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Bug 4830

Summary: Incorrect joystick button state after SYN_DROPPED
Product: SDL Reporter: Mathieu Eyraud <meyraud705>
Component: joystickAssignee: Sam Lantinga <slouken>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: icculus, sezeroz
Version: 2.0.10Keywords: target-2.0.14
Hardware: All   
OS: Linux   

Description Mathieu Eyraud 2019-10-17 15:14:46 UTC 
After receiving SYN_DROPPED event from a linux system, axis values are re-read but not the button values.
https://hg.libsdl.org/SDL/file/d1a5e3cd0ae8/src/joystick/linux/SDL_sysjoystick.c#l949
This may cause joystick button values to be in incorrect state and some joystick events to be missing.
Comment 1 Sam Lantinga 2019-10-19 20:05:09 UTC 
Good catch. Do you have a patch you've tested that polls button values as well?
Comment 2 Mathieu Eyraud 2019-10-19 21:40:04 UTC 
No, I Don't have a patch and I don't know linux's syscall to make one.
Comment 3 Ryan C. Gordon 2020-06-28 20:29:39 UTC 
This is fixed in https://hg.libsdl.org/SDL/rev/2173f488608f

Now we poll hats and buttons in addition to axes when we get a SYN_DROPPED, and we also follow the proper evdev procedure of throwing away the rest of the input packet (that is, until we see a SYN_REPORT) after a SYN_DROPPED event.

This can be tested by building the latest testjoystick and pressing the 'L' key to sleep for 3 seconds--and going wild with the joystick in that time--to overwhelm the kernel's event buffer. SDL will now recover and resync with the current joystick state.

--ryan.