drm/udl: Get rid of dev->struct_mutex usage

It's only used to protect our page list, and only when we know we have
a full reference. This means none of these code paths can ever race
with the final unref, and hence we do not need dev->struct_mutex
serialization and can simply switch to our own locking.

For more context the only magic the locked gem_free_object provides is
that it prevents concurrent final unref (and destruction) of gem
objects while anyone is holding dev->struct_mutex. This was used by
i915 (and other drivers) to implement eviction handling with less
headaches.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180327082356.24516-3-daniel.vetter@ffwll.ch

drivers/gpu/drm/udl/udl_dmabuf.c

@@ -76,6 +76,7 @@ static struct sg_table *udl_map_dma_buf(struct dma_buf_attachment *attach,
 	struct udl_drm_dmabuf_attachment *udl_attach = attach->priv;
 	struct udl_gem_object *obj = to_udl_bo(attach->dmabuf->priv);
 	struct drm_device *dev = obj->base.dev;
+	struct udl_device *udl = dev->dev_private;
 	struct scatterlist *rd, *wr;
 	struct sg_table *sgt = NULL;
 	unsigned int i;
@@ -112,7 +113,7 @@ static struct sg_table *udl_map_dma_buf(struct dma_buf_attachment *attach,
 		return ERR_PTR(-ENOMEM);
 	}
 
-	mutex_lock(&dev->struct_mutex);
+	mutex_lock(&udl->gem_lock);
 
 	rd = obj->sg->sgl;
 	wr = sgt->sgl;
@@ -137,7 +138,7 @@ static struct sg_table *udl_map_dma_buf(struct dma_buf_attachment *attach,
 	attach->priv = udl_attach;
 
 err_unlock:
-	mutex_unlock(&dev->struct_mutex);
+	mutex_unlock(&udl->gem_lock);
 	return sgt;
 }
 

drivers/gpu/drm/udl/udl_drv.c

@@ -53,7 +53,7 @@ static struct drm_driver driver = {
 	.unload = udl_driver_unload,
 
 	/* gem hooks */
-	.gem_free_object = udl_gem_free_object,
+	.gem_free_object_unlocked = udl_gem_free_object,
 	.gem_vm_ops = &udl_gem_vm_ops,
 
 	.dumb_create = udl_dumb_create,

drivers/gpu/drm/udl/udl_drv.h

@@ -54,6 +54,8 @@ struct udl_device {
 	struct usb_device *udev;
 	struct drm_crtc *crtc;
 
+	struct mutex gem_lock;
+
 	int sku_pixel_limit;
 
 	struct urb_list urbs;

drivers/gpu/drm/udl/udl_gem.c

@@ -214,9 +214,10 @@ int udl_gem_mmap(struct drm_file *file, struct drm_device *dev,
 {
 	struct udl_gem_object *gobj;
 	struct drm_gem_object *obj;
+	struct udl_device *udl = dev->dev_private;
 	int ret = 0;
 
-	mutex_lock(&dev->struct_mutex);
+	mutex_lock(&udl->gem_lock);
 	obj = drm_gem_object_lookup(file, handle);
 	if (obj == NULL) {
 		ret = -ENOENT;
@@ -236,6 +237,6 @@ int udl_gem_mmap(struct drm_file *file, struct drm_device *dev,
 out:
 	drm_gem_object_put(&gobj->base);
 unlock:
-	mutex_unlock(&dev->struct_mutex);
+	mutex_unlock(&udl->gem_lock);
 	return ret;
 }

drivers/gpu/drm/udl/udl_main.c

@@ -324,6 +324,8 @@ int udl_driver_load(struct drm_device *dev, unsigned long flags)
 	udl->ddev = dev;
 	dev->dev_private = udl;
 
+	mutex_init(&udl->gem_lock);
+
 	if (!udl_parse_vendor_descriptor(dev, udl->udev)) {
 		ret = -ENODEV;
 		DRM_ERROR("firmware not recognized. Assume incompatible device\n");