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(-)
