[PATCH 1/1] tty, serial, 8250: to avoid recv fifo overrun, read rx fifo during xmit

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From: Prasad Koya <prasad@xxxxxxxxxx>

At slow baud rates, say 9600, it is easy to run into tty buffer overrun
as interrupts could get disabled for a long time. To avoid that, if
UART_LSR_DR is set, pick up bytes from rx fifo while waiting for tx
buffer to be empty. Stop buffering if port->sysrq is true or
oops_in_progress is set.

George Spelvin reworked original patch quite a bit.

Tested on 16550 UART with 16byte FIFO

Signed-off-by: Prasad Koya <prasad@xxxxxxxxxx>
Signed-off-by: George Spelvin <linux@xxxxxxxxxxx>
---
 drivers/tty/serial/8250/8250_core.c | 65 +++++++++++++++++++++++++++++++++----
 drivers/tty/serial/8250/8250_fsl.c  |  2 +-
 include/linux/serial_8250.h         |  3 +-
 include/linux/serial_core.h         |  2 +-
 4 files changed, 63 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 11c6685..a9adf9d 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1434,9 +1434,12 @@ static void serial8250_enable_ms(struct uart_port *port)
  * serial8250_rx_chars: processes according to the passed in LSR
  * value, and returns the remaining LSR bits not handled
  * by this Rx routine.
+ * no_sysrq is normally false, but when set it indicates we should stop
+ * processing input after receiving a break, rather than passing the
+ * next character to uart_handle_sysrq_char.
  */
 unsigned char
-serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)
+serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr, bool no_sysrq)
 {
 	struct uart_port *port = &up->port;
 	unsigned char ch;
@@ -1472,8 +1475,11 @@ serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)
 				 * may get masked by ignore_status_mask
 				 * or read_status_mask.
 				 */
-				if (uart_handle_break(port))
+				if (uart_handle_break(port)) {
+					if (no_sysrq)
+						max_count = 0;
 					goto ignore_char;
+				}
 			} else if (lsr & UART_LSR_PE)
 				port->icount.parity++;
 			else if (lsr & UART_LSR_FE)
@@ -1609,7 +1615,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 			dma_err = up->dma->rx_dma(up, iir);
 
 		if (!up->dma || dma_err)
-			status = serial8250_rx_chars(up, status);
+			status = serial8250_rx_chars(up, status, false);
 	}
 	serial8250_modem_status(up);
 	if ((!up->dma || (up->dma && up->dma->tx_err)) &&
@@ -1962,9 +1968,30 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state)
 }
 
 /*
+ * In some special situations, we allow console output without necessarily
+ * locking the port.  The same situations also suppress processing input
+ * during polling output, because we aren't ready to deal with it.
+ * So just hope the hardware FIFO can hold until interrupts are
+ * re-enabled.
+ */
+static bool emergency_situation(const struct uart_port *port)
+{
+#ifdef SUPPORT_SYSRQ
+	return port->sysrq != 0 || oops_in_progress;
+#else
+	(void)port;
+	return oops_in_progress;
+#endif
+}
+
+/*
  *	Wait for transmitter & holding register to empty
+ *	If rx is true, then also poll for received characters if it's
+ *	safe to process them.  This is only used in the SUPPORT_SYSRQ
+ *	case, and hopefully the compiler can figure out it's dead code
+ *	if that's not the case.
  */
-static void wait_for_xmitr(struct uart_8250_port *up, int bits)
+static void __wait_for_xmitr(struct uart_8250_port *up, int bits, bool rx)
 {
 	unsigned int status, tmout = 10000;
 
@@ -1978,6 +2005,11 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
 			break;
 		if (--tmout == 0)
 			break;
+
+		if (rx && status & (UART_LSR_DR | UART_LSR_BI) &&
+		    !emergency_situation(&up->port))
+			serial8250_rx_chars(up, status, true);
+
 		udelay(1);
 	}
 
@@ -1989,12 +2021,29 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
 			up->msr_saved_flags |= msr & MSR_SAVE_FLAGS;
 			if (msr & UART_MSR_CTS)
 				break;
+
+			if (rx && !emergency_situation(&up->port)) {
+				status = serial_in(up, UART_LSR);
+
+				up->lsr_saved_flags |= status & LSR_SAVE_FLAGS;
+				if (status & (UART_LSR_DR | UART_LSR_BI))
+					serial8250_rx_chars(up, status, true);
+			}
+
 			udelay(1);
 			touch_nmi_watchdog();
 		}
 	}
 }
 
+/*
+ *	Wait for transmitter & holding register to empty
+ */
+static void wait_for_xmitr(struct uart_8250_port *up, int bits)
+{
+	__wait_for_xmitr(up, bits, false);
+}
+
 #ifdef CONFIG_CONSOLE_POLL
 /*
  * Console polling routines for writing and reading from the uart while
@@ -3168,7 +3217,7 @@ static void serial8250_console_putchar(struct uart_port *port, int ch)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
 
-	wait_for_xmitr(up, UART_LSR_THRE);
+	__wait_for_xmitr(up, UART_LSR_THRE, true);
 	serial_port_out(port, UART_TX, ch);
 }
 
@@ -3176,6 +3225,10 @@ static void serial8250_console_putchar(struct uart_port *port, int ch)
  *	Print a string to the serial port trying not to disturb
  *	any possible real use of the port...
  *
+ *	In order to avoid dropped input if the output is longer than
+ *	the available hardware FIFO space, we try to handle incoming
+ *	characters during this polling loop.
+ *
  *	The console_lock must be held when we get here.
  */
 static void
@@ -3214,7 +3267,7 @@ serial8250_console_write(struct console *co, const char *s, unsigned int count)
 	 *	Finally, wait for transmitter to become empty
 	 *	and restore the IER
 	 */
-	wait_for_xmitr(up, BOTH_EMPTY);
+	__wait_for_xmitr(up, BOTH_EMPTY, true);
 	serial_port_out(port, UART_IER, ier);
 
 	/*
diff --git a/drivers/tty/serial/8250/8250_fsl.c b/drivers/tty/serial/8250/8250_fsl.c
index c0533a5..25f0aa3 100644
--- a/drivers/tty/serial/8250/8250_fsl.c
+++ b/drivers/tty/serial/8250/8250_fsl.c
@@ -49,7 +49,7 @@ int fsl8250_handle_irq(struct uart_port *port)
 	lsr = orig_lsr = up->port.serial_in(&up->port, UART_LSR);
 
 	if (lsr & (UART_LSR_DR | UART_LSR_BI))
-		lsr = serial8250_rx_chars(up, lsr);
+		lsr = serial8250_rx_chars(up, lsr, false);
 
 	serial8250_modem_status(up);
 
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index e02acf0..8544bc5 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -128,7 +128,8 @@ extern void serial8250_do_pm(struct uart_port *port, unsigned int state,
 			     unsigned int oldstate);
 extern int fsl8250_handle_irq(struct uart_port *port);
 int serial8250_handle_irq(struct uart_port *port, unsigned int iir);
-unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr);
+unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr,
+				  bool no_sysrq);
 void serial8250_tx_chars(struct uart_8250_port *up);
 unsigned int serial8250_modem_status(struct uart_8250_port *up);
 
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 057038c..c93c5aa 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -410,7 +410,7 @@ uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
 	if (port->sysrq) {
 		if (ch && time_before(jiffies, port->sysrq)) {
 			handle_sysrq(ch);
-			port->sysrq = 0;
+			port->sysrq = 0;   /* Must be AFTER handle_sysrq */
 			return 1;
 		}
 		port->sysrq = 0;
-- 
1.8.1.4

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