Re: [PATCH] usbhid: Interpret 0 length ff effects as infinite (0xffff) length effects

From: Anssi Hannula
Date: Sun Oct 02 2022 - 07:35:00 EST

Paul Dino Jones kirjoitti 2022-10-02 01:16:

Hello, and thanks for looking into this!

I started using my Accuforce V2 sim wheel on Linux. I was getting no
response from racing simulators through wine, while native linux test
tools worked properly. It appears that many real-world applications will
send 0 as the replay length, which was resulting in the behavior I was
observing (nothing). The PID document does not explicitly state that 0
length effects should be interpreted as infinite, but it does mention
null effects being infinite effects.

Actually, it is Wine that is translating 0xFFFF from the application to 0x0000 for the Linux FF API:

Unfortunately "infinite" duration is not actually specified at all in our API currently.
input.h just says that the all durations are in msecs and values above 0x7fff cause unspecified results.

We have three places where the duration is handled:
- ff-memless: Considers 0 as infinite (in ml_get_combo_effect() and calculate_next_time()).
- iforce-ff: Just passes the duration to HW as-is - it is unknown what counts as infinite, if any.
- pidff: Just passes the duration to HW as-is, so using the unspecified-by-API 0xffff results in infinite duration (per USB HID PID spec).

So we probably want to specify some value to work as infinite, likely either 0 or 0xFFFF, and explicitly document that in input.h.
I suspect that ff-memless devices are currently the most popular, and e.g. Wine already assumes 0 is infinite, and I can't think of a reason to have an "actual" 0-duration effect, so I guess 0 would be the most sensible value.

Since iforce is an "ancestor" of HID PID of sorts, it may also support 0xffff = infinite.
I'll try to get hold of one to test, though it may take a couple of weeks...

This patch will interpret 0 length force feedback effects as 0xffff
(infinite) length effects, leaving other values for replay length

Signed-off-by: Paul Dino Jones <paul@xxxxxxxxxxxxxxxx>
drivers/hid/usbhid/hid-pidff.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hid/usbhid/hid-pidff.c b/drivers/hid/usbhid/hid-pidff.c
index 3b4ee21cd811..70653451c860 100644
--- a/drivers/hid/usbhid/hid-pidff.c
+++ b/drivers/hid/usbhid/hid-pidff.c
@@ -301,7 +301,7 @@ static void pidff_set_effect_report(struct
pidff_device *pidff,
pidff->set_effect_type->value[0] =
- pidff->set_effect[PID_DURATION].value[0] = effect->replay.length;
+ pidff->set_effect[PID_DURATION].value[0] = effect->replay.length ==
0 ? 0xffff : effect->replay.length;
pidff->set_effect[PID_TRIGGER_BUTTON].value[0] = effect->trigger.button;
pidff->set_effect[PID_TRIGGER_REPEAT_INT].value[0] =

Anssi Hannula