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serial: Z85C30: avoid a hang at console switch-over 

Changes to the generic console support code that happened a while ago
introduced a scenario where the initial console is used in parallel with
the final console during a brief period when switching between the two is
in progress.  During that time a message about the switch-over is printed.

With some combinations of chips, firmware and drivers, such as the Zilog
Z85C30 SCC used with the DECstation, a hang may happen because the
firmware used for the initial console may not expect the state of the chip
after it has been initialised by the driver.  This is not a bug in the
firmware, as some registers it would have to examine are write-only.

This is a workaround for the Z85C30 which reuses the power-management
callback to keep the transmitter of the line associated with the console
enabled.  It reflects the consensus reached in a discussion a while ago.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Jiri Slaby <jirislaby@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Maciej W. Rozycki 2008-07-23 21:29:48 -07:00 committed by Linus Torvalds
parent b76c5a0717
commit 3771359128
1 changed files with 19 additions and 2 deletions
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21
drivers/serial/zs.c
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@ -787,7 +787,6 @@ static int zs_startup(struct uart_port *uport)
zport->regs[1] &= ~RxINT_MASK;
zport->regs[1] |= RxINT_ALL | TxINT_ENAB | EXT_INT_ENAB;
zport->regs[3] |= RxENABLE;
zport->regs[5] |= TxENAB;
zport->regs[15] |= BRKIE;
write_zsreg(zport, R1, zport->regs[1]);
write_zsreg(zport, R3, zport->regs[3]);
@ -814,7 +813,6 @@ static void zs_shutdown(struct uart_port *uport)
spin_lock_irqsave(&scc->zlock, flags);
zport->regs[5] &= ~TxENAB;
zport->regs[3] &= ~RxENABLE;
write_zsreg(zport, R5, zport->regs[5]);
write_zsreg(zport, R3, zport->regs[3]);
@ -959,6 +957,23 @@ static void zs_set_termios(struct uart_port *uport, struct ktermios *termios,
spin_unlock_irqrestore(&scc->zlock, flags);
}
/*
* Hack alert!
* Required solely so that the initial PROM-based console
* works undisturbed in parallel with this one.
*/
static void zs_pm(struct uart_port *uport, unsigned int state,
unsigned int oldstate)
{
struct zs_port *zport = to_zport(uport);
if (state < 3)
zport->regs[5] |= TxENAB;
else
zport->regs[5] &= ~TxENAB;
write_zsreg(zport, R5, zport->regs[5]);
}
static const char *zs_type(struct uart_port *uport)
{
@ -1041,6 +1056,7 @@ static struct uart_ops zs_ops = {
.startup = zs_startup,
.shutdown = zs_shutdown,
.set_termios = zs_set_termios,
.pm = zs_pm,
.type = zs_type,
.release_port = zs_release_port,
.request_port = zs_request_port,
@ -1190,6 +1206,7 @@ static int __init zs_console_setup(struct console *co, char *options)
return ret;
zs_reset(zport);
zs_pm(uport, 0, -1);
if (options)
uart_parse_options(options, &baud, &parity, &bits, &flow);