linux-kernel - Re: [PATCH 2/2] serial: 8250: Allow to skip autoconfig

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Date:	Wed, 08 Jul 2015 10:00:27 -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 09:51 AM, Peter Hurley wrote:
> Hi Prarit,
> 
> On 07/08/2015 08:53 AM, Prarit Bhargava wrote:
>>
>>
>> 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.
> 
> For debugging purposes, can you use a kernel built with
> CONFIG_SERIAL_8250_DETECT_IRQ=n?
> 
> What is the platform and device type?
> 

Taichi?

P.

> Regards,
> Peter Hurley
> 
> 
>> 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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