acrn-kernel/Documentation/braille-console.txt

                       Linux Braille Console

To get early boot messages on a braille device (before userspace screen
readers can start), you first need to compile the support for the usual serial
console (see serial-console.txt), and for braille device (in Device Drivers -
Accessibility).

Then you need to specify a console=brl, option on the kernel command line, the
format is:

	console=brl,serial_options...

where serial_options... are the same as described in serial-console.txt

So for instance you can use console=brl,ttyS0 if the braille device is connected
to the first serial port, and console=brl,ttyS0,115200 to override the baud rate
to 115200, etc.

By default, the braille device will just show the last kernel message (console
mode).  To review previous messages, press the Insert key to switch to the VT
review mode.  In review mode, the arrow keys permit to browse in the VT content,
page up/down keys go at the top/bottom of the screen, and the home key goes back
to the cursor, hence providing very basic screen reviewing facility.

Sound feedback can be obtained by adding the braille_console.sound=1 kernel
parameter.

For simplicity, only one braille console can be enabled, other uses of
console=brl,... will be discarded.  Also note that it does not interfere with
the console selection mechanism described in serial-console.txt

For now, only the VisioBraille device is supported.

Samuel Thibault <samuel.thibault@ens-lyon.org>