Message-ID: <559D1D39.9070405@redhat.com>
Date:	Wed, 08 Jul 2015 08:53:13 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	Taichi Kageyama <t-kageyama@...jp.nec.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"jslaby@...e.cz" <jslaby@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH 2/2] serial: 8250: Allow to skip autoconfig_irq() for
 a console



On 07/08/2015 07:55 AM, Peter Hurley wrote:
> Hi Taichi,
> 
> On 06/05/2015 06:03 AM, Taichi Kageyama wrote:
>> 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
> 
> It completely depends on how long some other driver has interrupts disabled,
> which is a problem that needs fixed _in that driver_. autoconfig_irq() does not
> need fixing.
> 
> A typical cause of this behavior is printk spew from overly-instrumented
> debugging of some driver (trace_printk is better suited to heavy instrumentation).
> 

Peter, I understand what you're saying, however the problem is that this is the
serial driver which is one of, if not _the_ main debug output mechanism in the
kernel.

I'm not sure I agree with doing this patch, but perhaps an easier approach would
be to add a debug kernel parameter like "serial.force_irq=4" to force the irq to
a previously known good value?  That way the issue of the broken driver can be
debugged.

P.

> Regards,
> Peter Hurley
> 
> 
>> 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@...jp.nec.com>
>> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>> ---
>>   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,
>>
> 
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