Date: Tue, 16 Mar 2021 17:56:39 GMT From: Mitchell Horne <mhorne@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org Subject: git: a54c346ff3e8 - stable/12 - ns8250: don't drop IER_TXRDY on bus_grab/ungrab Message-ID: <202103161756.12GHudtJ085315@gitrepo.freebsd.org>
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The branch stable/12 has been updated by mhorne: URL: https://cgit.FreeBSD.org/src/commit/?id=a54c346ff3e80ff8f2f3d0ec56b5374a7dc34429 commit a54c346ff3e80ff8f2f3d0ec56b5374a7dc34429 Author: Mitchell Horne <mhorne@FreeBSD.org> AuthorDate: 2021-03-10 14:57:12 +0000 Commit: Mitchell Horne <mhorne@FreeBSD.org> CommitDate: 2021-03-16 17:56:03 +0000 ns8250: don't drop IER_TXRDY on bus_grab/ungrab It has been observed that some systems are often unable to resume from ddb after entering with debug.kdb.enter=1. Checking the status further shows the terminal is blocked waiting in tty_drain(), but it never makes progress in clearing the output queue, because sc->sc_txbusy is high. I noticed that when entering polling mode for the debugger, IER_TXRDY is set in the failure case. Since this bit is never tracked by the softc, it will not be restored by ns8250_bus_ungrab(). This creates a race in which a TX interrupt can be lost, creating the hang described above. Ensuring that this bit is restored is enough to prevent this, and resume from ddb as expected. The solution is to track this bit in the sc->ier field, for the same lifetime that TX interrupts are enabled. PR: 223917, 240122 Sponsored by: The FreeBSD Foundation (cherry picked from commit 7e7f7beee732810d3afcc83828341ac3e139b5bd) --- sys/dev/uart/uart_dev_ns8250.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c index 52775a9b8a44..997eca3ea6cd 100644 --- a/sys/dev/uart/uart_dev_ns8250.c +++ b/sys/dev/uart/uart_dev_ns8250.c @@ -735,6 +735,7 @@ ns8250_bus_ipend(struct uart_softc *sc) } else { if (iir & IIR_TXRDY) { ipend |= SER_INT_TXIDLE; + ns8250->ier &= ~IER_ETXRDY; uart_setreg(bas, REG_IER, ns8250->ier); uart_barrier(bas); } else @@ -1032,7 +1033,9 @@ ns8250_bus_transmit(struct uart_softc *sc) uart_setreg(bas, REG_DATA, sc->sc_txbuf[i]); uart_barrier(bas); } - uart_setreg(bas, REG_IER, ns8250->ier | IER_ETXRDY); + if (!broken_txfifo) + ns8250->ier |= IER_ETXRDY; + uart_setreg(bas, REG_IER, ns8250->ier); uart_barrier(bas); if (broken_txfifo) ns8250_drain(bas, UART_DRAIN_TRANSMITTER);
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