[PATCH net-next v1] ptp: document struct ptp_clock_request members

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

It's arguable most people interested in configuring a PPS signal
want it as external output, not as kernel input. PTP_CLK_REQ_PPS
is for input though. Add documentation to nudge readers into
the correct direction.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
Prompted by Richard's comment here:
https://lore.kernel.org/netdev/20180525170247.r4gn323udrucmyv6@localhost/
---
 include/linux/ptp_clock_kernel.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/ptp_clock_kernel.h b/include/linux/ptp_clock_kernel.h
index d3e8ba5c7125..0d47fd33b228 100644
--- a/include/linux/ptp_clock_kernel.h
+++ b/include/linux/ptp_clock_kernel.h
@@ -12,6 +12,19 @@
 #include <linux/pps_kernel.h>
 #include <linux/ptp_clock.h>
 
+/**
+ * struct ptp_clock_request - request PTP clock event
+ *
+ * @type:   The type of the request.
+ *	    EXTTS:  Configure external trigger timestamping
+ *	    PEROUT: Configure periodic output signal (e.g. PPS)
+ *	    PPS:    trigger internal PPS event for input
+ *	            into kernel PPS subsystem
+ * @extts:  describes configuration for external trigger timestamping.
+ *          This is only valid when event == PTP_CLK_REQ_EXTTS.
+ * @perout: describes configuration for periodic output.
+ *	    This is only valid when event == PTP_CLK_REQ_PEROUT.
+ */
 
 struct ptp_clock_request {
 	enum {
-- 
2.29.2