diff --git a/drivers/gpu/drm/rockchip/cdn-dp-core.c b/drivers/gpu/drm/rockchip/cdn-dp-core.c
index dcca05f4c2b9..367281ff343a 100644
--- a/drivers/gpu/drm/rockchip/cdn-dp-core.c
+++ b/drivers/gpu/drm/rockchip/cdn-dp-core.c
@@ -215,9 +215,9 @@ static int cdn_dp_start_hdcp1x_auth(struct cdn_dp_device *dp)
 	arm_smccc_smc(RK_SIP_HDCP_CONTROL, HDCP_KEY_DATA_START_TRANSFER,
 		      0, 0, 0, 0, 0, 0, &res);
 
-	buf = (uint64_t *)&dp->key;
-
-	for (; !res.a0 && (u8 *)buf - (u8 *)&dp->key < sizeof(dp->key);
+	BUILD_BUG_ON(sizeof(dp->key) % 6);
+	for (buf = (uint64_t *)&dp->key;
+	     !res.a0 && (u8 *)buf - (u8 *)&dp->key < sizeof(dp->key);
 	     buf += 6)
 		arm_smccc_smc(RK_SIP_HDCP_KEY_DATA64, buf[0], buf[1],
 			      buf[2], buf[3], buf[4], buf[5], 0, &res);
diff --git a/drivers/gpu/drm/rockchip/cdn-dp-core.h b/drivers/gpu/drm/rockchip/cdn-dp-core.h
index 8b5d283c66c6..22f059268afe 100644
--- a/drivers/gpu/drm/rockchip/cdn-dp-core.h
+++ b/drivers/gpu/drm/rockchip/cdn-dp-core.h
@@ -80,6 +80,9 @@ enum {
 	CDN_DP_HDCP_KEY_LEN = 308,
 };
 
+/*
+ * Size must be divisible by 6
+ */
 struct cdn_dp_hdcp_key_1x {
 	u8 ksv[CDN_DP_HDCP_KSV_LEN];
 	u8 device_key[CDN_DP_HDCP_DPK_LEN];