[RFC v2 1/2] pwm: sifive: Add DT documentation for SiFive PWM Controller

[Yash Shah <yash.shah@sifive.com>]

DT documentation for PWM controller added with updated compatible
string.

Signed-off-by: Wesley W. Terpstra <wesley@sifive.com>
[Atish: Compatible string update]
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Yash Shah <yash.shah@sifive.com>
---
 .../devicetree/bindings/pwm/pwm-sifive.txt         | 44 ++++++++++++++++++++++
 1 file changed, 44 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/pwm/pwm-sifive.txt

diff --git a/Documentation/devicetree/bindings/pwm/pwm-sifive.txt b/Documentation/devicetree/bindings/pwm/pwm-sifive.txt
new file mode 100644
index 0000000..250d8ee
--- /dev/null
+++ b/Documentation/devicetree/bindings/pwm/pwm-sifive.txt
@@ -0,0 +1,44 @@
+SiFive PWM controller
+
+Unlike most other PWM controllers, the SiFive PWM controller currently only
+supports one period for all channels in the PWM. This is set globally in DTS.
+The period also has significant restrictions on the values it can achieve,
+which the driver rounds to the nearest achievable frequency.
+
+Required properties:
+- compatible: should be something similar to "sifive,<chip>-pwm" for
+	      the PWM as integrated on a particular chip, and
+	      "sifive,pwm<version>" for the general PWM IP block
+	      programming model. Supported compatible strings are:
+	      "sifive,fu540-c000-pwm" for the SiFive PWM v0 as
+	      integrated onto the SiFive FU540 chip, and "sifive,pwm0"
+	      for the SiFive PWM v0 IP block with no chip integration
+	      tweaks.
+- reg: physical base address and length of the controller's registers
+- clocks: The frequency the controller runs at
+- #pwm-cells: Should be 2.
+  The first cell is the PWM channel number
+  The second cell is the PWM polarity
+- sifive,approx-period: the driver will get as close to this period as it can
+- interrupts: one interrupt per PWM channel
+
+PWM RTL that corresponds to the IP block version numbers can be found
+here:
+
+https://github.com/sifive/sifive-blocks/tree/master/src/main/scala/devices/pwm
+
+Further information on the format of the IP
+block-specific version numbers can be found in
+Documentation/devicetree/bindings/sifive/sifive-blocks-ip-versioning.txt
+
+Examples:
+
+pwm:  pwm@10020000 {
+	compatible = "sifive,fu540-c000-pwm","sifive,pwm0";
+	reg = <0x0 0x10020000 0x0 0x1000>;
+	clocks = <&tlclk>;
+	interrupt-parent = <&plic>;
+	interrupts = <42 43 44 45>;
+	#pwm-cells = <2>;
+	sifive,approx-period = <1000000>;
+};
-- 
1.9.1