drm/imx: Unconfuse preclose logic

So this one is special, since it tries to prevent races when userspace
crashes simply by disabling the vblank machinery. Well except that imx
always has vblanks enabled, and the disable_vblank hook actually just
tries to cancel a pending pageflip. Without any locking whatsoever. Of
course this is wrong, since it'll result in the hw not actually
displaying what drm thinks is the current frontbuffer.

Well since the core takes care of the disappearing DRM fd now. So we
can nuke all this confused code without ill side-effects.

Someone else needs to audit the locking for ->newfb and
->page_flip_event and fix it up. Common approach is to reuse
dev->event_lock for this.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-7-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/imx/imx-drm-core.c

@@ -171,18 +171,6 @@ static void imx_drm_disable_vblank(struct drm_device *drm, unsigned int pipe)
 	imx_drm_crtc->imx_drm_helper_funcs.disable_vblank(imx_drm_crtc->crtc);
 }
 
-static void imx_drm_driver_preclose(struct drm_device *drm,
-		struct drm_file *file)
-{
-	int i;
-
-	if (!file->is_master)
-		return;
-
-	for (i = 0; i < MAX_CRTC; i++)
-		imx_drm_disable_vblank(drm, i);
-}
-
 static const struct file_operations imx_drm_driver_fops = {
 	.owner = THIS_MODULE,
 	.open = drm_open,
@@ -463,7 +451,6 @@ static struct drm_driver imx_drm_driver = {
 	.load			= imx_drm_driver_load,
 	.unload			= imx_drm_driver_unload,
 	.lastclose		= imx_drm_driver_lastclose,
-	.preclose		= imx_drm_driver_preclose,
 	.set_busid		= drm_platform_set_busid,
 	.gem_free_object	= drm_gem_cma_free_object,
 	.gem_vm_ops		= &drm_gem_cma_vm_ops,

drivers/gpu/drm/imx/ipuv3-crtc.c

@@ -285,10 +285,6 @@ static int ipu_enable_vblank(struct drm_crtc *crtc)
 
 static void ipu_disable_vblank(struct drm_crtc *crtc)
 {
-	struct ipu_crtc *ipu_crtc = to_ipu_crtc(crtc);
-
-	ipu_crtc->page_flip_event = NULL;
-	ipu_crtc->newfb = NULL;
 }
 
 static int ipu_set_interface_pix_fmt(struct drm_crtc *crtc,