greybus: gb-beagleplay: Remove use of pad bytes

Make gb-beagleplay greybus spec compliant by moving cport information to
transport layer instead of using `header->pad` bytes.

Greybus HDLC frame now has the following payload:
1. le16 cport
2. gb_operation_msg_hdr msg_header
3. u8 *msg_payload

Fixes: ec558bbfea67 ("greybus: Add BeaglePlay Linux Driver")
Signed-off-by: Ayush Singh <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/greybus/gb-beagleplay.c

···
 	void *buf;
 };
 
+/**
+ * struct hdlc_greybus_frame - Structure to represent greybus HDLC frame payload
+ *
+ * @cport: cport id
+ * @hdr: greybus operation header
+ * @payload: greybus message payload
+ *
+ * The HDLC payload sent over UART for greybus address has cport preappended to greybus message
+ */
+struct hdlc_greybus_frame {
+	__le16 cport;
+	struct gb_operation_msg_hdr hdr;
+	u8 payload[];
+} __packed;
+
 static void hdlc_rx_greybus_frame(struct gb_beagleplay *bg, u8 *buf, u16 len)
 {
-	u16 cport_id;
-	struct gb_operation_msg_hdr *hdr = (struct gb_operation_msg_hdr *)buf;
-
-	memcpy(&cport_id, hdr->pad, sizeof(cport_id));
+	struct hdlc_greybus_frame *gb_frame = (struct hdlc_greybus_frame *)buf;
+	u16 cport_id = le16_to_cpu(gb_frame->cport);
+	u16 gb_msg_len = le16_to_cpu(gb_frame->hdr.size);
 
 	dev_dbg(&bg->sd->dev, "Greybus Operation %u type %X cport %u status %u received",
-		hdr->operation_id, hdr->type, cport_id, hdr->result);
+		gb_frame->hdr.operation_id, gb_frame->hdr.type, cport_id, gb_frame->hdr.result);
 
-	greybus_data_rcvd(bg->gb_hd, cport_id, buf, len);
+	greybus_data_rcvd(bg->gb_hd, cport_id, (u8 *)&gb_frame->hdr, gb_msg_len);
 }
 
 static void hdlc_rx_dbg_frame(const struct gb_beagleplay *bg, const char *buf, u16 len)
···
 	.write_wakeup = gb_tty_wakeup,
 };
 
+/**
+ * gb_message_send() - Send greybus message using HDLC over UART
+ *
+ * @hd: pointer to greybus host device
+ * @cport: AP cport where message originates
+ * @msg: greybus message to send
+ * @mask: gfp mask
+ *
+ * Greybus HDLC frame has the following payload:
+ * 1. le16 cport
+ * 2. gb_operation_msg_hdr msg_header
+ * 3. u8 *msg_payload
+ */
 static int gb_message_send(struct gb_host_device *hd, u16 cport, struct gb_message *msg, gfp_t mask)
 {
 	struct gb_beagleplay *bg = dev_get_drvdata(&hd->dev);
-	struct hdlc_payload payloads[2];
+	struct hdlc_payload payloads[3];
+	__le16 cport_id = cpu_to_le16(cport);
 
 	dev_dbg(&hd->dev, "Sending greybus message with Operation %u, Type: %X on Cport %u",
 		msg->header->operation_id, msg->header->type, cport);
 
-	if (msg->header->size > RX_HDLC_PAYLOAD)
+	if (le16_to_cpu(msg->header->size) > RX_HDLC_PAYLOAD)
 		return dev_err_probe(&hd->dev, -E2BIG, "Greybus message too big");
 
-	memcpy(msg->header->pad, &cport, sizeof(cport));
+	payloads[0].buf = &cport_id;
+	payloads[0].len = sizeof(cport_id);
+	payloads[1].buf = msg->header;
+	payloads[1].len = sizeof(*msg->header);
+	payloads[2].buf = msg->payload;
+	payloads[2].len = msg->payload_size;
 
-	payloads[0].buf = msg->header;
-	payloads[0].len = sizeof(*msg->header);
-	payloads[1].buf = msg->payload;
-	payloads[1].len = msg->payload_size;
-
-	hdlc_tx_frames(bg, ADDRESS_GREYBUS, 0x03, payloads, 2);
+	hdlc_tx_frames(bg, ADDRESS_GREYBUS, 0x03, payloads, 3);
 	greybus_message_sent(bg->gb_hd, msg, 0);
 
 	return 0;