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uart: Add primitive noise filtering on RX

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A long cable attached to the UART can act as an antenna if disconnected
from the other end.  This can cause noise on the receive side, possibly
as reflections from the transmit side, leading to an interrupt storm.
Filter this by adding a threshold of received characters without TX
ready, above which characters are dropped.  This is disabled by default,
but has been tested with a threshold of 1000+.  A high threshold is
recommended to avoid dropping characters during, for instance, a large
copy/paste from the other end.

Sponsored by:	Juniper Networks, Inc.
This commit is contained in:
Justin Hibbits 2024-11-20 12:08:26 -05:00
parent 11ba19ddbb
commit 09ef538713
2 changed files with 18 additions and 0 deletions
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17
sys/dev/uart/uart_dev_ns8250.c
View file

@ -77,6 +77,11 @@ static int broken_txfifo = 0;
SYSCTL_INT(_hw, OID_AUTO, broken_txfifo, CTLFLAG_RWTUN,
&broken_txfifo, 0, "UART FIFO has QEMU emulation bug");
static int uart_noise_threshold = 0;
SYSCTL_INT(_hw, OID_AUTO, uart_noise_threshold, CTLFLAG_RWTUN,
&uart_noise_threshold, 0,
"Number of UART RX interrupts where TX is not ready, before data is discarded");
/*
* To use early printf on x86, add the following to your kernel config:
*
@ -1012,6 +1017,7 @@ int
ns8250_bus_receive(struct uart_softc *sc)
{
struct uart_bas *bas;
struct ns8250_softc *ns8250 = (struct ns8250_softc *)sc;
int xc;
uint8_t lsr;
@ -1023,6 +1029,17 @@ ns8250_bus_receive(struct uart_softc *sc)
sc->sc_rxbuf[sc->sc_rxput] = UART_STAT_OVERRUN;
break;
}
/* Filter out possible noise on the line.
* Expect that the device should be able to transmit as well as
* receive, so if we receive too many characters before transmit
* is ready, it's probably noise.
*/
if ((lsr & (LSR_TXRDY | LSR_TEMT)) == 0 &&
uart_noise_threshold > 0) {
if (++ns8250->noise_count >= uart_noise_threshold)
break;
} else
ns8250->noise_count = 0;
xc = uart_getreg(bas, REG_DATA);
if (lsr & LSR_FE)
xc |= UART_STAT_FRAMERR;

1
sys/dev/uart/uart_dev_ns8250.h
View file

@ -41,6 +41,7 @@ struct ns8250_softc {
uint8_t ier_mask;
uint8_t ier_rxbits;
uint8_t busy_detect;
int noise_count;
};
extern struct uart_ops uart_ns8250_ops;