Message-ID: <55B00CF8.5070901@hurleysoftware.com>
Date:	Wed, 22 Jul 2015 17:36:56 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Alexander <alexhoppus111@...il.com>
CC:	yalin wang <yalin.wang2010@...il.com>,
	Arun KS <arunks.linux@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arun KS <getarunks@...il.com>
Subject: Re: Why linux console designed to work in polling mode?

Hi Alexander,

On 07/22/2015 04:08 PM, Alexander wrote:
> Hi. Thanks for respond.
> Don't understand how this affects described logic (deferred printk).
> Suppose you put bytes to UART FIFO in console_unlock() until you can,
> after this up_console_sem and move forward. In UART interrupt you will
> do the same thing: get char from log_buf, put into UART FIFO until it
> will be full then break. How the fact that printk could be called from any context
> interfere this? Other way round, this logic will eliminate busyloop
> and decrease printk time significantly (including printk in interrupts etc)
>> it is not easy to implement in a context that you can’t call any sleep function.
> Deferred printing is done in UART interrupt, there is no need to sleep anywhere ...

Here's an example kernel crash report on the serial console with 'deferred'
serial output on typical 16550A h/w:

[76330.588297] B

That's not much to diagnose the crash with.

Regards,
Peter Hurley

> On Wed, 22 Jul 2015 14:53:13 +0800
> yalin wang <yalin.wang2010@...il.com> wrote:
> 
>>
>>> On Jul 22, 2015, at 14:27, Arun KS <arunks.linux@...il.com> wrote:
>>>
>>> When i checked how kernel printing works, i mentioned that it takes
>>> messages from log_buffer in console_unlock and gives it to
>>> call_console_drivers -> ...-> some uart bsp function. Basically, as i
>>> see this BSP realization tries to flush all message chars in busyloop
>>> ... so it waits until FIFO_NOT_FULL bit will be dropped by UART and it
>>> will be able to push the next byte. Basically, as i see userspace
>>> printing do something different. It puts N_FIFO_BYTES and exits, next,
>>> when FIFO will be freed - interrupt will be generated, and other
>>> characters will be put into UART FIFO.
>>> Can we do something similar for kernel printing? i.e. do not busyloop
>>> sending char after char, but put N_FIFO chars and flush  other in
>>> interrupt. When panic will occur we can do busyloop printing again. Is
>>> it reliable? Suppose we have several cores.
>>>
>>>
>>
>> i think it is because printk is very different from other printf  function in user space,
>> printk()  can be called  in any context  atomic / non- atomic / irq /  soft-irq context ..
>>
>> so your  bsp->print function can’t be sleep, can’t call any sleep functions .
>>
>> another reason is that in console_unlock() function, it call bas->print like this:
>> 	call_console_drivers(level, ext_text, ext_len, text, len);
>> print expect your bsp function print all the text as showed in parameters.
>> and it doesn’t check the return value ,
>> so if your driver doesn’t use POLL mode, you should only print N_FIFO_BYTES bytes,
>> this need prink to check return value or need your bsp function to use some special method
>> to send the remaining bytes after received FIFO empty interrupt.
>> it is not easy to implement in a context that you can’t call any sleep function.
>>
>> Thanks
> 

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