linux-kernel - Re: [PATCH] vt: monochrome mode with 512 glyphs

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Message-ID: <2025082128-appraisal-wince-844f@gregkh>
Date: Thu, 21 Aug 2025 08:12:26 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Zsolt Kajtar <soci@....rulez.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vt: monochrome mode with 512 glyphs

On Thu, Aug 21, 2025 at 05:55:33AM +0200, Zsolt Kajtar wrote:
> In monochrome mode the lowest 2 bits of the attribute are used for the
> intensity. If a 512 glyph font is loaded the lowest attribute bit is
> used for the MSB.
> 
> The result is that with normal intensity (1) the MSB of the glyph can
> be set.
> 
> Due to this on my setup the erase character becomes 0x120 instead of
> 0x20. As 0x120 is allocated to U+0161 in the loaded font the erased area
> will be filled with "s" instead of spaces.
> 
> To make the 512 glyph case work in monochrome mode the intensity was cut
> down to one bit (bold only) in this patch.
> 
> Signed-off-by: Zsolt Kajtar <soci@....rulez.org>
> ---
>  drivers/tty/vt/vt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 62049ceb3..8eebb5f17 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -667,7 +667,7 @@ static u8 build_attr(struct vc_data *vc, u8 _color,
>  	{
>  	u8 a = _color;
>  	if (!vc->vc_can_do_color)
> -		return _intensity |
> +		return (_intensity & ~(vc->vc_hi_font_mask ? 1 : 0)) |

For some reason you didn't cc: all of the maintainers and the subsystem
mailing list for this code :(

Anyway, this is anything but obvious as to what is happening, can this
somehow be documented in the change to make it more clear?

thanks,

greg k-h