Hi Adam,

On Mo, 2016-11-21 at 21:17 +0100, Adam Borowski wrote:

On Sun, Nov 20, 2016 at 10:58:08PM +0100, Manuel Schölling wrote:

quoted

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.

First, big thanks for this fix, it's something that greatly annoyed me
since forever!

Yeah, me too! ;)

The thing about clear_console is unfortunate: they abused the bug you're
fixing.  I've asked to use \e[3J (https://bugs.debian.org/845177) so there's
hope it'll be applied in stretch; with Debian configuring its glibc to
support only kernels from two releases before (in jessie that's 2.6.32, in
stretch 3.2)[1] there's hope we can flip the default in several years.

Do you suspect any other program relies on VT switch to clear the
scrollback?

Not, AFAIK. Although I do not have a complete list of programs that are
suppose to do that.

But alas, this commit breaks that very \e[3J.  It does only a \e[2J, leaving
the scrollback uncleared.  For comparison, both mainline and with just your
preparatory commit, \e[3J works as expected.

Really? All my tests worked fine: I compiled the kernel with the latest patches, started the kernel in QEMU and then did

  $ openvt /bin/sh
  $ echo -e '\e[3J' # scrollback buffer was flushed correctly
  $ chvt 2
  $ echo -e '\e[3J' # scrollback buffer was flushed correctly

Can you tell me how you tested it? Maybe I can reproduce the bug.

Thanks for spending the time to test it!

Bye,

Manuel