Bug 217742 – HIDGamepadProvider adds an extra 50ms to all inputs

• Status: RESOLVED
• Resolution: FIXED
• Component: UI Events

[Justin Uberti]

When measuring Safari's button-to-photon latency with html5gamepad.com, I noticed that Safari has roughly double the latency of Chrome for wired USB gamepads (108ms vs 56ms).

I investigated this a bit and found this code in HIDGamepadProvider which I believe accounts for the difference:
https://github.com/WebKit/webkit/blob/950143da027e80924b4bb86defa8a3f21fd3fb1e/Source/WebCore/platform/gamepad/mac/HIDGamepadProvider.cpp#L39

50ms is enormous latency for a gamepad - gaming mice charge a premium in order to deliver ~10ms latency savings.

Could this delay be reduced or eliminated? The rationale around this code isn't entirely clear to me:

    // This isActive check is necessary as we want to delay input notifications from the time of the first input,
    // and not push the notification out on every subsequent input.
    if (!m_inputNotificationTimer.isActive())
        m_inputNotificationTimer.startOneShot(inputNotificationDelay);

[Radar WebKit Bug Importer]

<rdar://problem/70315838>

[Brady Eidson]

Definitely room for improvement here.

[Justin Uberti]

Note also that GameControllerGamepadProvider also has a 16ms notification delay:
https://github.com/WebKit/webkit/blob/950143da027e80924b4bb86defa8a3f21fd3fb1e/Source/WebCore/platform/gamepad/cocoa/GameControllerGamepadProvider.mm#L45

Could anyone comment on the intent behind these delays?

[Brady Eidson]

Created attachment 432706 [details]
Patch

[EWS]

Committed r279480 (239333@main): <https://commits.webkit.org/239333@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 432706 [details].