drm/fbdev-generic: Fix locking with drm_client_buffer_vmap_local()

Temporarily lock the fbdev buffer object during updates to prevent
memory managers from evicting/moving the buffer. Moving a buffer
object while update its content results in undefined behaviour.

Fbdev-generic updates its buffer object from a shadow buffer. Gem-shmem
and gem-dma helpers do not move buffer objects, so they are safe to be
used with fbdev-generic. Gem-vram and qxl are based on TTM, but pin
buffer objects are part of the vmap operation. So both are also safe
to be used with fbdev-generic.

Amdgpu and nouveau do not pin or lock the buffer object during an
update. Their TTM-based memory management could move the buffer object
while the update is ongoing.

The new vmap_local and vunmap_local helpers hold the buffer object's
reservation lock during the buffer update. This prevents moving the
buffer object on all memory managers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # virtio-gpu
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227113853.8464-11-tzimmermann@suse.de

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index e0f80c6a7096..d4bbc5d109c8 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -1227,6 +1227,18 @@ void drm_gem_vunmap(struct drm_gem_object *obj, struct iosys_map *map)
 }
 EXPORT_SYMBOL(drm_gem_vunmap);
 
+void drm_gem_lock(struct drm_gem_object *obj)
+{
+	dma_resv_lock(obj->resv, NULL);
+}
+EXPORT_SYMBOL(drm_gem_lock);
+
+void drm_gem_unlock(struct drm_gem_object *obj)
+{
+	dma_resv_unlock(obj->resv);
+}
+EXPORT_SYMBOL(drm_gem_unlock);
+
 int drm_gem_vmap_unlocked(struct drm_gem_object *obj, struct iosys_map *map)
 {
 	int ret;