drm/rockchip: vop: clear DMA stop bit on RK3066

The RK3066 VOP sets a dma_stop bit when it's done scanning out a frame
and needs the driver to acknowledge that by clearing the bit.

Unless we clear it "between" frames, the RGB output only shows noise
instead of the picture. atomic_flush is the place for it that least
affects other code (doing it on vblank would require converting all
other usages of the reg_lock to spin_(un)lock_irq, which would affect
performance for everyone).

This seems to be a redundant synchronization mechanism that was removed
in later iterations of the VOP hardware block.

Fixes: f4a6de85 ("drm: rockchip: vop: add rk3066 vop definitions")
Cc: stable@vger.kernel.org
Signed-off-by: Val Packett <val@packett.cool>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624204054.5524-2-val@packett.cool

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -1583,6 +1583,10 @@ static void vop_crtc_atomic_flush(struct drm_crtc *crtc,
 	VOP_AFBC_SET(vop, enable, s->enable_afbc);
 	vop_cfg_done(vop);
 
+	/* Ack the DMA transfer of the previous frame (RK3066). */
+	if (VOP_HAS_REG(vop, common, dma_stop))
+		VOP_REG_SET(vop, common, dma_stop, 0);
+
 	spin_unlock(&vop->reg_lock);
 
 	/*

drivers/gpu/drm/rockchip/rockchip_drm_vop.h

@@ -122,6 +122,7 @@ struct vop_common {
 	struct vop_reg lut_buffer_index;
 	struct vop_reg gate_en;
 	struct vop_reg mmu_en;
+	struct vop_reg dma_stop;
 	struct vop_reg out_mode;
 	struct vop_reg standby;
 };

drivers/gpu/drm/rockchip/rockchip_vop_reg.c

@@ -466,6 +466,7 @@ static const struct vop_output rk3066_output = {
 };
 
 static const struct vop_common rk3066_common = {
+	.dma_stop = VOP_REG(RK3066_SYS_CTRL0, 0x1, 0),
 	.standby = VOP_REG(RK3066_SYS_CTRL0, 0x1, 1),
 	.out_mode = VOP_REG(RK3066_DSP_CTRL0, 0xf, 0),
 	.cfg_done = VOP_REG(RK3066_REG_CFG_DONE, 0x1, 0),