net: ppp: Avoid wrapping each byte in muxing headers

When PPP is muxed, using uart_poll_out resulted in each byte getting
wrapped in a muxing header. This led to UART bombardment which
can quickly cause some modems to hang and panic. This was observed
regularly using a SIMCOM7600E modem.

A perfect fix would involve rewriting ppp.c, uart_mux.c and
modem_iface_uart.c to all use another UART API, but that would be more
invasive by several orders of magnitude than this one, which utilizes
the fact that the uart_mux implementation of uart_fifo_fill does NOT
require ISR context. Since the Zephyr UART API states that the behavior
of uart_fifo_fill outside of ISR context is implementation defined, this
should be kosher.

Signed-off-by: Benjamin Lindqvist <benjamin.lindqvist@endian.se>

drivers/console/uart_mux.c

@@ -516,6 +516,15 @@ static int uart_mux_fifo_fill(const struct device *dev,
 		return -ENOENT;
 	}
 
+	/* If we're not in ISR context, do the xfer synchronously. This
+	 * effectively let's applications use this implementation of fifo_fill
+	 * as a multi-byte poll_out which prevents each byte getting wrapped by
+	 * mux headers.
+	 */
+	if (!k_is_in_isr() && dev_data->dlci) {
+		return gsm_dlci_send(dev_data->dlci, tx_data, len);
+	}
+
 	LOG_DBG("dev_data %p len %d tx_ringbuf space %u",
 		dev_data, len, ring_buf_space_get(dev_data->tx_ringbuf));
 

drivers/modem/modem_iface_uart.c

@@ -125,6 +125,18 @@ static int modem_iface_uart_read(struct modem_iface *iface,
 	return 0;
 }
 
+static bool mux_is_active(struct modem_iface *iface)
+{
+	bool active = false;
+
+#if defined(CONFIG_UART_MUX_DEVICE_NAME)
+	const char *mux_name = CONFIG_UART_MUX_DEVICE_NAME;
+	active = (mux_name == iface->dev->name);
+#endif /* CONFIG_UART_MUX_DEVICE_NAME */
+
+	return active;
+}
+
 static int modem_iface_uart_write(struct modem_iface *iface,
 				  const uint8_t *buf, size_t size)
 {
@@ -136,9 +148,18 @@ static int modem_iface_uart_write(struct modem_iface *iface,
 		return 0;
 	}
 
-	do {
+	/* If we're using gsm_mux, We don't want to use poll_out because sending
-		uart_poll_out(iface->dev, *buf++);
+	 * one byte at a time causes each byte to get wrapped in muxing headers.
-	} while (--size);
+	 * But we can safely call uart_fifo_fill outside of ISR context when
+	 * muxing because uart_mux implements it in software.
+	 */
+	if (mux_is_active(iface)) {
+		uart_fifo_fill(iface->dev, buf, size);
+	} else {
+		do {
+			uart_poll_out(iface->dev, *buf++);
+		} while (--size);
+	}
 
 	return 0;
 }

drivers/net/ppp.c

@@ -194,9 +194,19 @@ static void ppp_change_state(struct ppp_driver_context *ctx,
 
 static int ppp_send_flush(struct ppp_driver_context *ppp, int off)
 {
-	if (!IS_ENABLED(CONFIG_NET_TEST)) {
+	if (IS_ENABLED(CONFIG_NET_TEST)) {
-		uint8_t *buf = ppp->send_buf;
+		return 0;
+	}
+	uint8_t *buf = ppp->send_buf;
 
+	/* If we're using gsm_mux, We don't want to use poll_out because sending
+	 * one byte at a time causes each byte to get wrapped in muxing headers.
+	 * But we can safely call uart_fifo_fill outside of ISR context when
+	 * muxing because uart_mux implements it in software.
+	 */
+	if (IS_ENABLED(CONFIG_GSM_MUX)) {
+		(void)uart_fifo_fill(ppp->dev, buf, off);
+	} else {
 		while (off--) {
 			uart_poll_out(ppp->dev, *buf++);
 		}