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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 23.09.26 um 14:57 schrieb Greg KH:
[...]
>> + * 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?

Yes. There's the "vt.cur_default" parameter on the kernel's command line 
[1], and apparently there's some escape sequence. [2]  The cursor size 
is the lowest 4 bits of that value, while

[1] https://elixir.bootlin.com/linux/v7.2.6/source/drivers/tty/vt/vt.c#L167
[2] https://elixir.bootlin.com/linux/v7.2.6/source/drivers/tty/vt/vt.c#L2523

Apart from that, I just noticed that this switch should certainly handle 
CUR_DEF without printing a warning.


>
>> +		if (cursor_size != cursor_default_size)
>> +			cursor_size = cursor_default_size;
>> +		else
>> +			cursor_size = CUR_UNDERLINE;
> Why underline as a default?

This has been the default all along. It is the default value of the 
cur_default parameter and picked up by vt when starting up a console. 
[3] VGA BIOSes tend to default to underline as well AFAICT. Only the old 
cursor code in fbcon falls back to full-size blocks if nothing else 
works. Using underline harmonizes behavior across components.

[3] https://elixir.bootlin.com/linux/v7.2.6/source/drivers/tty/vt/vt.c#L2297

Best regards
Thomas

>
> thanks,
>
> greg k-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)