Fix Linux haptic scaling, add 2.1 FIXME

include/SDL_haptic.h

@@ -117,6 +117,17 @@
 extern "C" {
 #endif /* __cplusplus */
 
+/* FIXME: For SDL 2.1, adjust all the magnitude variables to be Uint16 (0xFFFF).
+ *
+ * At the moment the magnitude variables are mixed between signed/unsigned, and
+ * it is also not made clear that ALL of those variables expect a max of 0x7FFF.
+ *
+ * Some platforms may have higher precision than that (Linux FF, Windows XInput)
+ * so we should fix the inconsistency in favor of higher possible precision,
+ * adjusting for platforms that use different scales.
+ * -flibit
+ */
+
 /**
  *  \typedef SDL_Haptic
  *

src/haptic/linux/SDL_syshaptic.c

@@ -801,7 +801,8 @@ SDL_SYS_ToFFEffect(struct ff_effect *dest, SDL_HapticEffect * src)
         else if (periodic->type == SDL_HAPTIC_SAWTOOTHDOWN)
             dest->u.periodic.waveform = FF_SAW_DOWN;
         dest->u.periodic.period = CLAMP(periodic->period);
-        dest->u.periodic.magnitude = periodic->magnitude;
+        /* Linux expects 0-65535, so multiply by 2 */
+        dest->u.periodic.magnitude = CLAMP(periodic->magnitude) * 2;
         dest->u.periodic.offset = periodic->offset;
         /* Linux phase is defined in interval "[0x0000, 0x10000[", corresponds with "[0deg, 360deg[" phase shift. */
         dest->u.periodic.phase = ((Uint32)periodic->phase * 0x10000U) / 36000;
@@ -908,9 +909,9 @@ SDL_SYS_ToFFEffect(struct ff_effect *dest, SDL_HapticEffect * src)
         dest->trigger.button = 0;
         dest->trigger.interval = 0;
 
-        /* Rumble */
-        dest->u.rumble.strong_magnitude = leftright->large_magnitude;
-        dest->u.rumble.weak_magnitude = leftright->small_magnitude;
+        /* Rumble (Linux expects 0-65535, so multiply by 2) */
+        dest->u.rumble.strong_magnitude = CLAMP(leftright->large_magnitude) * 2;
+        dest->u.rumble.weak_magnitude = CLAMP(leftright->small_magnitude) * 2;
 
         break;