[PATCH] HID: usbhid: skip interrupt IN polling for devices with no input reports

[Ahmed Yaseen <yaseen@ghoul.dev>]

usbhid starts polling a device's interrupt IN endpoint on open
(usbhid_open() -> hid_start_in()). If the report descriptor declares no
input reports there is nothing to read there, so the poll is useless,
and on some composite devices it is also harmful.

The ASUS ROG N-Key keyboards expose a second, input-less interface used
only for RGB control via feature reports. Opening its hidraw node (any
hidraw reader does, including SDL/Steam Input or a plain cat) starts the
pointless IN poll and keypress reports on the keyboard interface get
dropped for as long as the node stays open: a lost key-down drops a
letter, a lost key-up leaves the key stuck. usbmon shows the dropped
reports never reach the URB layer.

The useless poll itself is long-standing; commit 4ac74ea68f64 ("HID:
asus: early return for ROG devices") is what exposes it on these
devices by keeping the input-less interface alive instead of ejecting
it, so its hidraw node can be opened and the poll started.

Skip the poll in usbhid_open() when the device has no input reports.
Feature reports and hidraw output keep working over the control and OUT
endpoints, so the interface is otherwise unaffected.

Fixes: 4ac74ea68f64 ("HID: asus: early return for ROG devices")
Tested-by: Kerim Kabirov <the.privat33r+linux@pm.me>
Tested-by: GameBurrow <gameburrow@pm.me>
Signed-off-by: Ahmed Yaseen <yaseen@ghoul.dev>
---
 drivers/hid/usbhid/hid-core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
index 96b0181cf819..90a8b34d9305 100644
--- a/drivers/hid/usbhid/hid-core.c
+++ b/drivers/hid/usbhid/hid-core.c
@@ -688,7 +688,8 @@ static int usbhid_open(struct hid_device *hid)
 
 	set_bit(HID_OPENED, &usbhid->iofl);
 
-	if (hid->quirks & HID_QUIRK_ALWAYS_POLL) {
+	if ((hid->quirks & HID_QUIRK_ALWAYS_POLL) ||
+	    list_empty(&hid->report_enum[HID_INPUT_REPORT].report_list)) {
 		res = 0;
 		goto Done;
 	}
-- 
2.54.0