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

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Message-ID: <20190412073344.GA28707@localhost.localdomain>
Date:   Fri, 12 Apr 2019 07:34:02 +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 12, 2019 at 09:17:28AM +0200, Enrico Weigelt, metux IT consult wrote:
>On 11.04.19 15:05, Adamski, Krzysztof (Nokia - PL/Wroclaw) wrote:
>
>> 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.
>So, in your case, it doesn't need to be an actual tty, any file/chardev
>already do it, correct ? But then, why is that new driver needed ?
>

Well, that depends. If the program doing those writes expects /dev/console
to be a tty device, then it cannot be any file. But, yes, there are usually
other ways of achieving this goal that do not involve adding kernel driver
but none of them is as convenient as having this kernel support. With this,
we can only switch kernel cmdline without any modifications to the rootfs.
The discussed kernel driver is very simple so there is little effort required
for this convenience. Maintaining it would be very easy and, of course,
anybody not needing it, can simply disable it in config so there is no cost.

Krzysztof