linux-kernel - Re: [PATCH] tty: Add NULL TTY driver

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Message-ID: <20190411130458.GA11390@localhost.localdomain>
Date:   Thu, 11 Apr 2019 13:05:20 +0000
From:   "Adamski, Krzysztof (Nokia - PL/Wroclaw)" 
        <krzysztof.adamski@...ia.com>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
CC:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "jslaby@...e.com" <jslaby@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: Add NULL TTY driver

On Fri, Apr 05, 2019 at 02:32:41PM +0200, Enrico Weigelt, metux IT consult wrote:
>On 05.04.19 11:00, Vincent Whitchurch wrote:
>> On Fri, Apr 05, 2019 at 10:39:43AM +0200, Enrico Weigelt, metux IT consult wrote:
>>> On 03.04.19 16:11, Vincent Whitchurch wrote:
>>>
>>>> Especially on embedded systems, it would be convenient to have a simple
>>>> way to disable the console (both for kernel and userspace) on a system
>>>> which normally uses it, to free up the UART for other things.
>>>
>>> Just symlinking to /dev/null does not work ?
>>
>> No, /dev/null does not support the TTY ioctls.
>
>hmm, wo (which programs) do you need, that really need them ?
>
>>> OTOH, if you're introducing a dummy console, wouldn't a ringbuffer that,
>>> can be read out later, a better option ?
>>
>> There is already a ttyprintk driver in mainline to send these messages
>> to the printk ring buffer if one is actually intrested in what is
>> written to the console.  There's no option to enable it via console= in
>> mainline but I have a patch for that too.
>
>Great. IMHO, that would be the better way.

There are (embedded) cases where the kernel ring buffer is stored for
log inspection and all the logs that are *not* wanted there (like
interactive debug logs, some progress bar, etc) are send specifically to
/dev/console to avoid sending them to normal log store (as they are only
useful when debugging over console). In non-debug mode /dev/console
becomes /dev/null to speed things up.

I would personally benefit from Vincents patch. In fact I found this
thread because I was considering doing exactly the same for our system.

Krzysztof