From 09ef538713e08a98584dbcf66ff80290cc1b41a2 Mon Sep 17 00:00:00 2001 From: Justin Hibbits Date: Wed, 20 Nov 2024 12:08:26 -0500 Subject: [PATCH] uart: Add primitive noise filtering on RX 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. --- sys/dev/uart/uart_dev_ns8250.c | 17 +++++++++++++++++ sys/dev/uart/uart_dev_ns8250.h | 1 + 2 files changed, 18 insertions(+) diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c index 3da2e8b8c75..1df39b49953 100644 --- a/sys/dev/uart/uart_dev_ns8250.c +++ b/sys/dev/uart/uart_dev_ns8250.c @@ -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; diff --git a/sys/dev/uart/uart_dev_ns8250.h b/sys/dev/uart/uart_dev_ns8250.h index 324ff72f6e5..e8a17e96c26 100644 --- a/sys/dev/uart/uart_dev_ns8250.h +++ b/sys/dev/uart/uart_dev_ns8250.h @@ -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;