drm/i915/skl: SKL Watermark Computation

This patch implements the watermark algorithm and its necessary
functions. Two function pointers skl_update_wm and
skl_update_sprite_wm are provided. The skl_update_wm will update
the watermarks for the crtc provided as an argument and then
checks for change in DDB allocation for other active pipes and
recomputes the watermarks for those Pipes and planes as well.
Finally it does the register programming for all dirty pipes.
The trigger of the Watermark double buffer registers will have
to be once the plane configurations are done by the caller.

v2: fixed the divide-by-0 error in the results computation func.
    Also reworked the PLANE_WM register values computation func to
    make it more compact. Incorporated all other review comments
    from Damien.

v3: Changed the skl_compute_plane_wm function to now return success
    or failure. Also the result blocks and lines are computed here
    instead of in skl_compute_wm_results function.

v4: Adjust skl_ddb_alloc_changed() to the new planes/cursor split
    (Damien)

v5: Reworked the affected functions to implement new plane/cursor
    split.

v6: Rework the logic that triggers the DDB allocation and WM computation
    of skl_update_other_pipe_wm() to not depend on non-computed DDB
    values.
    Always give a valid cursor_width (at boot it's 0) to keep the
    invariant that we consider the cursor plane always enabled.
    Otherwise we end up dividing by 0 in skl_compute_plane_wm()
    (Damien Lespiau)

v7: Spell out allocation
    skl_ddb_ functions should have the ddb as first argument
    Make the skl_ddb_alloc_changed() parameters const
    (Damien)

v8: Rebase on top of the crtc->primary changes

v9: Split the staging results structure to not exceed the 1Kb stack
    allocation in skl_update_wm()

v10: Make skl_pipe_pixel_rate() take a pointer to the pipe config
     Add a comment about overflow considerations for skl_wm_method1()
     Various additions of const
     Various use of sizeof(variable) instead of sizeof(type)
     Various move of variable definitons to a narrower scope
     Zero initialize some stack allocated structures to make sure we
     don't have garbage in case we don't write all the values
     (Ville)

v11: Remove non-necessary default number of blocks/lines when the plane
     is disabled (Ville)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -1722,8 +1722,18 @@ struct drm_i915_private {
 		 */
 		uint16_t skl_latency[8];
 
+		/*
+		 * The skl_wm_values structure is a bit too big for stack
+		 * allocation, so we keep the staging struct where we store
+		 * intermediate results here instead.
+		 */
+		struct skl_wm_values skl_results;
+
 		/* current hardware state */
-		struct ilk_wm_values hw;
+		union {
+			struct ilk_wm_values hw;
+			struct skl_wm_values skl_hw;
+		};
 	} wm;
 
 	struct i915_runtime_pm pm;