[BRLTTY] ‘empty console’

[Nicolas Pitre]

On Wed, 27 Nov 2019, Sebastian Humenda wrote:
> Nicolas Pitre schrieb am Tue, Nov 26, 2019 at 04:44:42PM -0500:
> > What does "stty -a" give you, with and without the font change? More
> > specifically, what are the reported rows and columns values on the first
> > output line?
> 
> Font size 8 x 16 (non-frame buffer font)
> 
> speed 38400 baud; rows 90; columns 320; line = 0;
> 
> From small to large, the smallest working font size is  11 x 22, terminus:
> 
> speed 38400 baud; rows 65; columns 232; line = 0;

OK! That's it.

The /dev/vcsa device from which BRLTTY gets screen dimensions and the 
cursor position uses a header which fields are only 8-bit wide.  When 
this header was created (must be more than 20 years ago at this point) 
no one expected screens to hold more than 255 rows and/or lines.

More recently, to avoid returning random garbage, the kernel code 
started clamping those values to 255 if the actual value is larger. Your 
320 columns therefore gets clamped to 255 and that's what BRLTTY uses.

So, in practice, your braille window is probably located in the middle 
of the actual screen where there is no content.  For example, BRLTTY 
would seek content for line 2 column 1 from the screen location that 
actually corresponds to line 1 column 256.

I bet that if you move the window to the top left corner, or if you 
clear the screen at a shell prompt, then you'll see something.

On the BRLTTY's side, there could possibly be a way to use TIOCGWINSZ on 
the console device when vcsa returns 255 on the screen dimension. But 
that won't help with cursor position beyond 255 though. At least the 
user could then be warned.


Nicolas