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net: ppp: Avoid wrapping each byte in muxing headers

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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>
This commit is contained in:
Benjamin Lindqvist 2020-10-15 14:10:08 +02:00 • committed by Carles Cufí
commit 449fc7bb1c
3 changed files with 45 additions and 5 deletions
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14
drivers/net/ppp.c
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@ -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)) {
uint8_t *buf = ppp->send_buf;
if (IS_ENABLED(CONFIG_NET_TEST)) {
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++);
}