Confusion 2: the raw_event callback of the hid_driver structure returns
an int, but over time non-negative values are ignored by
hid_input_report. This happened in the following change:
--------------------------------------------------
commit b1a1442a23776756b254b69786848a94d92445ba
Author: Jiri Kosina [off-list ref]
Date:   Mon Jun 3 11:27:48 2013 +0200

    HID: core: fix reporting of raw events
    
    hdrw->raw event can return three different return value types:
    
    - ret < 0   indicates that the hdrv driver found an error while parsing
    - ret == 0  indicates no error has been encountered, and the driver has
                processed the report
    - ret > 0   indicates that there was no parsing error, and the driver hasn't
                processed the event.
    
    Calling hid_report_raw_event() has to be called appropriately so that it
    reflects what has been done by ->raw_event() callback, otherwise we might
    updates of the in-kernel structure are lost upon arrival of the report, which
    is wrong.
    
    Reported-and-tested-by: Srinivas Pandruvada [off-list ref]
    Reported-and-tested-by: Daniel Leung [off-list ref]
    Signed-off-by: Jiri Kosina [off-list ref]

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index c272078..8f616bd 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c

@@ -1293,7 +1293,7 @@ int hid_input_report(struct hid_device *hid, int type, u8 *data, int size, int i
 
        if (hdrv && hdrv->raw_event && hid_match_report(hid, report)) {
                ret = hdrv->raw_event(hid, report, data, size);
-               if (ret != 0) {
+               if (ret < 0) {
                        ret = ret < 0 ? ret : 0;
                        goto unlock;
                }
--------------------------------------------------

So now it does not matter whether the raw_event method returns 0 or 1.
The documentation says:

    raw_event and event should return 0 on no action performed, 1 when
    no further processing should be done and negative on error

What does "no further processing" mean? I guess that the intention was
to signal from a HID driver to ignore the packet, but that the above
change was done to update the state and allow hidraw to pick up the
report.