diff --git a/arch/powerpc/platforms/cell/spufs/backing_ops.c b/arch/powerpc/platforms/cell/spufs/backing_ops.c
index 07a0e815abf5..ec01214e51ee 100644
--- a/arch/powerpc/platforms/cell/spufs/backing_ops.c
+++ b/arch/powerpc/platforms/cell/spufs/backing_ops.c
@@ -162,7 +162,8 @@ static int spu_backing_wbox_write(struct spu_context *ctx, u32 data)
 		BUG_ON(avail != (4 - slot));
 		ctx->csa.spu_mailbox_data[slot] = data;
 		ctx->csa.spu_chnlcnt_RW[29] = ++slot;
-		ctx->csa.prob.mb_stat_R = (((4 - slot) & 0xff) << 8);
+		ctx->csa.prob.mb_stat_R &= ~(0x00ff00);
+		ctx->csa.prob.mb_stat_R |= (((4 - slot) & 0xff) << 8);
 		gen_spu_event(ctx, MFC_SPU_MAILBOX_WRITTEN_EVENT);
 		ret = 4;
 	} else {