>From 859779819fe0490daaddeff5cb3cdda7a4372584 Mon Sep 17 00:00:00 2001
From: Prasad Koya <prasad@xxxxxxxxxx>
Date: Wed, 14 Jan 2015 11:07:41 -0800
Subject: [PATCH 1/1] tty, serial, 8250: to avoid recv fifo overrun, read rx fifo during xmit
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>
---
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
--
To unsubscribe from this list: send the line "unsubscribe linux-serial" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
