Patch is tested and based on 3.19-rc2.
On Wed, Jan 14, 2015 at 11:47 AM, <prasad.koya@xxxxxxxxx> wrote:
> 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
>
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