Re: [PATCH] vt_ioctl: make VT_RESIZEX behave like VT_RESIZE

Maciej W. Rozycki
2021-04-12T13:30:35Z

On Mon, 12 Apr 2021, Daniel Vetter wrote:

> > Note that it's entirely possible that things continue to work well
> > despite this warning. It's unclear to me from your email if you
> > actually see any difference (and apparently you're not able to see it
> > right now due to not being close to the machine).
>
> Original search didn't turn up any users of VT_RESIZEX, this is the
> first. And looking at the source code I think we could outright remove
> support for VT_RESIZEX (but make it silent) and everything should keep
> working:
>
> /*
> * ALWAYS do a VT_RESIZE, even if we already did a VT_RESIZEX
> on a 1.3.3 or higher kernel,
> * until those kernel programmers make this unambiguous
> */
>
> if (do_VT_RESIZE(curr_textmode->cols, curr_textmode->rows,
> resize1x1)) sresize=TRUE;
>
> if (check_kernel_version(1,3,3, "VT_RESIZEX"))
> {
> /*
> * VDisplay must de divided by 2 for DoubleScan modes,
> * or VT_RESIZEX will fail -- until someone fixes the kernel
> * so it understands about doublescan modes.
> */
> if (do_VT_RESIZEX(curr_textmode->cols,
> curr_textmode->rows,
> curr_textmode->VDisplay /
> (MOFLG_ISSET(curr_textmode, ATTR_DOUBLESCAN) ? 2 : 1),
> curr_textmode->FontHeight,
> curr_textmode->HDisplay/8*curr_textmode->FontWidth,
> curr_textmode->FontWidth, resize1x1)) sresize=TRUE;
> }
>
> The functions are just straightforward wrappers. There's also no cvs
> repo, changelog or old releases before 2000 that would shed some light
> on why this code even exists.

I did some archaeology then, using a local copy of the linux-mips.org
Linux tree that has historic information imported from the old oss.sgi.com
MIPS/Linux CVS repo. According to that the ioctl was added with or
shortly before 2.1.14:

commit beb116954b9b7f3bb56412b2494b562f02b864b1
Author: Ralf Baechle <ralf@linux-mips.org>
Date: Tue Jan 7 02:33:00 1997 +0000

Import of Linux/MIPS 2.1.14

and, importantly, it was used to set some parameters:

if ( vlin )
video_scan_lines = vlin;
if ( clin )
video_font_height = clin;

used by `con_adjust_height' in drivers/char/vga.c: `video_scan_lines' to
set the vertical display limit (so that it is a whole multiple of the new
font height) and `video_font_height' to set the cursor scan lines in the
CRTC. The function was used by the PIO_FONTX and PIO_FONTRESET VT ioctls
at that point.

That code was moved to `vgacon_adjust_height' in drivers/video/vgacon.c
and then drivers/video/console/vgacon.c. The code is still there, serving
the KDFONTOP ioctl.

With:

commit 9736a3546de7b6a2b16ad93539e4b3ac72b385bb
Author: Ralf Baechle <ralf@linux-mips.org>
Date: Thu Jun 5 10:06:35 2003 +0000

Merge with Linux 2.5.66.

the parameters were moved into `struct vc_data':

if (vlin)
- video_scan_lines = vlin;
+ vc->vc_scan_lines = vlin;
if (clin)
- video_font_height = clin;
+ vc->vc_font.height = clin;

and this piece of code to set them only removed with the change discussed
here.

So without even looking at the VT, which I'll surely get to eventually, I
conclude this change regresses font resizing (KD_FONT_OP_SET) once a new
resolution has been set with svgatextmode. I think this change needs to
be reverted, especially as the problematic PIO_FONT ioctl referred has
been since removed with commit ff2047fb755d ("vt: drop old FONT ioctls").

Maciej