[PATCH] serial: zs: Fix a transmit lockup in console output

[Maciej W. Rozycki]

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 <ma...@linux-mips.org>
---
linux-mips-dec-zs-console-transmit.diff
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);
 }