pwm: Document signal polarity convention

The PWM subsystem defines normal and inversed PWM signal polarity in an
unambiguous way. In addition to the documentation in the linux/pwm.h
header file, add a paragraph in Documentation/pwm.txt because people are
likely to look there for guidance.

Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/pwm.txt b/Documentation/pwm.txt
index 93cb979..0527f61 100644
--- a/Documentation/pwm.txt
+++ b/Documentation/pwm.txt
@@ -97,6 +97,13 @@ pwm_chip as argument which provides a description of the PWM chip, the
 number of PWM devices provided by the chip and the chip-specific
 implementation of the supported PWM operations to the framework.
 
+When implementing polarity support in a PWM driver, make sure to respect the
+signal conventions in the PWM framework. By definition, normal polarity
+characterizes a signal starts high for the duration of the duty cycle and
+goes low for the remainder of the period. Conversely, a signal with inversed
+polarity starts low for the duration of the duty cycle and goes high for the
+remainder of the period.
+
 Locking
 -------