8250: fix boot hang with serial console when using with Serial Over Lan port

Intel 8257x Ethernet boards have a feature called Serial Over Lan.

This feature works by emulating a serial port, and it is detected by
kernel as a normal 8250 port.  However, this emulation is not perfect, as
also noticed on changeset 7500b1f602aad75901774a67a687ee985d85893f.

Before this patch, the kernel were trying to check if the serial TX is
capable of work using IRQ's.

This were done with a code similar this:

        serial_outp(up, UART_IER, UART_IER_THRI);
        lsr = serial_in(up, UART_LSR);
        iir = serial_in(up, UART_IIR);
        serial_outp(up, UART_IER, 0);

        if (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT)
		up->bugs |= UART_BUG_TXEN;

This works fine for other 8250 ports, but, on 8250-emulated SoL port, the
chip is a little lazy to down UART_IIR_NO_INT at UART_IIR register.

Due to that, UART_BUG_TXEN is sometimes enabled.  However, as TX IRQ keeps
working, and the TX polling is now enabled, the driver miss-interprets the
IRQ received later, hanging up the machine until a key is pressed at the
serial console.

This is the 6 version of this patch.  Previous versions were trying to
introduce a large enough delay between serial_outp and serial_in(up,
UART_IIR), but not taking forever.  However, the needed delay couldn't be
safely determined.

At the experimental tests, a delay of 1us solves most of the cases, but
still hangs sometimes.  Increasing the delay to 5us was better, but still
doesn't solve.  A very high delay of 50 ms seemed to work every time.

However, poking around with delays and pray for it to be enough doesn't
seem to be a good approach, even for a quirk.

So, instead of playing with random large arbitrary delays, let's just
disable UART_BUG_TXEN for all SoL ports.

[akpm@linux-foundation.org: fix warnings]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 0d934bfbdd9b..b4b39811b445 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -2083,6 +2083,20 @@ static int serial8250_startup(struct uart_port *port)
 
 	serial8250_set_mctrl(&up->port, up->port.mctrl);
 
+	/* Serial over Lan (SoL) hack:
+	   Intel 8257x Gigabit ethernet chips have a
+	   16550 emulation, to be used for Serial Over Lan.
+	   Those chips take a longer time than a normal
+	   serial device to signalize that a transmission
+	   data was queued. Due to that, the above test generally
+	   fails. One solution would be to delay the reading of
+	   iir. However, this is not reliable, since the timeout
+	   is variable. So, let's just don't test if we receive
+	   TX irq. This way, we'll never enable UART_BUG_TXEN.
+	 */
+	if (up->port.flags & UPF_NO_TXEN_TEST)
+		goto dont_test_tx_en;
+
 	/*
 	 * Do a quick test to see if we receive an
 	 * interrupt when we enable the TX irq.
@@ -2102,6 +2116,7 @@ static int serial8250_startup(struct uart_port *port)
 		up->bugs &= ~UART_BUG_TXEN;
 	}
 
+dont_test_tx_en:
 	spin_unlock_irqrestore(&up->port.lock, flags);
 
 	/*