drivers: modem: sara-r4: adjust send and receive lengths

- limit max. number of bytes when sending to socket
  The number of bytes sent to a socket in one transaction
  is limited to 512 in HEX mode (Sara-R4), and to 1024
  otherwise. This corresponds to numbers given in the
  manual for ublox cellular modems.

- report number of bytes actually sent, as reported by modem
  After writing data to a socket, we now return the number of
  bytes actually written, as reported by the modem.

Signed-off-by: Hans Wilmers <hans@wilmers.no>

drivers/modem/ublox-sara-r4.c

@@ -144,6 +144,9 @@ struct modem_data {
 	/* modem state */
 	int ev_creg;
 
+	/* bytes written to socket in last transaction */
+	int sock_written;
+
 	/* response semaphore */
 	struct k_sem sem_response;
 };
@@ -250,6 +253,24 @@ static ssize_t send_socket_data(struct modem_socket *sock,
 		return -EINVAL;
 	}
 
+#if defined(CONFIG_MODEM_UBLOX_SARA_R4)
+	/* Hex mode allows sending 512 bytes to the socket in one command */
+	if (buf_len > (MDM_MAX_DATA_LENGTH / 2)) {
+		buf_len = (MDM_MAX_DATA_LENGTH / 2);
+	}
+#else
+	/*
+	 * Binary and ASCII mode allows sending MDM_MAX_DATA_LENGTH bytes to
+	 * the socket in one command
+	 */
+	if (buf_len > MDM_MAX_DATA_LENGTH) {
+		buf_len = MDM_MAX_DATA_LENGTH;
+	}
+#endif
+
+	/* The number of bytes written will be reported by the modem */
+	mdata.sock_written = 0;
+
 	if (sock->ip_proto == IPPROTO_UDP) {
 		ret = modem_context_get_addr_port(dst_addr, &dst_port);
 		snprintk(send_buf, sizeof(send_buf),
@@ -318,7 +339,7 @@ exit:
 		return ret;
 	}
 
-	return buf_len;
+	return mdata.sock_written;
 }
 
 /*
@@ -464,9 +485,8 @@ MODEM_CMD_DEFINE(on_cmd_sockcreate)
 /* Handler: +USO[WR|ST]: <socket_id>[0],<length>[1] */
 MODEM_CMD_DEFINE(on_cmd_sockwrite)
 {
-	/* TODO: check length against original send length*/
+	mdata.sock_written = ATOI(argv[1], 0, "length");
-
+	LOG_DBG("bytes written: %d", mdata.sock_written);
-	/* don't give back semaphore -- OK to follow */
 }
 
 /* Common code for +USOR[D|F]: "<hex_data>" */