Recent Codemasters "DiRT" series games check for game controller changes every 2 seconds, which can cause a spike in frame times, leading to gameplay glitches. DirtFix suspends this background polling to eliminate the glitches it causes.

• DiRT Rally
• DiRT 4
• GRID Autosport

• DiRT Rally 2.0 (resolved in v1.10.1)

So far Codemasters seem unwilling to apply the known fix to older titles. If you're unhappy with that, be sure to let them know your opinion:

• Web: Codemasters Community - General Discussion
• Twitter: @dirtgame or @Codemasters
• Email: custservice@codemasters.com

• Download and run the installer for the latest version.
• Select the games to fix (or Add any DiRT game locations that are missing).
• Click OK to apply the settings.
• Launch the DiRT game as normal.

To change settings for a game, re-launch DirtFix from the Start Menu shortcut. To completely deactivate, uninstall from "Add or Remove Programs".

To upgrade an earlier version simply over-install with the latest version.

Before installing, the SteamVR profiler shows CPU frame time spikes at ~2 second intervals that often exceed the 11ms budget for 90fps in VR:

After installing, the spikes are eliminated, for smoother gameplay:

Below is a 3 second thread activity snapshot from DiRT Rally. It was recorded in VR after pressing Esc to pause the start of a rally. DirtFix was not installed.

The coloured markers show when each of the ~30 threads are actively executing. Thread 5532 is the DirectInput polling thread, and the solid blue block shows it polling for game controller changes using IDirectInput8::EnumDevices. The bulk of this time is spent parsing device INF files in cfgmg32!ParseNewInf. Note that when the input thread is active, most of the other game threads are not running!

Examining the end of idle gap we can see how long the threads were asleep, and what ultimately woke them up. In many cases the threads were waiting for each other, and one of them was waiting for the input thread:

The stack on the left shows the main game thread, which is currently dispatching windows messages. The DINPUT8 module is hooked into the window procedure as part of a IDirectInputDevice8::SetCooperativeLevel call, and before its processing it acquires the DINPUT8!g_crstDll critical section. This is owned by another thread, so it goes back to sleep to wait for it.

The stack on the right shows that the input thread owns the critical section during device enumeration, and it's only released when that completes. This lock contention is stalling the main game thread processing whenever the input thread is enumerating devices, which happens every ~2 seconds. This typically leads to multiple dropped frames.

While DirtFix suspending the input polling does avoid the problem, it would be better if polling only occurred when PnP notifications indicated a change. This could be done using RegisterDeviceNotification and handling WM_DEVICECHANGE.

DirtFix uses a shim version of a DirectInput module (dinput8.dll), which is copied into the game directory. This allows allows it to sit between the game and DirectInput API, and change its behaviour.

DirtFix passes through the first 2 calls to IDirectInput8::EnumDevices on each thread, before failing the call. The failure code causes the game to skip any post-processing of the results, so no further controller changes are seen. This both saves CPU time and avoids the main thead lock contention, to prevents the glitches.

Source code is available from the DirtFix project page on GitHub. Includes VS2019 solution, but requires detours.lib from vcpkg.

• hide and deactivate fixed versions DiRT Rally 2.0.
• added GRID Autosport to affected games list.

• auto-detect Microsoft Store games installed on drives other than C:

• auto-detect supported games bought from Oculus and Microsoft stores.
• added support for games in protected locations that require elevation.

• added support for hot-plugging controllers while the game is running.

• fixed re-entrant hook crash, possibly related to Logitech drivers.
  Special thanks to GitHub users @digitalmonk1973, @br3ndanp, and @caerphoto.

• added support for DiRT Rally 2.0 and DiRT 4.
• completely rewritten to avoid using thread injection.

• first public test release.

Simon Owen
https://github.com/simonowen/dirtfix