drm/ttm: add multihop infrastrucutre (v3)

Currently drivers get called to move a buffer, but if they have to
move it temporarily through another space (SYSTEM->VRAM via TT)
then they can end up with a lot of ttm->driver->ttm call stacks,
if the temprorary space moves requires eviction.

Instead of letting the driver do all the placement/space for the
temporary, allow it to report back (-EMULTIHOP) and a placement (hop)
to the move code, which will then do the temporary move, and the
correct placement move afterwards.

This removes a lot of code from drivers, at the expense of
adding some midlayering. I've some further ideas on how to turn
it inside out, but I think this is a good solution to the call
stack problems.

v2: separate out the driver patches, add WARN for getting
MULTHOP in paths we shouldn't (Daniel)
v3: use memset (Christian)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: hristian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201109005432.861936-2-airlied@gmail.com

include/drm/ttm/ttm_bo_driver.h

@@ -121,6 +121,8 @@ struct ttm_bo_driver {
 	 * Return the bo flags for a buffer which is not mapped to the hardware.
 	 * These will be placed in proposed_flags so that when the move is
 	 * finished, they'll end up in bo->mem.flags
+	 * This should not cause multihop evictions, and the core will warn
+	 * if one is proposed.
 	 */
 
 	void (*evict_flags)(struct ttm_buffer_object *bo,
@@ -134,12 +136,15 @@ struct ttm_bo_driver {
 	 * the graphics address space
 	 * @ctx: context for this move with parameters
 	 * @new_mem: the new memory region receiving the buffer
+	 @ @hop: placement for driver directed intermediate hop
 	 *
 	 * Move a buffer between two memory regions.
+	 * Returns errno -EMULTIHOP if driver requests a hop
 	 */
 	int (*move)(struct ttm_buffer_object *bo, bool evict,
 		    struct ttm_operation_ctx *ctx,
-		    struct ttm_resource *new_mem);
+		    struct ttm_resource *new_mem,
+		    struct ttm_place *hop);
 
 	/**
 	 * struct ttm_bo_driver_member verify_access