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Subject[PATCH] serial: zs: Fix a transmit lockup in console output
Transmit interrupts are disabled and the transmit buffer drained in the 
course of console output so that polled transmission is possible. That
however causes a lost transmit interrupt as the TxIP bit in RR3 is only
set on a transmit buffer full-to-empty transition and then iff transmit
interrupts are enabled at the same time. Consequently if console output
disturbs a regular transmission in progress, the TxIP bit is never set
again and the transmission locks up waiting for a transmit interrupt.

Fix the problem by restarting transmission manually rather than waiting
for a transmit interrupt that will never happen.

Signed-off-by: Maciej W. Rozycki <>
Index: linux-20150524-4maxp64/drivers/tty/serial/zs.c
--- linux-20150524-4maxp64.orig/drivers/tty/serial/zs.c
+++ linux-20150524-4maxp64/drivers/tty/serial/zs.c
@@ -1181,6 +1181,10 @@ static void zs_console_write(struct cons
if (txint & TxINT_ENAB) {
zport->regs[1] |= TxINT_ENAB;
write_zsreg(zport, R1, zport->regs[1]);
+ /* Resume any transmission as the TxIP bit won't be set. */
+ if (!zport->tx_stopped)
+ zs_raw_transmit_chars(zport);
spin_unlock_irqrestore(&scc->zlock, flags);
 \ /
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