linux-kernel - Re: [PATCH v2 2/2] vt: add VT_GETCONSIZECSRPOS to retrieve console size and cursor position

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Message-ID: <8fb2c16f-0e9b-402d-a7f2-4881de8c7bd9@kernel.org>
Date: Thu, 15 May 2025 07:47:32 +0200
From: Jiri Slaby <jirislaby@...nel.org>
To: Nicolas Pitre <nico@...xnic.net>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Nicolas Pitre <npitre@...libre.com>, linux-serial@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] vt: add VT_GETCONSIZECSRPOS to retrieve console
 size and cursor position

On 14. 05. 25, 21:42, Nicolas Pitre wrote:
> From: Nicolas Pitre <npitre@...libre.com>
> 
> The console dimension and cursor position are available through the
> /dev/vcsa interface already. However the /dev/vcsa header format uses
> single-byte fields therefore those values are clamped to 255.
> 
> As surprizing as this may seem, some people do use 240-column 67-row
> screens (a 1920x1080 monitor with 8x16 pixel fonts) which is getting
> close to the limit. Monitors with higher resolution are not uncommon
> these days (3840x2160 producing a 480x135 character display) and it is
> just a matter of time before someone with, say, a braille display using
> the Linux VT console and BRLTTY on such a screen reports a bug about
> missing and oddly misaligned screen content.
> 
> Let's add VT_GETCONSIZECSRPOS for the retrieval of console size and cursor
> position without byte-sized limitations. The actual console size limit as
> encoded in vt.c is 32767x32767 so using a short here is appropriate. Then
> this can be used to get the cursor position when /dev/vcsa reports 255.
> 
> The screen dimension may already be obtained using TIOCGWINSZ and adding
> the same information to VT_GETCONSIZECSRPOS might be redundant. However
> applications that care about cursor position also care about display
> size and having 2 separate system calls to obtain them separately is
> wasteful. Also, the cursor position can be queried by writing "\e[6n" to
> a tty and reading back the result but that may be done only by the actual
> application using that tty and not a sideline observer.
> 
> Signed-off-by: Nicolas Pitre <npitre@...libre.com>
> ---
>   drivers/tty/vt/vt_ioctl.c | 16 ++++++++++++++++
>   include/uapi/linux/vt.h   |  9 +++++++++
>   2 files changed, 25 insertions(+)
> 
> diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
> index 4b91072f3a4e..83a3d49535e5 100644
> --- a/drivers/tty/vt/vt_ioctl.c
> +++ b/drivers/tty/vt/vt_ioctl.c
> @@ -951,6 +951,22 @@ int vt_ioctl(struct tty_struct *tty,
>   					(unsigned short __user *)arg);
>   	case VT_WAITEVENT:
>   		return vt_event_wait_ioctl((struct vt_event __user *)arg);
> +
> +	case VT_GETCONSIZECSRPOS:
> +	{
> +		struct vt_consizecsrpos concsr;
> +
> +		console_lock();
> +		concsr.con_cols = vc->vc_cols;
> +		concsr.con_rows = vc->vc_rows;
> +		concsr.csr_col = vc->state.x;
> +		concsr.csr_row = vc->state.y;
> +		console_unlock();

Makes a lot of sense!

> +		if (copy_to_user(up, &concsr, sizeof(concsr)))
> +			return -EFAULT;
> +		return 0;
> +	}
> +
>   	default:
>   		return -ENOIOCTLCMD;
>   	}
> diff --git a/include/uapi/linux/vt.h b/include/uapi/linux/vt.h
> index e9d39c48520a..e93c8910133b 100644
> --- a/include/uapi/linux/vt.h
> +++ b/include/uapi/linux/vt.h
> @@ -84,4 +84,13 @@ struct vt_setactivate {
>   
>   #define VT_SETACTIVATE	0x560F	/* Activate and set the mode of a console */
>   
> +struct vt_consizecsrpos {
> +	unsigned short con_rows;	/* number of console rows */
> +	unsigned short con_cols;	/* number of console columns */
> +	unsigned short csr_row;		/* current cursor's row */
> +	unsigned short csr_col;		/* current cursor's column */

Use __u16 pls.

> +};
> +
> +#define VT_GETCONSIZECSRPOS 0x5610  /* get console size and cursor position */

Can we define that properly as
   _IOR(0x56, 0x10, struct vt_consizecsrpos)
? Note this would still differ from "conflicting":
#define VIDIOC_G_FBUF            _IOR('V', 10, struct v4l2_framebuffer)

thanks,
-- 
js
suse labs