drm/shmem-helpers: Don't call get/put_pages on imported dma-buf in vmap

There's no direct harm, because for the shmem helpers these are noops
on imported buffers. The trouble is in the locks these take - I want
to change dma_buf_vmap locking, and so need to make sure that we only
ever take certain locks on one side of the dma-buf interface: Either
for exporters, or for importers.

v2: Change the control flow less compared to what's there (Thomas)
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514202256.490926-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -252,15 +252,15 @@ static void *drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem)
 	if (shmem->vmap_use_count++ > 0)
 		return shmem->vaddr;
 
-	ret = drm_gem_shmem_get_pages(shmem);
-	if (ret)
-		goto err_zero_use;
-
 	if (obj->import_attach) {
 		shmem->vaddr = dma_buf_vmap(obj->import_attach->dmabuf);
 	} else {
 		pgprot_t prot = PAGE_KERNEL;
 
+		ret = drm_gem_shmem_get_pages(shmem);
+		if (ret)
+			goto err_zero_use;
+
 		if (!shmem->map_cached)
 			prot = pgprot_writecombine(prot);
 		shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT,
@@ -276,6 +276,7 @@ static void *drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem)
 	return shmem->vaddr;
 
 err_put_pages:
+	if (!obj->import_attach)
 		drm_gem_shmem_put_pages(shmem);
 err_zero_use:
 	shmem->vmap_use_count = 0;