hog: ignore UHID-setup events and document why

The UHID_{START,STOP,OPEN,CLOSE} events should be ignored by us to avoid
triggering the warn(). It is safe to do that. Add few comments that
explain why we don't have to handle these.

diff --git a/profiles/input/hog.c b/profiles/input/hog.c
index 91c2802..4647fef 100644 (file)
--- a/profiles/input/hog.c
+++ b/profiles/input/hog.c
@@ -621,12 +621,43 @@ static gboolean uhid_event_cb(GIOChannel *io, GIOCondition cond,
        DBG("uHID event type %d received", ev.type);
 
        switch (ev.type) {
+       case UHID_START:
+       case UHID_STOP:
+               /* These are called to start and stop the underlying hardware.
+                * For HoG we open the channels before creating the device so
+                * the hardware is always ready. No need to handle these.
+                * Note that these are also called when the kernel switches
+                * between device-drivers loaded on the HID device. But we can
+                * simply keep the hardware alive during transitions and it
+                * works just fine.
+                * The kernel never destroys a device itself! Only an explicit
+                * UHID_DESTROY request can remove a device. */
+               break;
+       case UHID_OPEN:
+       case UHID_CLOSE:
+               /* OPEN/CLOSE are sent whenever user-space opens any interface
+                * provided by the kernel HID device. Whenever the open-count
+                * is non-zero we must be ready for I/O. As long as it is zero,
+                * we can decide to drop all I/O and put the device
+                * asleep This is optional, though. Moreover, some
+                * special device drivers are buggy in that regard, so
+                * maybe we just keep I/O always awake like HIDP in the
+                * kernel does. */
+               break;
        case UHID_OUTPUT:
        case UHID_FEATURE:
                forward_report(hogdev, &ev);
                break;
        case UHID_OUTPUT_EV:
-               DBG("uHID output event: type %d code %d value %d",
+               /* This is only sent by kernels prior to linux-3.11. It
+                * requires us to parse HID-descriptors in user-space to
+                * properly handle it. This is redundant as the kernel
+                * does it already. That's why newer kernels assemble
+                * the output-reports and send it to us via UHID_OUTPUT.
+                * We never implemented this, so we rely on users to use
+                * recent-enough kernels if they want this feature. No reason
+                * to implement this for older kernels. */
+               DBG("Unsupported uHID output event: type %d code %d value %d",
                        ev.u.output_ev.type, ev.u.output_ev.code,
                        ev.u.output_ev.value);
                break;