drm/vc4: hdmi: Fix register offset with longer CEC messages

The code prior to 311e305f ("drm/vc4: hdmi: Implement a register
layout abstraction") was relying on the fact that the register offset
was incremented by 4 for each readl call. That worked since the register
width is 4 bytes.

However, since that commit the HDMI_READ macro is now taking an enum,
and the offset doesn't increment by 4 but 1 now. Divide the index by 4
to fix this.

Fixes: 311e305f ("drm/vc4: hdmi: Implement a register layout abstraction")
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111142309.193441-4-maxime@cerno.tech
(cherry picked from commit e9c9481f)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -1444,13 +1444,20 @@ static irqreturn_t vc4_cec_irq_handler_thread(int irq, void *priv)
 
 static void vc4_cec_read_msg(struct vc4_hdmi *vc4_hdmi, u32 cntrl1)
 {
+	struct drm_device *dev = vc4_hdmi->connector.dev;
 	struct cec_msg *msg = &vc4_hdmi->cec_rx_msg;
 	unsigned int i;
 
 	msg->len = 1 + ((cntrl1 & VC4_HDMI_CEC_REC_WRD_CNT_MASK) >>
 					VC4_HDMI_CEC_REC_WRD_CNT_SHIFT);
+
+	if (msg->len > 16) {
+		drm_err(dev, "Attempting to read too much data (%d)\n", msg->len);
+		return;
+	}
+
 	for (i = 0; i < msg->len; i += 4) {
-		u32 val = HDMI_READ(HDMI_CEC_RX_DATA_1 + i);
+		u32 val = HDMI_READ(HDMI_CEC_RX_DATA_1 + (i >> 2));
 
 		msg->msg[i] = val & 0xff;
 		msg->msg[i + 1] = (val >> 8) & 0xff;
@@ -1543,11 +1550,17 @@ static int vc4_hdmi_cec_adap_transmit(struct cec_adapter *adap, u8 attempts,
 				      u32 signal_free_time, struct cec_msg *msg)
 {
 	struct vc4_hdmi *vc4_hdmi = cec_get_drvdata(adap);
+	struct drm_device *dev = vc4_hdmi->connector.dev;
 	u32 val;
 	unsigned int i;
 
+	if (msg->len > 16) {
+		drm_err(dev, "Attempting to transmit too much data (%d)\n", msg->len);
+		return -ENOMEM;
+	}
+
 	for (i = 0; i < msg->len; i += 4)
-		HDMI_WRITE(HDMI_CEC_TX_DATA_1 + i,
+		HDMI_WRITE(HDMI_CEC_TX_DATA_1 + (i >> 2),
 			   (msg->msg[i]) |
 			   (msg->msg[i + 1] << 8) |
 			   (msg->msg[i + 2] << 16) |