joystick: Use a better heuristic to guess what is a joystick

Previously we only checked for at least one button or key and at least the X and Y absolute axes, but this has both false positives and false negatives. Graphics tablets, trackpads and touchscreens all have buttons and absolute X and Y axes, but we don't want to detect those as joysticks. On normal Linux systems ordinary users do not have access to these device nodes, but members of the 'input' group do. Conversely, some game controllers only have digital buttons and no analogue axes (the Nintendo Wiimote is an example), and some have axes and no buttons (steering wheels or flight simulator rudders might not have buttons). Use the more elaborate heuristic factored out from SDL's udev code path to handle these cases. In an ideal world we could use exactly the same heuristic as udev's input_id builtin, but that isn't under a suitable license for inclusion in SDL, so we have to use a parallel implementation of something vaguely similar. Signed-off-by: Simon McVittie <smcv@collabora.com>
libsdl-org · Nov 12, 2020 · fdd945f · fdd945f
1 parent 8db3171
commit fdd945f
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Showing 3 changed files with 11 additions and 7 deletions.
diff --git a/src/core/linux/SDL_evdev_capabilities.c b/src/core/linux/SDL_evdev_capabilities.c
@@ -22,7 +22,7 @@
 
 #include "SDL_evdev_capabilities.h"
 
-#if HAVE_LIBUDEV_H
+#if HAVE_LIBUDEV_H || defined(SDL_JOYSTICK_LINUX)
 
 extern int
 SDL_EVDEV_GuessDeviceClass(unsigned long bitmask_ev[NBITS(EV_MAX)],

diff --git a/src/core/linux/SDL_evdev_capabilities.h b/src/core/linux/SDL_evdev_capabilities.h
@@ -25,7 +25,7 @@
 #ifndef SDL_evdev_capabilities_h_
 #define SDL_evdev_capabilities_h_
 
-#if HAVE_LIBUDEV_H
+#if HAVE_LIBUDEV_H || defined(SDL_JOYSTICK_LINUX)
 
 #include <linux/input.h>
 
@@ -51,7 +51,7 @@ extern int SDL_EVDEV_GuessDeviceClass(unsigned long bitmask_ev[NBITS(EV_MAX)],
                                       unsigned long bitmask_key[NBITS(KEY_MAX)],
                                       unsigned long bitmask_rel[NBITS(REL_MAX)]);
 
-#endif /* HAVE_LIBUDEV_H */
+#endif /* HAVE_LIBUDEV_H || defined(SDL_JOYSTICK_LINUX) */
 
 #endif /* SDL_evdev_capabilities_h_ */
 

diff --git a/src/joystick/linux/SDL_sysjoystick.c b/src/joystick/linux/SDL_sysjoystick.c
@@ -160,19 +160,23 @@ GuessIsJoystick(int fd)
     unsigned long evbit[NBITS(EV_MAX)] = { 0 };
     unsigned long keybit[NBITS(KEY_MAX)] = { 0 };
     unsigned long absbit[NBITS(ABS_MAX)] = { 0 };
+    unsigned long relbit[NBITS(REL_MAX)] = { 0 };
+    int devclass;
 
     if ((ioctl(fd, EVIOCGBIT(0, sizeof(evbit)), evbit) < 0) ||
         (ioctl(fd, EVIOCGBIT(EV_KEY, sizeof(keybit)), keybit) < 0) ||
+        (ioctl(fd, EVIOCGBIT(EV_REL, sizeof(relbit)), relbit) < 0) ||
         (ioctl(fd, EVIOCGBIT(EV_ABS, sizeof(absbit)), absbit) < 0)) {
         return (0);
     }
 
-    if (!(test_bit(EV_KEY, evbit) && test_bit(EV_ABS, evbit) &&
-          test_bit(ABS_X, absbit) && test_bit(ABS_Y, absbit))) {
-        return 0;
+    devclass = SDL_EVDEV_GuessDeviceClass(evbit, absbit, keybit, relbit);
+
+    if (devclass & SDL_UDEV_DEVICE_JOYSTICK) {
+        return 1;
     }
 
-    return 1;
+    return 0;
 }
 
 static int