[PATCH v2 0/4] have the vt console preserve unicode characters

[Nicolas Pitre <nicolas.pitre@linaro.org>]

The vt code translates UTF-8 strings into glyph index values and stores
those glyph values directly in the screen buffer. Because there can only
be at most 512 glyphs, it is impossible to represent most unicode
characters, in which case a default glyph (often '?') is displayed
instead. The original unicode value is then lost.

This also means that the /dev/vcs* devices only provide user space with
glyph index values, and then user applications must get hold of the
unicode-to-glyph table the kernel is using in order to back-translate
those into actual characters. It is not possible to get back the original
unicode value when multiple unicode characters map to the same glyph,
especially for the vast majority that maps to the default replacement
glyph.

The 512-glyph limitation is inherent to VGA displays, but users of
/dev/vcs* shouldn't have to be restricted to a narrow unicode space from
lossy screen content because of that. This is especially true for
accessibility applications such as BRLTTY that rely on /dev/vcs to rander
screen content onto braille terminals.

This patch series introduces unicode support to /dev/vcs* devices,
allowing full unicode access from userspace to the vt console which
can, amongst other purposes, appropriately translate actual unicode
screen content into braille. Memory is allocated, and possible CPU
overhead introduced, only if /dev/vcsu is read at least once.

I'm a prime user of this feature, as well as the BRLTTY maintainer Dave Mielke
who implemented support for this in BRLTTY. There is therefore a vested
interest in maintaining this feature as necessary. And this received
extensive testing as well at this point.

Patch #4 was used for validation and is included for completeness, however
if people think it is unappropriate for mainline then it can be dropped.

This is also available on top of v4.18-rc1 here:

  git://git.linaro.org/people/nicolas.pitre/linux vt-unicode

Changes from v1:

- Rebased to v4.18-rc1.
- Dropped first patch (now in mainline as commit 4b4ecd9cb8).
- Removed a printk instance from an error path easily triggerable
  from user space.
- Minor cleanup.

Diffstat:

 drivers/tty/vt/vc_screen.c     |  90 +++++++--
 drivers/tty/vt/vt.c            | 347 +++++++++++++++++++++++++++++++++--
 include/linux/console_struct.h |   2 +
 include/linux/selection.h      |   5 +
 4 files changed, 419 insertions(+), 25 deletions(-)