Re: [PATCH v3 1/5] vt: Add cursor-size helpers

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 28.09.26 um 11:07 schrieb Jani Nikula:
> On Mon, 28 Sep 2026, Thomas Zimmermann <tzimmermann@suse.de> 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.
>>
>> Users control the cursor size with the vt module's parameter cur_default
>> or with the ESC sequence \e[?Nc, where N is the cursor-size constant. In
>> case of an invalid setting, the new helpers fall back to cur_default and
>> then underline cursors; in this order.
>>
>> 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. VGA BIOSes also tend to use underline by.
>> default. Upon initialization vt applies its default to the console, but
>> each console might fall back to it own default. For example, running the
>> ESC code from above with the invalid constant of 8 magically flips the
>> cursor from underline to block size on vgacon. Another call with N set
>> to 0 (i.e., default) magically flips it back to underlyine. Making
>> underline the new default everywhere harmonizes vt, fbcon and most VGA
>> BIOSes.
>>
>> v3:
>> - mention user interfaces to control cursor size in commit message (GregKH)
>> - elaborate defaults (GregKH)
>> - move retry logic into helper
>> - handle CUR_DEF
>> 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            | 104 +++++++++++++++++++++++++++++++++
>>   include/linux/console_struct.h |   7 +++
>>   2 files changed, 111 insertions(+)
>>
>> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
>> index 57edf37495a8..8141296ccad0 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 +265,109 @@ unsigned int vc_font_size(const struct vc_font *font)
>>   }
>>   EXPORT_SYMBOL_GPL(vc_font_size);
>>   
>> +/*
>> + * Cursors
>> + */
>> +
>> +static unsigned int vc_cursor_size(unsigned int cursor_size)
>> +{
>> +	const unsigned int cursor_default_size = CUR_SIZE(READ_ONCE(cur_default));
>> +
>> +retry:
>> +	switch (cursor_size) {
>> +	case CUR_NONE:
>> +	case CUR_UNDERLINE:
>> +	case CUR_LOWER_THIRD:
>> +	case CUR_LOWER_HALF:
>> +	case CUR_TWO_THIRDS:
>> +	case CUR_BLOCK:
>> +		return cursor_size;
>> +	default:
>> +		pr_warn_once("Unknown cursor %u\n", cursor_size);
>> +		fallthrough;
>> +	case CUR_DEF:
>> +		/*
>> +		 * Use user-given default size, or underline if
>> +		 * the given default is invalid.
>> +		 */
>> +		if (cursor_size != cursor_default_size)
>> +			cursor_size = cursor_default_size;
>> +		else
>> +			cursor_size = CUR_UNDERLINE;
>> +		goto retry;
> Complete bikeshed, but IMO goto for retries is ugly. It's fine for error
> handling, but this could trivially be a loop.

Ok. I have no strong opinion about it.

Best regards
Thomas

>
> BR,
> Jani.
>
>> +	}
>> +}
>> +
>> +/**
>> + * 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)
>> +{
>> +	switch (vc_cursor_size(cursor_size)) {
>> +	case CUR_NONE:
>> +		return cell_height;
>> +	case CUR_UNDERLINE:
>> +	default:
>> +		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;
>> +	}
>> +}
>> +EXPORT_SYMBOL_GPL(vc_cursor_start);
>> +
>> +/**
>> + * vc_cursor_end - Calculates the first scanline after the cursor 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 first scanline after the cursor within the character cell
>> + */
>> +unsigned int vc_cursor_end(unsigned int cell_height, unsigned int cursor_size)
>> +{
>> +	switch (vc_cursor_size(cursor_size)) {
>> +	case CUR_UNDERLINE:
>> +	default:
>> +		if (cell_height < 10)
>> +			return cell_height;
>> +		else
>> +			return cell_height - 1;
>> +	case CUR_NONE:
>> +	case CUR_LOWER_THIRD:
>> +	case CUR_LOWER_HALF:
>> +	case CUR_TWO_THIRDS:
>> +	case CUR_BLOCK:
>> +		return cell_height;
>> +	}
>> +}
>> +EXPORT_SYMBOL_GPL(vc_cursor_end);
>> +
>>   /*
>>    * /sys/class/tty/tty0/
>>    *
>> diff --git a/include/linux/console_struct.h b/include/linux/console_struct.h
>> index fe915afdece5..1d37c7898ea0 100644
>> --- a/include/linux/console_struct.h
>> +++ b/include/linux/console_struct.h
>> @@ -204,6 +204,10 @@ struct vc {
>>   extern struct vc vc_cons [MAX_NR_CONSOLES];
>>   extern void vc_SAK(struct work_struct *work);
>>   
>> +/*
>> + * Cursors
>> + */
>> +
>>   #define CUR_MAKE(size, change, set)	((size) | ((change) << 8) |	\
>>   		((set) << 16))
>>   #define CUR_SIZE(c)		 ((c) & 0x00000f)
>> @@ -222,6 +226,9 @@ extern void vc_SAK(struct work_struct *work);
>>   #define CUR_CHANGE(c)		 ((c) & 0x00ff00)
>>   #define CUR_SET(c)		(((c) & 0xff0000) >> 8)
>>   
>> +unsigned int vc_cursor_start(unsigned int cell_height, unsigned int cursor_size);
>> +unsigned int vc_cursor_end(unsigned int cell_height, unsigned int cursor_size);
>> +
>>   bool con_is_visible(const struct vc_data *vc);
>>   
>>   #endif /* _LINUX_CONSOLE_STRUCT_H */

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Stefan Gaiser, Jochen Jaser, Abhinav Puri, (HRB 36809, AG Nürnberg)