[PATCH] serial: 8250_dw: Improve unwritable LCR workaround

[Tim Kryger]

The Designware UART has a limitation where it ignores writes into the
LCR if the UART is busy.  The current workaround stashes a copy of the
last written LCR and writes it back down to the hardware if it receives
a special busy interrupt which is raised when a write was ignored.

Unfortunately, interrupts are typically disabled prior to performing a
sequence of register writes that include the LCR so the point at which
the retry occurs is too late.  An example is serial8250_do_set_termios()
where an ignored LCR write results in the baud divisor not being set and
instead a garbage character is sent out the transmitter.

Furthermore, since serial_port_out() offers no way to indicate failure,
a serious effort must be made to ensure that the LCR is actually updated
before returning back to the caller.  This is difficult, however, as a
UART that was busy during the first attempt is likely to still be busy
when a subsequent attempt is made unless some extra action is taken.

This updated workaround takes the extreme action of clearing the TX/RX
FIFOs and reading the receive buffer before writing down the LCR in the
hope that doing so will force the UART into an idle state.  While this
may seem unnecessarily aggressive, writes to the LCR are used to change
the baud rate, parity, stop bit, or data length so the data that may be
lost is likely not important.  Admittedly, this is far from ideal but it
seems to be the best that can be done given the hardware limitations.

Lastly, the revised workaround doesn't touch the LCR in the ISR, so it
avoids the possibility of a "serial8250: too much work for irq" lock up.
This problem is rare in real situations but can be reproduced easily by
wiring up two UARTs and running the following commands.

  # stty -F /dev/ttyS1 echo
  # stty -F /dev/ttyS2 echo
  # cat /dev/ttyS1 &
  [1] 375
  # echo asdf > /dev/ttyS1
  asdf

  [   27.700000] serial8250: too much work for irq96
  [   27.700000] serial8250: too much work for irq96
  [   27.710000] serial8250: too much work for irq96
  [   27.710000] serial8250: too much work for irq96
  [   27.720000] serial8250: too much work for irq96
  [   27.720000] serial8250: too much work for irq96
  [   27.730000] serial8250: too much work for irq96
  [   27.730000] serial8250: too much work for irq96
  [   27.740000] serial8250: too much work for irq96

Signed-off-by: Tim Kryger <tim.kryger@xxxxxxxxxx>
Reviewed-by: Matt Porter <matt.porter@xxxxxxxxxx>
Reviewed-by: Markus Mayer <markus.mayer@xxxxxxxxxx>
---
 drivers/tty/serial/8250/8250_dw.c | 57 +++++++++++++++++++++++++++------------
 1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index daf710f..ffbb69c 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -56,7 +56,6 @@
 
 
 struct dw8250_data {
-	int		last_lcr;
 	int		last_mcr;
 	int		line;
 	struct clk	*clk;
@@ -76,17 +75,35 @@ static inline int dw8250_modify_msr(struct uart_port *p, int offset, int value)
 	return value;
 }
 
+/* The UART will ignore writes to LCR when busy we take aggressive steps
+ * to ensure that it is idle before attempting to write to LCR */
+static void dw8250_force_idle(struct uart_port *p)
+{
+	serial8250_clear_and_reinit_fifos(container_of
+					  (p, struct uart_8250_port, port));
+	(void)p->serial_in(p, UART_LSR);
+	(void)p->serial_in(p, UART_MSR);
+	(void)p->serial_in(p, UART_RX);
+}
+
 static void dw8250_serial_out(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = p->private_data;
 
-	if (offset == UART_LCR)
-		d->last_lcr = value;
-
-	if (offset == UART_MCR)
-		d->last_mcr = value;
-
-	writeb(value, p->membase + (offset << p->regshift));
+	if (offset == UART_LCR) {
+		int tries = 1000;
+		while (tries--) {
+			if (value == p->serial_in(p, UART_LCR))
+				return;
+			dw8250_force_idle(p);
+			writeb(value, p->membase + (UART_LCR << p->regshift));
+		}
+		dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+	} else {
+		if (offset == UART_MCR)
+			d->last_mcr = value;
+		writeb(value, p->membase + (offset << p->regshift));
+	}
 }
 
 static unsigned int dw8250_serial_in(struct uart_port *p, int offset)
@@ -107,13 +124,20 @@ static void dw8250_serial_out32(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = p->private_data;
 
-	if (offset == UART_LCR)
-		d->last_lcr = value;
-
-	if (offset == UART_MCR)
-		d->last_mcr = value;
-
-	writel(value, p->membase + (offset << p->regshift));
+	if (offset == UART_LCR) {
+		int tries = 1000;
+		while (tries--) {
+			if (value == p->serial_in(p, UART_LCR))
+				return;
+			dw8250_force_idle(p);
+			writel(value, p->membase + (UART_LCR << p->regshift));
+		}
+		dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+	} else {
+		if (offset == UART_MCR)
+			d->last_mcr = value;
+		writel(value, p->membase + (offset << p->regshift));
+	}
 }
 
 static unsigned int dw8250_serial_in32(struct uart_port *p, int offset)
@@ -131,9 +155,8 @@ static int dw8250_handle_irq(struct uart_port *p)
 	if (serial8250_handle_irq(p, iir)) {
 		return 1;
 	} else if ((iir & UART_IIR_BUSY) == UART_IIR_BUSY) {
-		/* Clear the USR and write the LCR again. */
+		/* Clear the USR */
 		(void)p->serial_in(p, d->usr_reg);
-		p->serial_out(p, UART_LCR, d->last_lcr);
 
 		return 1;
 	}
-- 
1.8.0.1


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