Just the context: SDL2 is creating Mouse (resp. Touch.) 'synthetic' events when the user move the real Touch (resp. Mouse) device. They appear with an id -1.
And only if the hints are enabled.

https://hg.libsdl.org/SDL/file/1b1fae168557/include/SDL_touch.h#l60
/* Used as the device ID for mouse events simulated with touch input */
#define SDL_TOUCH_MOUSEID ((Uint32)-1)
/* Used as the SDL_TouchID for touch events simulated with mouse input */
#define SDL_MOUSE_TOUCHID ((Sint64)-1)

https://hg.libsdl.org/SDL/file/1b1fae168557/include/SDL_hints.h#l350
See the hints:
SDL_HINT_MOUSE_TOUCH_EVENTS
SDL_HINT_TOUCH_MOUSE_EVENTS

...

you can add traces in SDLActivity.java: public boolean onTouch(View v, MotionEvent event):
trace the value of "touchDevId", it should be a real value, not -1.

( https://developer.android.com/reference/android/view/MotionEvent#getDeviceId() )

But maybe the simulator reports a virtual device. And maybe you can dump the "event.getSource()" too.

(If needed, other places:
src/video/android/SDL_androidtouch.c:  Android_OnTouchAndroid_OnTouch()
src/video/android/SDL_androidmouse.c:  Android_OnMouse()   )


Maybe there are stuff in the bug report:
"Android, some issue with InputDevice checks"
https://bugzilla.libsdl.org/show_bug.cgi?id=3958
I saw various strange checks.

If Android just send -1 as a valid id, the easiest patch for SDL2, would be to add:

"if (touchDevId < 0) touchDevId -= 1;"

So that -1 is restrict to the synthetic events.


https://hg.libsdl.org/SDL/file/1b1fae168557/android-project/app/src/main/java/org/libsdl/app/SDLActivity.java#l1952



Also, maybe please dump stuffs from "initTouch()"

https://hg.libsdl.org/SDL/file/1b1fae168557/android-project/app/src/main/java/org/libsdl/app/SDLActivity.java#l1220

I tried with Android 11 emulator rev8, and I also got -1 id when using adb shell.
to send mouse and touch events.

When using the real mouse or real touch screen, it worked though.


Here's a fix, can you double check:
https://hg.libsdl.org/SDL/rev/e1c4081a20db

I think just make sure that touchDevId is not equal to SDL_MOUSE_TOUCHID should be enough, so your fix should work. I will test today.

Though in theory it's possible on other platforms too, so maybe better use additional parameter, something like this?

int SDL_SendTouch(SDL_TouchID id, SDL_FingerID fingerid, SDL_Window * window, SDL_bool down, float x, float y, float pressure, bool simulated = false);

And are you able to test real gamepad device? It seems that it returns -1 too...

- We don't want to drop the -1 id coming from Android, it's valid even if it's a virtual device. we re-affect it to -2, and other n negative to n - 1.  We keep -1 for internal sdl synthetic id.
We could change the function, as well, not sure why we didn't do that.
  

I have no real android 11 so I used the emulator. Also I can't connect a gamepad and only use the shell commands.
id -1 seems to be the joystick named Virtual

This probably need to be changed to accept -1 :
https://hg.libsdl.org/SDL/file/6b1b46ba0a2d/android-project/app/src/main/java/org/libsdl/app/SDLControllerManager.java#l95

I see the joystick appearing, but I could get it working neither with android 11 nor old android.

The check here might be wrong
https://hg.libsdl.org/SDL/file/6b1b46ba0a2d/android-project/app/src/main/java/org/libsdl/app/SDLControllerManager.java#l95
could be:
int source = event.getSource();
if (source == InputDevice.SOURCE_JOYSTICK || source == InputDevice.SOURCE_GAMEPAD || source == InputDevice.SOURCE_DPAD) {

Just after, all MotionEvent.ACTION_* are not handle.
so, we might want to return "true" for ACTION_MOVE and false otherwise... 
and/or move the code onNativePad{Up,Down} from SDLActivity.
Not really sure ...

If you get Virtual -1, then it's the same what gets that guy on his Pixel 4 XL Phone with Android 11 and Logitech USB controller (see the screenshot).

And he already tested my quick fix with changing SDL_MOUSE_TOUCHID to different value and it fixed these mouse events for him. I can ask him to test your patch if you need to try it on real device.

I also tested your patch on android emulator and it works for me too.

Created attachment 4474 [details]
screenshot gamepad

Please don't use your patch (eg. 
As a quick fix we changed SDL_MOUSE_TOUCHID to different value, so that it doesn't conflict with the id=-1. )


Ask the user to test:
with this first patch in https://hg.libsdl.org/SDL/rev/e1c4081a20db 


Also, in addition, with this second patch (to be made) to accept -1:
https://hg.libsdl.org/SDL/file/6b1b46ba0a2d/android-project/app/src/main/java/org/libsdl/app/SDLControllerManager.java#l95

replace:
 if ((device == null) || (deviceId < 0)) {
by
 if (device == null) {

Please tell if all is ok with those two patches, thanks

NB: by default, even on non Android 10, and without gamepad connected, 
my phone report a device id with -1, 

Virtual id=-1
device id == 769  (512 + 256 + 1)

which is CLASS={ BUTTON } 
source(s): DPAD and KEYBOARD

So maybe your issue with gamepad, is totally different: HID not compiled ? gamecontrollerdb.h entry missing ?

I asked that guy to test your patches, will tell you when I will get some information.

The "if (device == null)" will not work for multiple gamepads if all gamepads will have device id -1 (would be useful eg. on Android TV). But not sure how to solve it... if device id = -1 then generate id based on device name?

And I wonder if it's maybe a bug in android, because it's a bit strange behavior... I had a quick look and there is one related commit

https://android.googlesource.com/platform/frameworks/native/+/0d8ed6e8ade20652cea20f579f40b1d698ce8fc0

But it's for injected events, so I'm not sure if/how it affects real touch/gamepad events.