clk: Add PWM clock driver

Some board designers, when running out of clock output pads, decide to
(mis)use PWM output pads to provide a clock to external components.
This driver supports this practice by providing an adapter between the
PWM and clock bindings in the device tree. As the PWM bindings specify
the period in the device tree, this is a fixed clock.

Tested-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Michael Turquette <mturquette@linaro.org>

1 files changed, 26 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/clock/pwm-clock.txt b/Documentation/devicetree/bindings/clock/pwm-clock.txt
new file mode 100644
index 000000000000..83db876b3b90
--- /dev/null
+++ b/Documentation/devicetree/bindings/clock/pwm-clock.txt
@@ -0,0 +1,26 @@
+Binding for an external clock signal driven by a PWM pin.
+
+This binding uses the common clock binding[1] and the common PWM binding[2].
+
+[1] Documentation/devicetree/bindings/clock/clock-bindings.txt
+[2] Documentation/devicetree/bindings/pwm/pwm.txt
+
+Required properties:
+- compatible : shall be "pwm-clock".
+- #clock-cells : from common clock binding; shall be set to 0.
+- pwms : from common PWM binding; this determines the clock frequency
+  via the period given in the PWM specifier.
+
+Optional properties:
+- clock-output-names : From common clock binding.
+- clock-frequency : Exact output frequency, in case the PWM period
+  is not exact but was rounded to nanoseconds.
+
+Example:
+        clock {
+                compatible = "pwm-clock";
+                #clock-cells = <0>;
+                clock-frequency = <25000000>;
+                clock-output-names = "mipi_mclk";
+                pwms = <&pwm2 0 40>; /* 1 / 40 ns = 25 MHz */
+        };