[PATCH 2/2] serial: 8250: Allow to skip autoconfig_irq() for a console

[Taichi Kageyama]

This patch provides a new parameter as a workaround of the following
problem. It allows us to skip autoconfig_irq() and to use a well-known irq
number for a console even if CONFIG_SERIAL_8250_DETECT_IRQ is defined.

There're cases where autoconfig_irq() fails during boot.
In these cases, the console doesn't work in interrupt mode,
the mode cannot be changed anymore, and "input overrun"
(which can make operation mistakes) happens easily.
This problem happens with high rate every boot once it occurs
because the boot sequence is always almost same.

autoconfig_irq() assumes that a CPU can handle an interrupt from a serial
during the waiting time, but there're some cases where the CPU cannot
handle the interrupt for longer than the time. It completely depends on
how other functions work on the CPU. Ideally, autoconfig_irq() should be
fixed to control these cases but as long as auto_irq algorithm is used,
it's hard to know which CPU can handle an interrupt from a serial and
to assure the interrupt of the CPU is enabled during auto_irq.
Meanwhile for legacy consoles, they actually don't require auto_irq
because they basically use well-known irq number.
For non-console serials, this workaround is not required
because setserial command can kick autoconfig_irq() again for them.

Signed-off-by: Taichi Kageyama <t-kageyama@xxxxxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
  drivers/tty/serial/8250/8250_core.c |   17 +++++++++++++++++
  1 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 6bf31f2..60fda28 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -65,6 +65,8 @@ static int serial_index(struct uart_port *port)

  static unsigned int skip_txen_test; /* force skip of txen test at init time */

+static unsigned int skip_cons_autoirq; /* force skip autoirq for console */
+
  /*
   * Debugging.
   */
@@ -3336,6 +3338,9 @@ serial8250_register_ports(struct uart_driver *drv, struct device *dev)
  		if (skip_txen_test)
  			up->port.flags |= UPF_NO_TXEN_TEST;

+		if (uart_console(&up->port) && skip_cons_autoirq)
+			up->port.flags &= ~UPF_AUTO_IRQ;
+
  		uart_add_one_port(drv, &up->port);
  	}
  }
@@ -3875,6 +3880,9 @@ int serial8250_register_8250_port(struct uart_8250_port *up)
  		if (skip_txen_test)
  			uart->port.flags |= UPF_NO_TXEN_TEST;

+		if (uart_console(&uart->port) && skip_cons_autoirq)
+			uart->port.flags &= ~UPF_AUTO_IRQ;
+
  		if (up->port.flags & UPF_FIXED_TYPE)
  			uart->port.type = up->port.type;

@@ -3936,6 +3944,10 @@ void serial8250_unregister_port(int line)
  		uart->port.flags &= ~UPF_BOOT_AUTOCONF;
  		if (skip_txen_test)
  			uart->port.flags |= UPF_NO_TXEN_TEST;
+
+		if (uart_console(&uart->port) && skip_cons_autoirq)
+			uart->port.flags &= ~UPF_AUTO_IRQ;
+
  		uart->port.type = PORT_UNKNOWN;
  		uart->port.dev = &serial8250_isa_devs->dev;
  		uart->capabilities = 0;
@@ -4044,6 +4056,9 @@ MODULE_PARM_DESC(nr_uarts, "Maximum number of UARTs supported. (1-" __MODULE_STR
  module_param(skip_txen_test, uint, 0644);
  MODULE_PARM_DESC(skip_txen_test, "Skip checking for the TXEN bug at init time");

+module_param(skip_cons_autoirq, uint, 0644);
+MODULE_PARM_DESC(skip_cons_autoirq, "Skip auto irq for console during boot");
+
  #ifdef CONFIG_SERIAL_8250_RSA
  module_param_array(probe_rsa, ulong, &probe_rsa_count, 0444);
  MODULE_PARM_DESC(probe_rsa, "Probe I/O ports for RSA");
@@ -4070,6 +4085,8 @@ static void __used s8250_options(void)
  	module_param_cb(share_irqs, &param_ops_uint, &share_irqs, 0644);
  	module_param_cb(nr_uarts, &param_ops_uint, &nr_uarts, 0644);
  	module_param_cb(skip_txen_test, &param_ops_uint, &skip_txen_test, 0644);
+	module_param_cb(skip_cons_autoirq, &param_ops_uint,
+		&skip_cons_autoirq, 0644);
  #ifdef CONFIG_SERIAL_8250_RSA
  	__module_param_call(MODULE_PARAM_PREFIX, probe_rsa,
  		&param_array_ops, .arr = &__param_arr_probe_rsa,
-- 
1.7.1