linux-kernel - Re: [PATCH v10 3/4] console: Add persistent scrollback buffers for all VGA consoles

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Date:   Thu, 19 Jan 2017 18:20:52 +0100
From:   Adam Borowski <kilobyte@...band.pl>
To:     Manuel Schölling <manuel.schoelling@....de>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
        lkml14@...tdoyle.com, rdunlap@...radead.org, shorne@...il.com,
        andrey_utkin@...tmail.com, akpm@...ux-foundation.org,
        paul.burton@...tec.com, daniel.vetter@...ll.ch, tj@...nel.org,
        hdegoede@...hat.com, linux-kernel@...r.kernel.org,
        linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v10 3/4] console: Add persistent scrollback buffers for
 all VGA consoles

On Thu, Jan 19, 2017 at 05:12:15PM +0100, Manuel Schölling wrote:
> On Thu, 2017-01-19 at 14:23 +0100, Greg KH wrote:
> > On Fri, Jan 13, 2017 at 09:07:57PM +0100, Manuel Schölling wrote:
> > > Add a scrollback buffers for each VGA console.  The benefit is that
> > > the scrollback history is not flushed when switching between consoles
> > > but is persistent.  The buffers are allocated on demand when a new
> > > console is opened.
> > > 
> > > This breaks tools like clear_console that rely on flushing the
> > > scrollback history by switching back and forth between consoles
> > > which is why this feature is disabled by default.
> > > Use the escape sequence \e[3J instead for flushing the buffer.
> > 
> > This issue is the one that makes me the most worried.  Why doesn't
> > clear_console() work anymore?  Why doesn't it use \e[3J ?
> 
> Well, clear_console() just switches from one console to another and
> back again. It just assumes that the scrollback buffer is flushed when
> switching.
> My plan is to make a patch for clear_console() as soon as these patches
> are in the kernel - it's chicken-and-egg problem.

No need to wait, \e[3J is supported since Linux 2.6.39; the problem I
spotted was that a previous version of your patch would break that.

It is also safe to output that sequence to a terminal unconditionally: I've
tested a number of terminals, they all either support it (most X terminals,
our console) or silently ignore it.

We can't, though, rely on terminfo to do so: it knows about this capability
(which it calls "E3") for TERM=linux only since very recently.  The TERM
variable is also unreliable: it fails to carry over a serial link while
blindly printing \e[3J works.

As for patching clear_console, https://bugs.debian.org/845177 has a minimal
fix; although for all setups supported by Debian that program could be be
better replaced with just "printf '\e[3J\e[2J'" which would make it work on
strictly more terminals than current code does.  Same for distributions
which copied clear_console (it originates from Ubuntu, maintained in Debian
since then).

Meow!
-- 
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