Hello, Gentoo.

Yes, console soft scrolling is back!  That essential feature that was
stripped out of the kernel at around 5.4.x has returned!

Only this time, it's even better!  Instead of one scrollback buffer
shared between all tty's, there's now a buffer for each tty.

How to get it working:
(i) Extract the enclosed patch file to your /usr/src/linux-5.10.27-gentoo
directory (or probably any 5.10 kernel's).
(ii) From that directory run $ patch -p0 < diff.20210405.diff.
(iii)(a) If you've already got a working 5.10.27, do a $ make oldconfig.
  That should bring up the new configuration items, for which you can
  accept the defaults.  These two items are a flag to enable the
  scrollback, and a buffer size defaulting to 128kB.
(iii)(b) If you haven't yet got a 5.10.27, just configure your kernel in
  the usual way.  The two new items are under Device Drivers/Graphics
  support/Console display driver support.
(iv) Build the kernel.
(v) Put the new kernel into your usual boot manager.
(vi) Reboot and enjoy!

Admittedly, the exercise isn't quite finished - the patched source files
still have my personal change markings in them, to make debugging easier.
But the problems I reported here a few days ago are now solved.  One or
two features haven't (yet) been implemented - having a single scroll
buffer shared amongst all tty's isn't there, and there're no kernel
command line parameters to control the feature.

Also, you may wonder about how safe the patch is.  All I can say is that
there is nothing malicious in it, and I am known on this list, but of
course if it breaks I won't be to blame.

Bug reports and other comments are welcome, of course.

If anybody would like the corresponding patch which works on 5.4.n, for n
>= 80, that is available, too.

-- 
Alan Mackenzie (Nuremberg, Germany).