drm/vc4: hdmi: Clear unused infoframe packet RAM registers

Using a hdmi analyser the bytes in packet ram
registers beyond the length were visible in the
infoframes and it flagged the checksum as invalid.

Zeroing unused words of packet RAM avoids this

Fixes: 21317b3f ("drm/vc4: Set up the AVI and SPD infoframes.")
Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Link: https://lore.kernel.org/r/20220613144800.326124-20-maxime@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -461,9 +461,11 @@ static void vc4_hdmi_write_infoframe(struct drm_encoder *encoder,
 	const struct vc4_hdmi_register *ram_packet_start =
 		&vc4_hdmi->variant->registers[HDMI_RAM_PACKET_START];
 	u32 packet_reg = ram_packet_start->offset + VC4_HDMI_PACKET_STRIDE * packet_id;
+	u32 packet_reg_next = ram_packet_start->offset +
+		VC4_HDMI_PACKET_STRIDE * (packet_id + 1);
 	void __iomem *base = __vc4_hdmi_get_field_base(vc4_hdmi,
 						       ram_packet_start->reg);
-	uint8_t buffer[VC4_HDMI_PACKET_STRIDE];
+	uint8_t buffer[VC4_HDMI_PACKET_STRIDE] = {};
 	unsigned long flags;
 	ssize_t len, i;
 	int ret;
@@ -499,6 +501,13 @@ static void vc4_hdmi_write_infoframe(struct drm_encoder *encoder,
 		packet_reg += 4;
 	}
 
+	/*
+	 * clear remainder of packet ram as it's included in the
+	 * infoframe and triggers a checksum error on hdmi analyser
+	 */
+	for (; packet_reg < packet_reg_next; packet_reg += 4)
+		writel(0, base + packet_reg);
+
 	HDMI_WRITE(HDMI_RAM_PACKET_CONFIG,
 		   HDMI_READ(HDMI_RAM_PACKET_CONFIG) | BIT(packet_id));