These changes allow for a working experience out of the box with SDL_GameController.

libdrc_input_mods.diff

diff --git a/src/uinput-feeder.cpp b/src/uinput-feeder.cpp
index 6288a06..b832fac 100644
--- a/src/uinput-feeder.cpp
+++ b/src/uinput-feeder.cpp
@@ -67,11 +67,27 @@ int InitUinput() {
   dev.id.bustype = BUS_VIRTUAL;
 
   // Enable joysticks: 4 axis (2 for each stick).
+#ifdef DRC_RAW_AXIS_VALUES
   for (int i = 0; i < 4; ++i) {
     dev.absmin[i] = -kStickRadius;
     dev.absmax[i] = kStickRadius;
     ioctl(uinput_fd, UI_SET_ABSBIT, i);
   }
+#else
+  // These values are largely fudged based on my hardware's raw values. -flibit
+  dev.absmin[0] = -108;
+  dev.absmax[0] =  108;
+  ioctl(uinput_fd, UI_SET_ABSBIT, 0);
+  dev.absmin[1] = -108;
+  dev.absmax[1] =  108;
+  ioctl(uinput_fd, UI_SET_ABSBIT, 1);
+  dev.absmin[2] =  -98;
+  dev.absmax[2] =   98;
+  ioctl(uinput_fd, UI_SET_ABSBIT, 2);
+  dev.absmin[3] =  -98;
+  dev.absmax[3] =   98;
+  ioctl(uinput_fd, UI_SET_ABSBIT, 3);
+#endif
 
   // Enable 32 buttons (we only use 19 of these, but it makes things easier).
   for (int i = 0; i < 32; ++i) {
@@ -133,6 +149,8 @@ void FeedDrcInputToUinput(const InputData& inp) {
     evt[i].type = EV_ABS;
     evt[i].code = i - 32;
     evt[i].value = static_cast<int>(sticks[i - 32] * kStickRadius);
+    if (((i - 32) % 2) == 1)
+      evt[i].value *= -1; // Invert the Y axes.
     memcpy(&evt[i].time, &tv, sizeof (tv));
   }
 

SDL_GameController.txt

export SDL_GAMECONTROLLERCONFIG="0600000057696920552047616d655000,Wii U GamePad (libdrc),platform:Linux,x:b12,a:b14,b:b15,y:b13,back:b2,guide:b1,start:b3,dpleft:b11,dpdown:b8,dpright:b10,dpup:b9,leftshoulder:b5,lefttrigger:b7,rightshoulder:b4,righttrigger:b6,leftstick:b23,rightstick:b22,leftx:a0,lefty:a1,rightx:a2,righty:a3,"