Date:	Mon, 20 Jul 2015 12:36:11 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Taichi Kageyama <t-kageyama@...jp.nec.com>
CC:	Prarit Bhargava <prarit@...hat.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

Hi Taichi,

On 07/16/2015 05:58 AM, Taichi Kageyama wrote:
> On 2015/07/15 4:29, Peter Hurley wrote:
>> On 07/13/2015 09:16 PM, Taichi Kageyama wrote:
>>> On 2015/07/11 9:12, Peter Hurley wrote:
>>>> On 07/09/2015 01:32 AM, Taichi Kageyama wrote:
>>>>> On 2015/07/08 23:00, Prarit Bhargava wrote:
>>>>>> On 07/08/2015 09:51 AM, Peter Hurley wrote:
>>>>>>> On 07/08/2015 08:53 AM, Prarit Bhargava wrote:
>>>>>>>> On 07/08/2015 07:55 AM, Peter Hurley wrote:
>>>>>>>>> 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
>>>>>>>>>
>>>>>
>>>>> Thank you for your comments.
>>>>>
>>>>>>>>> It completely depends on how long some other driver has interrupts disabled,
>>>>>
>>>>> Agree.
>>>>>
>>>>>>>>> which is a problem that needs fixed _in that driver_. autoconfig_irq() does not
>>>>>>>>> need fixing.
>>>>>
>>>>> Peter, ideally, you're right.
>>>>> However, we cannot assume how long other drivers disable interrupts.
>>>>> That's why I introduced this workaround.
>>>>> In my opinion, a console is important and always should be available
>>>>> even if other drivers have a bad behavior.
>>>>
>>>> I have no problem with wanting to make the console more robust, but
>>>> rather with the hacky way this is being done.
>>>
>>> Hi Peter,
>>>
>>> Thank you for your advice.
>>> If there is other way to fix this problem simply,
>>> I also think it's better than the dirty hack.
>>
>> While module parameters seem like "simple" solutions at the time,
>> they add real maintenance burden, because they establish userspace
>> requirements that must be preserved forever to avoid breakage.
> 
> Yeah, I agree with you.
> 
>>>> Better solutions:
>>>> 1. Fix autoprobing to force irq affinity to autoprobing cpu
>>>
>>> I couldn't make sure which CPU handled serial interrupt
>>> on all platforms before irq# was not known.
>>> Do you know the way to detect which CPU is used for console serial?
>>
>>
>> The basic idea would be:
>> 1. disable preemption
>> 2. for each irq descriptor selected for autoprobing, set the irq
>>     affinity to the current processor.
>> 3. probe the i/o port as is done now
>> 4. stop probing
>> 5. re-enable preemption.
> 
> Thanks, I think it works.
> 
>> With this solution, your patch 1/2 wouldn't be required either
>> because the worker thread that disabled interrupts wouldn't be
>> running on the cpu detecting the triggered irq(s).
> 
> I still need my patch 1/2 which fixes also other cases (see case2 & 3).
> I think both port->lock and console_lock are required in your solution.
> to avoid deadlock because printk() can be called on every context.
> 
>> I would imagine most or all of this would be done in
>> probe_irq_on(), possibly refactored to perform the preemption
>> disable and irq affinity.
> 
> I think introducing new function like "probe_irq_set_affinity()" is better
> than modifying probe_irq_on(). I cannot test all legacy devices and
> I don't have any reason to break the code which works for other devices.

That's fine, although most of the arguments for fixing this in the serial
driver apply equally to other users of probe_irq_on().


>>> The way is safe for all platforms?
>>
>> Please understand though, autoprobing is not safe, period.
>> Even says so in Kconfig.
> 
> OK, I'll try to create new patch which makes autoprobing safer as possible.
> New patch is going to be like below.
>   1. console and port lock
>   2. probe_irq_on()
>   3. probe_irq_set_affinity(&cpumask)
>   4. probe_irq_off()
>   5. port and console unlock

The port->lock can't be taken in this context because hard irq
has to be disabled with port->lock which defeats the purpose of
pinning the irq affinity to the current cpu.

What are you concerned about being concurrent with autoconfig_irq()?
Many operations are excluded by the port->mutex.


> I think we don't have to care saving original affinity value and restore it.
> Probably serial8250_startup->... setup_affinity() resets irq affinity
> as default every after autoconfig_irq() is called.
> 
>>>> 2. Fix how the 8250 driver behaves with no irq
>>>
>>> The console works with polling-mode if irq==0.
>>> I think this behavior should not be changed.
>>
>> So another solution would be to use setserial to set the irq during
>> boot, right?
> 
> Right, if it works. Users have to kick setserial before some programs
> which opens the console and make sure its irq# is valid.

Ubuntu does this as part of boot (restores saved serial setup) but I
understand other distros might not do this.

Regards,
Peter Hurley
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