Re: [PATCH v2 1/5] vt: Add cursor-size helpers

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Sep 15, 2026 at 02:29:12PM +0200, Thomas Zimmermann wrote:
> Cursors in the VT subsystem are blocks within a character cell that are
> filled with the foreground color. The new helpers vc_cursor_start() and
> vc_cursor_end() return the scanlines in which the cursor block starts rsp.
> ends. This is compatible with VGA hardware
> 
> In terms of cursor design, the new cursor-size helpers follow established
> styles in fbcon. The only exception is in underline cursors for fonts with
> a size larger than 10. The underlining dash is now one pixel closer to
> the font-glyph data, so that the cursor looks less detached. This follows
> the style used by vgacon.
> 
> Similar code in vgacon and fbcon ignores the cursor's default size stored
> in vt.cur_default. The consoles default to full-block cursors, while vt
> defaults to underline cursors. The new helper fall back to cur_default and
> then underline cursors; in this order.
> 
> v2:
> - export non-font interface for vgacon
> - avoid interference from concurrent user space (Sashiko)
> - fix function docs (Sashiko)
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Reviewed-by: Helge Deller <deller@gmx.de>
> ---
>  drivers/tty/vt/vt.c            | 93 ++++++++++++++++++++++++++++++++++
>  include/linux/console_struct.h |  7 +++
>  2 files changed, 100 insertions(+)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 57edf37495a8..5d2583cd6e79 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -71,6 +71,7 @@
>   * by Adam Tla/lka <atlka@pg.gda.pl>, Aug 2006
>   */
>  
> +#include <linux/compiler.h>
>  #include <linux/module.h>
>  #include <linux/types.h>
>  #include <linux/sched/signal.h>
> @@ -264,6 +265,98 @@ unsigned int vc_font_size(const struct vc_font *font)
>  }
>  EXPORT_SYMBOL_GPL(vc_font_size);
>  
> +/*
> + * Cursors
> + */
> +
> +/**
> + * vc_cursor_start - Calculates the cursor's first scanline within a character cell
> + * @cell_height: The overall height of the character cell
> + * @cursor_size: The size constant of cursor pattern
> + *
> + * The parameter @cell_height is the height of the character cell as
> + * displayed by the console. The argument given in @cursor_size is one
> + * of the CUR_ constants, as stored in struct @vc_data.vc_cursor_type.
> + * For unknown values, the helper draws the default cursor or an underline
> + * dash.
> + *
> + * Returns:
> + * The index of the cursor's first scanline within the character cell
> + */
> +unsigned int vc_cursor_start(unsigned int cell_height, unsigned int cursor_size)
> +{
> +	const unsigned int cursor_default_size = CUR_SIZE(READ_ONCE(cur_default));
> +
> +retry:
> +	switch (cursor_size) {
> +	case CUR_NONE:
> +		return cell_height;
> +	case CUR_UNDERLINE:
> +		if (cell_height < 10)
> +			return cell_height - 1;
> +		else
> +			return cell_height - 3;
> +	case CUR_LOWER_THIRD:
> +		return cell_height - cell_height / 3;
> +	case CUR_LOWER_HALF:
> +		return cell_height - cell_height / 2;
> +	case CUR_TWO_THIRDS:
> +		return cell_height - (cell_height * 2) / 3;
> +	case CUR_BLOCK:
> +		return 0;
> +	default:
> +		pr_warn_once("Unknown cursor %u\n", cursor_size);

Can userspace trigger this?  If not, who can?

> +		if (cursor_size != cursor_default_size)
> +			cursor_size = cursor_default_size;
> +		else
> +			cursor_size = CUR_UNDERLINE;

Why underline as a default?

thanks,

greg k-h