git: 7e7f7beee732 - main - ns8250: don't drop IER_TXRDY on bus_grab/ungrab

[Mitchell Horne]

The branch main has been updated by mhorne:

URL: https://cgit.FreeBSD.org/src/commit/?id=7e7f7beee732810d3afcc83828341ac3e139b5bd

commit 7e7f7beee732810d3afcc83828341ac3e139b5bd
Author:     Mitchell Horne <mhorne at FreeBSD.org>
AuthorDate: 2021-03-10 14:57:12 +0000
Commit:     Mitchell Horne <mhorne at FreeBSD.org>
CommitDate: 2021-03-10 15:04:42 +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
    Reviewed by:    imp, manu
    Tested by:      bz
    MFC after:      5 days
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D29130
---
 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 d920a76ae275..45b4d315c3d5 100644
--- a/sys/dev/uart/uart_dev_ns8250.c
+++ b/sys/dev/uart/uart_dev_ns8250.c
@@ -738,6 +738,7 @@ 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
@@ -1035,7 +1036,9 @@ 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);