leds: Add driver for Qualcomm LPG

The Light Pulse Generator (LPG) is a PWM-block found in a wide range of
PMICs from Qualcomm. These PMICs typically comes with 1-8 LPG instances,
with their output being routed to various other components, such as
current sinks or GPIOs.

Each LPG instance can operate on fixed parameters or based on a shared
lookup-table, altering the duty cycle over time. This provides the means
for hardware assisted transitions of LED brightness.

A typical use case for the fixed parameter mode is to drive a PWM
backlight control signal, the driver therefor allows each LPG instance
to be exposed to the kernel either through the LED framework or the PWM
framework.

A typical use case for the LED configuration is to drive RGB LEDs in
smartphones etc, for which the driver supports multiple channels to be
ganged up to a MULTICOLOR LED. In this configuration the pattern
generators will be synchronized, to allow for multi-color patterns.

The idea of modelling this as a LED driver ontop of a PWM driver was
considered, but setting the properties related to patterns does not fit
in the PWM API. Similarly the idea of just duplicating the lower bits in
a PWM and LED driver separately was considered, but this would not allow
the PWM channels and LEDs to be configured on a per-board basis. The
driver implements the more complex LED interface, and provides a PWM
interface on the side of that, in the same driver.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Tested-by: Marijn Suijten <marijn.suijten@somainline.org>
[On the Sony Xperia Nile Discovery, SDM630]
Signed-off-by: Pavel Machek <pavel@ucw.cz>

1 files changed, 76 insertions, 0 deletions

diff --git a/Documentation/leds/leds-qcom-lpg.rst b/Documentation/leds/leds-qcom-lpg.rst
new file mode 100644
index 000000000000..f12416f02dd8
--- /dev/null
+++ b/Documentation/leds/leds-qcom-lpg.rst
@@ -0,0 +1,76 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============================
+Kernel driver for Qualcomm LPG
+==============================
+
+Description
+-----------
+
+The Qualcomm LPG can be found in a variety of Qualcomm PMICs and consists of a
+number of PWM channels, a programmable pattern lookup table and a RGB LED
+current sink.
+
+To facilitate the various use cases, the LPG channels can be exposed as
+individual LEDs, grouped together as RGB LEDs or otherwise be accessed as PWM
+channels. The output of each PWM channel is routed to other hardware
+blocks, such as the RGB current sink, GPIO pins etc.
+
+The each PWM channel can operate with a period between 27us and 384 seconds and
+has a 9 bit resolution of the duty cycle.
+
+In order to provide support for status notifications with the CPU subsystem in
+deeper idle states the LPG provides pattern support. This consists of a shared
+lookup table of brightness values and per channel properties to select the
+range within the table to use, the rate and if the pattern should repeat.
+
+The pattern for a channel can be programmed using the "pattern" trigger, using
+the hw_pattern attribute.
+
+/sys/class/leds/<led>/hw_pattern
+--------------------------------
+
+Specify a hardware pattern for a Qualcomm LPG LED.
+
+The pattern is a series of brightness and hold-time pairs, with the hold-time
+expressed in milliseconds. The hold time is a property of the pattern and must
+therefor be identical for each element in the pattern (except for the pauses
+described below).
+
+Simple pattern::
+
+    "255 500 0 500"
+
+        ^
+        |
+    255 +----+    +----+
+        |    |    |    |      ...
+      0 |    +----+    +----
+        +---------------------->
+        0    5   10   15     time (100ms)
+
+The LPG supports specifying a longer hold-time for the first and last element
+in the pattern, the so called "low pause" and "high pause".
+
+Low-pause pattern::
+
+    "255 1000 0 500 255 500 0 500"
+
+        ^
+        |
+    255 +--------+    +----+    +----+    +--------+
+        |        |    |    |    |    |    |        |      ...
+      0 |        +----+    +----+    +----+        +----
+        +----------------------------->
+        0    5   10   15  20   25   time (100ms)
+
+Similarily, the last entry can be stretched by using a higher hold-time on the
+last entry.
+
+In order to save space in the shared lookup table the LPG supports "ping-pong"
+mode, in which case each run through the pattern is performed by first running
+the pattern forward, then backwards. This mode is automatically used by the
+driver when the given pattern is a palindrome. In this case the "high pause"
+denotes the wait time before the pattern is run in reverse and as such the
+specified hold-time of the middle item in the pattern is allowed to have a
+different hold-time.