drm/mgag200: Fix big-endian support

Unlike the original, deleted Matrox mga driver, the new mgag200 driver
has the XRGB frame-buffer byte swapped on big-endian "RISC"
systems. Fix by enabling byte swapping "PowerPC" OPMODE for any
__BIG_ENDIAN config.

Fixes: 414c453 ("mgag200: initial g200se driver (v2)")
Signed-off-by: René Rebe <rene@exactco.de>
Cc: stable@kernel.org
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20251208.141827.965103015954471168.rene@exactco.de

Author: rxrbln

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -161,6 +161,30 @@ static void mgag200_set_startadd(struct mga_device *mdev,
 	WREG_ECRT(0x00, crtcext0);
 }
 
+/*
+ * Set the opmode for the hardware swapper for Big-Endian processor
+ * support for the frame buffer aperture and DMAWIN space.
+ */
+static void mgag200_set_datasiz(struct mga_device *mdev, u32 format)
+{
+#if defined(__BIG_ENDIAN)
+	u32 opmode = RREG32(MGAREG_OPMODE);
+
+	opmode &= ~(GENMASK(17, 16) | GENMASK(9, 8) | GENMASK(3, 2));
+
+	/* Big-endian byte-swapping */
+	switch (format) {
+	case DRM_FORMAT_RGB565:
+		opmode |= 0x10100;
+		break;
+	case DRM_FORMAT_XRGB8888:
+		opmode |= 0x20200;
+		break;
+	}
+	WREG32(MGAREG_OPMODE, opmode);
+#endif
+}
+
 void mgag200_init_registers(struct mga_device *mdev)
 {
 	u8 crtc11, misc;
@@ -496,6 +520,7 @@ void mgag200_primary_plane_helper_atomic_update(struct drm_plane *plane,
 	struct drm_atomic_helper_damage_iter iter;
 	struct drm_rect damage;
 
+	mgag200_set_datasiz(mdev, fb->format->format);
 	drm_atomic_helper_damage_iter_init(&iter, old_plane_state, plane_state);
 	drm_atomic_for_each_plane_damage(&iter, &damage) {
 		mgag200_handle_damage(mdev, shadow_plane_state->data, fb, &damage);