drm/mgag200: Add module parameter to pin all buffers at offset 0

For hardware that does not interpret the startadd field correctly,
add the module parameter 'hw_bug_no_startadd', which enables the
workaround.

v3:
	* style and typo fixes
v2:
	* ask user for feedback if the option is active
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: John Donnelly <john.p.donnelly@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126101950.11989-1-tzimmermann@suse.de

drivers/gpu/drm/mgag200/mgag200_drv.c

@@ -27,6 +27,10 @@ int mgag200_modeset = -1;
 MODULE_PARM_DESC(modeset, "Disable/Enable modesetting");
 module_param_named(modeset, mgag200_modeset, int, 0400);
 
+int mgag200_hw_bug_no_startadd = -1;
+MODULE_PARM_DESC(modeset, "HW does not interpret scanout-buffer start address correctly");
+module_param_named(hw_bug_no_startadd, mgag200_hw_bug_no_startadd, int, 0400);
+
 static struct drm_driver driver;
 
 static const struct pci_device_id pciidlist[] = {
@@ -97,6 +101,16 @@ DEFINE_DRM_GEM_FOPS(mgag200_driver_fops);
 
 static bool mgag200_pin_bo_at_0(const struct mga_device *mdev)
 {
+	if (mgag200_hw_bug_no_startadd > 0) {
+		DRM_WARN_ONCE("Option hw_bug_no_startradd is enabled. Please "
+			      "report the output of 'lspci -vvnn' to "
+			      "<dri-devel@lists.freedesktop.org> if this "
+			      "option is required to make mgag200 work "
+			      "correctly on your system.\n");
+		return true;
+	} else if (!mgag200_hw_bug_no_startadd) {
+		return false;
+	}
 	return mdev->flags & MGAG200_FLAG_HW_BUG_NO_STARTADD;
 }