79 lines
2.9 KiB
ReStructuredText
79 lines
2.9 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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==============================
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Kernel driver for Qualcomm LPG
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==============================
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Description
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-----------
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The Qualcomm LPG can be found in a variety of Qualcomm PMICs and consists of a
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number of PWM channels, a programmable pattern lookup table and a RGB LED
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current sink.
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To facilitate the various use cases, the LPG channels can be exposed as
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individual LEDs, grouped together as RGB LEDs or otherwise be accessed as PWM
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channels. The output of each PWM channel is routed to other hardware
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blocks, such as the RGB current sink, GPIO pins etc.
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The each PWM channel can operate with a period between 27us and 384 seconds and
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has a 9 bit resolution of the duty cycle.
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In order to provide support for status notifications with the CPU subsystem in
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deeper idle states the LPG provides pattern support. This consists of a shared
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lookup table of brightness values and per channel properties to select the
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range within the table to use, the rate and if the pattern should repeat.
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The pattern for a channel can be programmed using the "pattern" trigger, using
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the hw_pattern attribute.
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/sys/class/leds/<led>/hw_pattern
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--------------------------------
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Specify a hardware pattern for a Qualcomm LPG LED.
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The pattern is a series of brightness and hold-time pairs, with the hold-time
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expressed in milliseconds. The hold time is a property of the pattern and must
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therefor be identical for each element in the pattern (except for the pauses
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described below). As the LPG hardware is not able to perform the linear
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transitions expected by the leds-trigger-pattern format, each entry in the
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pattern must be followed a zero-length entry of the same brightness.
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Simple pattern::
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"255 500 255 0 0 500 0 0"
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^
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255 +----+ +----+
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0 | +----+ +----
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+---------------------->
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0 5 10 15 time (100ms)
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The LPG supports specifying a longer hold-time for the first and last element
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in the pattern, the so called "low pause" and "high pause".
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Low-pause pattern::
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"255 1000 255 0 0 500 0 0 255 500 255 0 0 500 0 0"
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^
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255 +--------+ +----+ +----+ +--------+
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| | | | | | | | ...
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0 | +----+ +----+ +----+ +----
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+----------------------------->
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0 5 10 15 20 25 time (100ms)
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Similarily, the last entry can be stretched by using a higher hold-time on the
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last entry.
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In order to save space in the shared lookup table the LPG supports "ping-pong"
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mode, in which case each run through the pattern is performed by first running
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the pattern forward, then backwards. This mode is automatically used by the
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driver when the given pattern is a palindrome. In this case the "high pause"
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denotes the wait time before the pattern is run in reverse and as such the
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specified hold-time of the middle item in the pattern is allowed to have a
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different hold-time.
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