs are set to corresponding values.  These
+values are stored and not written until the sync file is written.
+
+cd /sys/class/leds/rgb:grouped_leds/colors
+
+echo 1 > sync_enable
+
+echo 100 > red/brightness
+echo 80 > green/brightness
+echo 180 > blue/brightness
+
+* LED device driver has not been updated and the LED states have not changed.
+* Writing the LED brightness files again will only change the stored value and
+* not the device driver value.
+
+echo 1 > sync
+
+* LED device driver has been updated; the LEDs should present the brightness
+* levels that have been set.  Since sync_enable is still enabled writing to the
+* LED brightness files will not change the current brightnesses.
+
+Example of Writing LEDs with Sync Disabled
+------------------------------------------
+Below the values of each LED are written to the device driver immediately upon
+request.
+
+cd /sys/class/leds/rgb:grouped_leds/colors
+
+echo 0 > sync_enable
+
+echo 100 > red/brightness // Red LED should be on with the current brightness
+echo 80 > green/brightness // Green LED should be on with the current brightness
+echo 180 > blue/brightness // Blue LED should be on with the current brightness
+.
+.
+.
+echo 0 > green/brightness // Green LED should be off
+
+Multicolor Class Brightness Control
+-----------------------------------
+The multicolor class will redirect the device drivers call back
+function for brightness control to the multicolor class brightness
+control function.
+
+The brightness level for each LED is calculated based on the color LED
+brightness setting divided by the color LED max brightness setting multiplied by
+the requested value.
+
+led_brightness = requested_value*(led_color_brightness/led_color_max_brightness)
+
+Example:
+Three LEDs are present in the group as defined in "Directory Layout Example"
+within this document.
+
+A user first writes the color LED brightness file with the brightness level that
+is neccesary to achieve a blueish violet output from the RGB LED group.
+
+echo 138 > /sys/class/leds/rgb:grouped_leds/red/brightness
+echo 43 > /sys/class/leds/rgb:grouped_leds/green/brightness
+echo 226 > /sys/class/leds/rgb:grouped_leds/blue/brightness
+
+red -
+	brightness = 138
+	max_brightness = 255
+green -
+	brightness = 43
+	max_brightness = 255
+blue -
+	brightness = 226
+	max_brightness = 255
+
+The user can control the brightness of that RGB group by writing the parent
+'brightness' control.  Assuming a parent max_brightness of 255 the user may want
+to dim the LED color group to half.  The user would write a value of 128 to the
+parent brightness file then the values written to each LED will be adjusted
+base on this value
+
+cat /sys/class/leds/rgb:grouped_leds/max_brightness
+255
+echo 128 > /sys/class/leds/rgb:grouped_leds/brightness
+
+adjusted_red_value = 128 * (138/255) = 69
+adjusted_green_value = 128 * (43/255) = 21
+adjusted_blue_value = 128 * (226/255) = 113
+
+Reading the parent brightness file will return the current brightness value of
+the color LED group.
+
+cat /sys/class/leds/rgb:grouped_leds/max_brightness
+255
+
+echo 128 > /sys/class/leds/rgb:grouped_leds/brightness
+
+cat /sys/class/leds/rgb:grouped_leds/max_brightness
+128
+
+