drm/nouveau/clk/gk20a: only restore divider to 1:1 if needed

Only restore the 1:1 divider if it is not set already. Also use the
proper masks for this operation and add a second write as done in the
Android code.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c

@@ -429,9 +429,16 @@ _gk20a_pllg_program_mnp(struct gk20a_clk *clk, bool allow_slide)
 
 	/* restore out divider 1:1 */
 	val = nvkm_rd32(device, GPC2CLK_OUT);
+	if ((val & GPC2CLK_OUT_VCODIV_MASK) !=
+	    (GPC2CLK_OUT_VCODIV1 << GPC2CLK_OUT_VCODIV_SHIFT)) {
 		val &= ~GPC2CLK_OUT_VCODIV_MASK;
+		val |= GPC2CLK_OUT_VCODIV1 << GPC2CLK_OUT_VCODIV_SHIFT;
 		udelay(2);
 		nvkm_wr32(device, GPC2CLK_OUT, val);
+		/* Intentional 2nd write to assure linear divider operation */
+		nvkm_wr32(device, GPC2CLK_OUT, val);
+		nvkm_rd32(device, GPC2CLK_OUT);
+	}
 
 	/* slide up to new NDIV */
 	return allow_slide ? gk20a_pllg_slide(clk, clk->n) : 0;